  
  [1X7 [33X[0;0YFinite Classical Polar Spaces[133X[101X
  
  [33X[0;0YIn  this chapter we describe how to use [5XFinInG[105X to work with finite classical
  polar spaces.[133X
  
  
  [1X7.1 [33X[0;0YFinite Classical Polar Spaces[133X[101X
  
  [33X[0;0YA  [13Xpolar  space[113X  is  a  point-line incidence geometry, satisfying the famous
  one-or-all  axiom,  i.e.  for  any point [22XP[122X, not incident with a line [22Xl[122X, [22XP[122X is
  collinear with exactly one point of [22Xl[122X or with all points of [22Xl[122X. The axiomatic
  treatment  of  polar  spaces  has  its  foundations in [Vel59], [Tit74], and
  [BS74],  the latter in which the one-or-all axiom is described. Polar spaces
  are axiomatically, point-line geometries, but may contain higher dimensional
  projective  subspaces  too.  All  maximal subspaces have the same projective
  dimension, and this determines the rank of the polar space.[133X
  
  [33X[0;0YWell  known  examples  of [13Xfinite[113X polar spaces are the geometries attached to
  sesquilinear and quadratic forms of vector spaces over a finite field, these
  geometries  are  called  the  [13Xfinite  classical  polar  spaces[113X.  For a given
  sesquilinear, respectively quadratic, form [22Xf[122X, the elements of the associated
  geometry are the totally isotropic, respectively totally singular, subspaces
  of the vectors space with relation to the form [22Xf[122X. The treatment of the forms
  is done through the package [5XForms[105X.[133X
  
  [33X[0;0YFrom  the  axiomatic  point of view, a polar space is a point-line geometry,
  and  has rank at least 2. Considering a sesquilinear or quadratic form [22Xf[122X, of
  Witt  index  1,  the associated geometry consists only of projective points,
  and  is  then  in the axiomatic treatment, not a polar space. However, as is
  the  case  for  projective  spaces, we will consider the rank one geometries
  associated  to  forms of Witt index 1 as examples of classical polar spaces.
  Even  the  elliptic quadric on the projective line, a [13Xgeometry[113X associated to
  an  elliptic  quadratic form on a two dimensional vector space over a finite
  field,  is  considered as a classical polar space, though it has no singular
  subspaces.  The  reason for this treatment is that most, if not all, methods
  for  operations  applicable  on these geometries, rely on the same algebraic
  methodology.  So,  in [5XFinInG[105X, a classical polar space (sometimes abbreviated
  to polar space), is the geometry associated with a sesquilinear or quadratic
  form on a finite dimensional vector space over a finite field.[133X
  
  [1X7.1-1 IsClassicalPolarSpace[101X
  
  [29X[2XIsClassicalPolarSpace[102X[32X Category
  
  [33X[0;0YThis  category  is  a  subcategory  of  [10XIsLieGeometry[110X,  and contains all the
  geometries associated to a non-degenerate sesquilinear or quadratic form.[133X
  
  [33X[0;0YThe  underlying  vector  space  and matrix group are to our advantage in the
  treatment  of  classical  polar  spaces.  We  refer the reader to [HT91] and
  [Cam00b]  for  the  necessary  background  theory  (if  it  is not otherwise
  provided), and we follow the approach of [Cam00b] to introduce all different
  flavours.[133X
  
  [33X[0;0YConsider the projective space [22XPG(n,q)[122X with underlying vector space [22XV(n+1,q)[122X.
  Consider  a  non-degenerate  sesquilinear  form  [22Xf[122X.  Then  [22Xf[122X  is  Hermitian,
  alternating  or  symmetric.  When  the  characteristic  of the field is odd,
  respectively   even,   a  symmetric  bilinear  form  is  called  orthogonal,
  respectively, pseudo. We do not consider the pseudo case, so we suppose that
  [22Xf[122X  is  Hermitian,  symplectic  or  orthogonal.  The  classical  polar  space
  associated  with  [22Xf[122X  is  the  incidence  geometry  whose elements are of the
  subspaces  of  [22XPG(n,q)[122X whose underlying vector subspace is totally isotropic
  with  relation  to  [22Xf[122X.  We  call  a  polar  space  [13XHermitian[113X,  respectively,
  [13Xsymplectic[113X,  [13Xorthogonal[113X,  if  the underlying sesquilinear form is Hermitian,
  respectively, symplectic, orthogonal.[133X
  
  [33X[0;0YSymmetric  bilinear  forms have completely different geometric properties in
  even  characteristic  than  in  odd characteristic. On the other hand, polar
  spaces   geometrically   comparable   to  orthogonal  polar  spaces  in  odd
  characteristic, do exist in even characteristic. The algebraic background is
  now  established  by  quadratic  forms on a vector space instead of bilinear
  forms.  Consider a non-singular quadratic form [22Xq[122X on a vector space [22XV(n+1,q)[122X.
  The  classical polar space associated with [22Xf[122X is the incidence geometry whose
  elements  are  the  subspaces of [22XPG(n,q)[122X whose underlying vector subspace is
  totally  singular  with  relation to [22Xq[122X. The connection with orthogonal polar
  spaces  in  odd  characteristic  is  clear,  since  in  odd  characteristic,
  quadratic  forms  and symmetric bilinear forms are equivalent. Therefore, we
  call  polar  spaces with an underlying quadratic form in even characteristic
  also [13Xorthogonal[113X polar spaces.[133X
  
  [1X7.1-2 PolarSpace[101X
  
  [29X[2XPolarSpace[102X( [3Xform[103X ) [32X operation
  [29X[2XPolarSpace[102X( [3Xpol[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya classical polar space[133X
  
  [33X[0;0Y[3Xform[103X  must  be  a  sesquilinear  or quadratic form created by use of the GAP
  package [5XForms[105X. In the second variant, the argument [3Xpol[103X must be a polarity of
  a  projective  space.  An error message will be displayed if [3Xpol[103X is a pseudo
  polarity.  We  refer  to  Chapter  [14X6[114X  for  more information on polarities of
  projective  spaces,  and more particularly to Section [14X6.3[114X for the connection
  between polarities and forms.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmat := [[0,0,0,1],[0,0,-2,0],[0,2,0,0],[-1,0,0,0]]*Z(5)^0;[127X[104X
    [4X[28X[ [ 0*Z(5), 0*Z(5), 0*Z(5), Z(5)^0 ], [ 0*Z(5), 0*Z(5), Z(5)^3, 0*Z(5) ], [128X[104X
    [4X[28X  [ 0*Z(5), Z(5), 0*Z(5), 0*Z(5) ], [ Z(5)^2, 0*Z(5), 0*Z(5), 0*Z(5) ] ][128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(25));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X3,GF(5^2)): x1*y4+Z(5)^3*x2*y3+Z(5)*x3*y2-x4*y1=0 >[128X[104X
    [4X[25Xgap>[125X [27Xr := PolynomialRing(GF(32),4);[127X[104X
    [4X[28XGF(2^5)[x_1,x_2,x_3,x_4][128X[104X
    [4X[25Xgap>[125X [27Xpoly := r.3*r.2+r.1*r.4;[127X[104X
    [4X[28Xx_1*x_4+x_2*x_3[128X[104X
    [4X[25Xgap>[125X [27Xform := QuadraticFormByPolynomial(poly,r);[127X[104X
    [4X[28X< quadratic form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(3,GF(2^5)): x_1*x_4+x_2*x_3=0 >[128X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(5,GF(7));[127X[104X
    [4X[28X[ [ Z(7)^0, 0*Z(7), 0*Z(7), 0*Z(7), 0*Z(7) ], [128X[104X
    [4X[28X  [ 0*Z(7), Z(7)^0, 0*Z(7), 0*Z(7), 0*Z(7) ], [128X[104X
    [4X[28X  [ 0*Z(7), 0*Z(7), Z(7)^0, 0*Z(7), 0*Z(7) ], [128X[104X
    [4X[28X  [ 0*Z(7), 0*Z(7), 0*Z(7), Z(7)^0, 0*Z(7) ], [128X[104X
    [4X[28X  [ 0*Z(7), 0*Z(7), 0*Z(7), 0*Z(7), Z(7)^0 ] ][128X[104X
    [4X[25Xgap>[125X [27Xphi := PolarityOfProjectiveSpace(mat,GF(7));[127X[104X
    [4X[28X<polarity of PG(4, GF(7)) >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(phi);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(4,GF(7)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2=0 >[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [33X[0;0Y[5XFinInG[105X  relies  on  the package [5XForms[105X for its facility with sesquilinear and
  quadratic forms. One can specify a polar space with a user-defined form, and
  we  refer  to  the  documentation  for  [5XForms[105X for information on how one can
  create and use forms. Here we just display a worked example.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xid := IdentityMat(7, GF(3));;[127X[104X
    [4X[25Xgap>[125X [27Xform := QuadraticFormByMatrix(id, GF(3));[127X[104X
    [4X[28X< quadratic form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace( form );[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X6,GF(3)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2=0 >[128X[104X
    [4X[25Xgap>[125X [27Xpsl32 := PSL(3,2);[127X[104X
    [4X[28XGroup([ (4,6)(5,7), (1,2,4)(3,6,5) ])[128X[104X
    [4X[25Xgap>[125X [27Xreps:=[[1,1,1,0,0,0,0], [-1,1,1,0,0,0,0], [1,-1,1,0,0,0,0], [1,1,-1,0,0,0,0]]*Z(3)^0;;[127X[104X
    [4X[25Xgap>[125X [27Xovoid := Union( List(reps, x-> Orbit(psl32, x, Permuted)) );;[127X[104X
    [4X[25Xgap>[125X [27Xovoid := List(ovoid, x -> VectorSpaceToElement(ps, x));;[127X[104X
    [4X[25Xgap>[125X [27Xplanes := AsList( Planes( ps ) );;[127X[104X
    [4X[28X#I  Computing collineation group of canonical polar space...[128X[104X
    [4X[25Xgap>[125X [27XForAll(planes, p -> Number(ovoid, x -> x in p) = 1);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.2 [33X[0;0YCanonical and standard Polar Spaces[133X[101X
  
  [33X[0;0YTo  introduce  the classification of polar spaces, we use the classification
  of the underlying forms in similarity classes. We follow mostly the approach
  and terminology of [KL90], as we did in the manual of the package [5XForms[105X.[133X
  
  [33X[0;0YConsider  a vector space [22XV=V(n+1,q)[122X and a sesquilinear form [22Xf[122X on [22XV[122X. The pair
  [22X(V,f)[122X  is  called  a  formed space. Consider now two formed spaces [22X(V,f)[122X and
  [22X(V,f')[122X,  where  [22Xf[122X  and  [22Xf'[122X  are  two sesquilinear forms on [22XV[122X. A non-singular
  linear map [22Xϕ[122X from [22XV[122X to itself induces a [13Xsimilarity[113X of the formed space [22X(V,f)[122X
  to the formed space [22X(V,f')[122X if and only if[133X
  
  [33X[0;0Y[22Xf(v,w) = λ f'(ϕ(v),ϕ(w)),[122X[133X
  
  [33X[0;0Yfor all vectors [22Xv,w[122X and some non-zero [22Xλ ∈ GF(q)[122X . Up to similarity, there is
  only  one  class  of  non-degenerate  Hermitian  forms,  and  one  class  of
  non-degenerate  symplectic  forms  on  a given vector space [22XV[122X. For symmetric
  bilinear  forms  in  odd  characteristic,  the  number of similarity classes
  depends  on  the  dimension  of  [22XV[122X.  In  odd  dimension,  there  is only one
  similarity  class,  and  non-degenerate  forms  in  this  class  are  called
  parabolic  (bilinear)  forms.  In  even  dimension, there are two similarity
  classes,  and  non-degenerate  forms are either elliptic (bilinear) forms or
  hyperbolic (bilinear) forms.[133X
  
  [33X[0;0YConsider now a vector space [22XV[122X and a quadratic form [22Xq[122X on [22XV[122X. The pair [22X(V,q)[122X is
  called  a  formed  space.  Consider  now two formed spaces [22X(V,q)[122X and [22X(V,q')[122X,
  where  [22Xq[122X  and [22Xq'[122X are two quadratic forms on [22XV[122X. A non-degenerate linear map [22Xϕ[122X
  from  [22XV[122X  to  itself  induces  a  [13Xsimilarity[113X of the formed space [22X(V,q)[122X to the
  formed space [22X(V,q')[122X if and only if[133X
  
  [33X[0;0Y[22Xq(v) = λ q'(ϕ(v)))[122X,[133X
  
  [33X[0;0Yfor  all vectors [22Xv[122X and some non-zero [22Xλ ∈ GF(q)[122X . For quadratic forms in even
  characteristic, the number of similarity classes depends on the dimension of
  [22XV[122X.  In odd dimension, there is only one similarity class, and non-degenerate
  forms  in  this  class  are  called  parabolic  (bilinear)  forms.  In  even
  dimension,  there  are  two similarity classes, and non-degenerate forms are
  either elliptic (bilinear) forms or hyperbolic (bilinear) forms.[133X
  
  [33X[0;0YIf [22Xϕ[122X induces a similarity of a formed vector space such that [22Xλ = 1[122X, then the
  similarity  is  called an [13Xisometry[113X of the formed vector space. In almost all
  cases,  each  similarity class contains exactly one isometry class. Only the
  orthogonal  sesquilinear  forms  (in  odd  characteristic) have two isometry
  classes.  Consequently,  if an isometry exists between formed vector spaces,
  they  are  called  [13Xisometric[113X.  Projectively, a formed vector space becomes a
  classical  polar  space  embedded in a projective space. Obviously, forms in
  the  same similarity class determine exactly the same classical polar space.
  Conversely,  it is well known that a classical polar space determines a form
  up  to a constant factor, i.e. it determines a similarity class of forms. In
  [5XFinInG[105X,  the  word  [13Xcanonical[113X  is  used  in  the  mathematical sense, i.e. a
  classical  polar  space  is  [13Xcanonical[113X  if its determining form belongs to a
  fixed  similarity  class.  A classical polar space is called [13Xstandard[113X if its
  determining  form  is  the  fixed representative of the canonical similarity
  class.  Hence  a  [13Xstandard[113X  classical  polar  space  is  always  a [13Xcanonical[113X
  classical  polar  space, a canonical polar space is determined by a standard
  form up to a constant factor. In the following table, we summerise the above
  information  on  polar  spaces,  together  with  the standard forms that are
  chosen  in  [5XFinInG[105X. Note that Tr refers to the absolute trace map from [22XGF(q)[122X
  to [22XGF(p)[122X.[133X
  
      ┌───────────────────────┬──────────────────────────────────────────────────────────────────────────┬─────────────────────┬──────────────────────┐
      │ polar space           │ standard form                                                            │ characteristic [22Xp[122X    │ projective dimension │ 
      ├───────────────────────┼──────────────────────────────────────────────────────────────────────────┼─────────────────────┼──────────────────────┤
      │ hermitian polar space │ [22XX_0^q+1 + X_1^q+1 + ... + X_n^q+1[122X                                        │ odd or even         │ odd or even          │ 
      │ symplectic space      │ [22XX_0 Y_1 - Y_0X_1+ ... + X_n-1Y_n - Y_n-1X_n[122X                              │ odd or even         │ odd                  │ 
      │ hyperbolic quadric    │ [22XX_0 X_1 + ... + X_n-1X_n[122X                                                 │ [22Xp ≡ 3 mod 4[122X or [22Xp=2[122X  │ odd                  │ 
      │ hyperbolic quadric    │ [22X2(X_0 X_1 + ... + X_n-1X_n)[122X                                              │ [22Xp ≡ 1 mod 4[122X         │ odd                  │ 
      │ parabolic quadric     │ [22XX_0^2 + X_1X_2 + ... + X_n-1X_n[122X                                          │ [22Xp ≡ 1,3 mod 8[122Xor [22Xp=2[122X │ even                 │ 
      │ parabolic quadric     │ [22Xt(X_0^2 + X_1X_2 + ... + X_n-1X_n)[122X, t a primitve element of GF(p)        │ [22Xp ≡ 5,7 mod 8[122X       │ even                 │ 
      │ elliptic quadric      │ [22XX_0^2 + X_1^2 + X_2X_3 + ... + X_n-1X_n[122X                                  │ [22Xp ≡ 3 mod 4[122X         │ odd                  │ 
      │ elliptic quadric      │ [22XX_0^2 + tX_1^2 + X_2X_3 + ... + X_n-1X_n[122X, [22Xt[122X a primitive element of [22XGF(p)[122X │ [22Xp ≡ 1 mod 4[122X         │ odd                  │ 
      │ elliptic quadric      │ [22XX_0^2 + X_0X_1 + dX_1^2 + X_2X_3 + ... + X_n-1X_n[122X, [22XTr(d)=1[122X               │ even                │ odd                  │ 
      └───────────────────────┴──────────────────────────────────────────────────────────────────────────┴─────────────────────┴──────────────────────┘
  
       [1XTable:[101X finite classical polar spaces
  
  
  [33X[0;0YWe refer to Appendix [14XB[114X for information on the operations that construct gram
  matrices that are used to obtain the above standard forms.[133X
  
  [33X[0;0YThe  [5XFinInG[105X  provides  a wealth of flexibility in constructing polar spaces.
  The  user  may  choose  a particular quadratic or sesquilinear form, but may
  also  chose  to construct polars spaces that have one of the above mentioned
  forms  as underlying form. Furthermore, [5XFinInG[105X will detect when necessary if
  the user-constructed polar space is canonical. This mechanism gives the user
  complete  flexibility  while  avoiding  unnecessary  computations  when, for
  example, constructing the collineation group of a user-defined polar space.[133X
  
  [33X[0;0YThe  following  five operations always return polar spaces induced by one of
  the above standard forms.[133X
  
  [1X7.2-1 SymplecticSpace[101X
  
  [29X[2XSymplecticSpace[102X( [3Xd[103X, [3XF[103X ) [32X operation
  [29X[2XSymplecticSpace[102X( [3Xd[103X, [3Xq[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya symplectic polar space[133X
  
  [33X[0;0YThis function returns the symplectic polar space of dimension [3Xd[103X over [3XF[103X for a
  field [3XF[103X or over GF([3Xq[103X) for a prime power [3Xq[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[28XSymplectic form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . 1 . .[128X[104X
    [4X[28X 1 . . .[128X[104X
    [4X[28X . . . 1[128X[104X
    [4X[28X . . 1 .[128X[104X
    [4X[28XWitt Index: 2[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.2-2 HermitianPolarSpace[101X
  
  [29X[2XHermitianPolarSpace[102X( [3Xd[103X, [3XF[103X ) [32X operation
  [29X[2XHermitianPolarSpace[102X( [3Xd[103X, [3Xq[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya Hermitian polar space[133X
  
  [33X[0;0YThis  function returns the Hermitian polar space of dimension [3Xd[103X over [3XF[103X for a
  field [3XF[103X or over GF([3Xq[103X) for a prime power [3Xq[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(2,25);[127X[104X
    [4X[28XH(2, 5^2)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XH(2, 25)[128X[104X
    [4X[28XHermitian form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X 1 . .[128X[104X
    [4X[28X . 1 .[128X[104X
    [4X[28X . . 1[128X[104X
    [4X[28XPolynomial: [ [  x_1^6+x_2^6+x_3^6 ] ][128X[104X
    [4X[28XWitt Index: 1[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.2-3 ParabolicQuadric[101X
  
  [29X[2XParabolicQuadric[102X( [3Xd[103X, [3XF[103X ) [32X operation
  [29X[2XParabolicQuadric[102X( [3Xd[103X, [3Xq[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya parabolic quadric[133X
  
  [33X[0;0Y[3Xd[103X  must  be  an  even  positive integer. This function returns the parabolic
  quadric  of dimension [3Xd[103X over [3XF[103X for a field [3XF[103X or over GF([3Xq[103X) for a prime power
  [3Xq[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(2,9);[127X[104X
    [4X[28XQ(2, 9)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ(2, 9)[128X[104X
    [4X[28XParabolic bilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X 1 . .[128X[104X
    [4X[28X . . 2[128X[104X
    [4X[28X . 2 .[128X[104X
    [4X[28XPolynomial: [ [  x_1^2+x_2*x_3 ] ][128X[104X
    [4X[28XWitt Index: 1[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,16);[127X[104X
    [4X[28XQ(4, 16)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ(4, 16)[128X[104X
    [4X[28XParabolic quadratic form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X 1 . . . .[128X[104X
    [4X[28X . . 1 . .[128X[104X
    [4X[28X . . . . .[128X[104X
    [4X[28X . . . . 1[128X[104X
    [4X[28X . . . . .[128X[104X
    [4X[28XPolynomial: [ [  x_1^2+x_2*x_3+x_4*x_5 ] ][128X[104X
    [4X[28XWitt Index: 2[128X[104X
    [4X[28XBilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . . . . .[128X[104X
    [4X[28X . . 1 . .[128X[104X
    [4X[28X . 1 . . .[128X[104X
    [4X[28X . . . . 1[128X[104X
    [4X[28X . . . 1 .[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.2-4 HyperbolicQuadric[101X
  
  [29X[2XHyperbolicQuadric[102X( [3Xd[103X, [3XF[103X ) [32X operation
  [29X[2XHyperbolicQuadric[102X( [3Xd[103X, [3Xq[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya hyperbolic quadric[133X
  
  [33X[0;0Y[3Xd[103X  must  be  an  odd  positive integer. This function returns the hyperbolic
  quadric  of dimension [3Xd[103X over [3XF[103X for a field [3XF[103X or over GF([3Xq[103X) for a prime power
  [3Xq[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,3);[127X[104X
    [4X[28XQ+(5, 3)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ+(5, 3)[128X[104X
    [4X[28XHyperbolic bilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . 2 . . . .[128X[104X
    [4X[28X 2 . . . . .[128X[104X
    [4X[28X . . . 2 . .[128X[104X
    [4X[28X . . 2 . . .[128X[104X
    [4X[28X . . . . . 2[128X[104X
    [4X[28X . . . . 2 .[128X[104X
    [4X[28XPolynomial: [ [  x_1*x_2+x_3*x_4+x_5*x_6 ] ][128X[104X
    [4X[28XWitt Index: 3[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(3,4);[127X[104X
    [4X[28XQ+(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ+(3, 4)[128X[104X
    [4X[28XHyperbolic quadratic form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . 1 . .[128X[104X
    [4X[28X . . . .[128X[104X
    [4X[28X . . . 1[128X[104X
    [4X[28X . . . .[128X[104X
    [4X[28XPolynomial: [ [  x_1*x_2+x_3*x_4 ] ][128X[104X
    [4X[28XWitt Index: 2[128X[104X
    [4X[28XBilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . 1 . .[128X[104X
    [4X[28X 1 . . .[128X[104X
    [4X[28X . . . 1[128X[104X
    [4X[28X . . 1 .[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.2-5 EllipticQuadric[101X
  
  [29X[2XEllipticQuadric[102X( [3Xd[103X, [3XF[103X ) [32X operation
  [29X[2XEllipticQuadric[102X( [3Xd[103X, [3Xq[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan elliptic quadric[133X
  
  [33X[0;0Y[3Xd[103X  must  be  an  odd  positive  integer.  This function returns the elliptic
  quadric  of dimension [3Xd[103X over [3XF[103X for a field [3XF[103X or over GF([3Xq[103X) for a prime power
  [3Xq[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,27);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[28XElliptic bilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X 1 . . .[128X[104X
    [4X[28X . 1 . .[128X[104X
    [4X[28X . . . 2[128X[104X
    [4X[28X . . 2 .[128X[104X
    [4X[28XPolynomial: [ [  x_1^2+x_2^2+x_3*x_4 ] ][128X[104X
    [4X[28XWitt Index: 1[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,8);[127X[104X
    [4X[28XQ-(5, 8)[128X[104X
    [4X[25Xgap>[125X [27XDisplay(ps);[127X[104X
    [4X[28XQ-(5, 8)[128X[104X
    [4X[28XElliptic quadratic form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X 1 1 . . . .[128X[104X
    [4X[28X . 1 . . . .[128X[104X
    [4X[28X . . . 1 . .[128X[104X
    [4X[28X . . . . . .[128X[104X
    [4X[28X . . . . . 1[128X[104X
    [4X[28X . . . . . .[128X[104X
    [4X[28XPolynomial: [ [  x_1^2+x_1*x_2+x_2^2+x_3*x_4+x_5*x_6 ] ][128X[104X
    [4X[28XWitt Index: 2[128X[104X
    [4X[28XBilinear form[128X[104X
    [4X[28XGram Matrix:[128X[104X
    [4X[28X . 1 . . . .[128X[104X
    [4X[28X 1 . . . . .[128X[104X
    [4X[28X . . . 1 . .[128X[104X
    [4X[28X . . 1 . . .[128X[104X
    [4X[28X . . . . . 1[128X[104X
    [4X[28X . . . . 1 .[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [33X[0;0YThe  following  operations  are  applicable  on any classical polar space in
  [5XFinInG[105X.[133X
  
  [1X7.2-6 IsCanonicalPolarSpace[101X
  
  [29X[2XIsCanonicalPolarSpace[102X( [3Xps[103X ) [32X attribute
  [6XReturns:[106X  [33X[0;10Ytrue or false[133X
  
  [33X[0;0YThis  attribute returns true when a polar space with a particular underlying
  form  is  canonical.  The  execution  of  this  attribute  on a general user
  constructed  polar  space  needs to check the type of [3Xps[103X. The obtained extra
  information  is stored automatically as attribute for [3Xps[103X, as can be noted by
  the different printing of [3Xps[103X before and after execution.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmat := [[0,1,0,0],[0,0,0,0],[0,0,0,1],[0,0,0,0]]*Z(5)^0;[127X[104X
    [4X[28X[ [ 0*Z(5), Z(5)^0, 0*Z(5), 0*Z(5) ], [ 0*Z(5), 0*Z(5), 0*Z(5), 0*Z(5) ], [128X[104X
    [4X[28X  [ 0*Z(5), 0*Z(5), 0*Z(5), Z(5)^0 ], [ 0*Z(5), 0*Z(5), 0*Z(5), 0*Z(5) ] ][128X[104X
    [4X[25Xgap>[125X [27Xform := QuadraticFormByMatrix(mat,GF(5));[127X[104X
    [4X[28X< quadratic form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(3,GF(5)): x_1*x_2+x_3*x_4=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsCanonicalPolarSpace(ps);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xps;[127X[104X
    [4X[28XQ+(3, 5): x_1*x_2+x_3*x_4=0[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[1,0,0],[0,0,1],[0,1,0]]*Z(3)^0;[127X[104X
    [4X[28X[ [ Z(3)^0, 0*Z(3), 0*Z(3) ], [ 0*Z(3), 0*Z(3), Z(3)^0 ], [128X[104X
    [4X[28X  [ 0*Z(3), Z(3)^0, 0*Z(3) ] ][128X[104X
    [4X[25Xgap>[125X [27Xform := QuadraticFormByMatrix(mat,GF(3));[127X[104X
    [4X[28X< quadratic form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(2,GF(3)): x_1^2-x_2*x_3=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsCanonicalPolarSpace(ps);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xps;[127X[104X
    [4X[28XQ(2, 3): x_1^2-x_2*x_3=0[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.2-7 CanonicalPolarSpace[101X
  
  [29X[2XCanonicalPolarSpace[102X( [3Xform[103X ) [32X operation
  [29X[2XCanonicalPolarSpace[102X( [3XP[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya classical polar space[133X
  
  [33X[0;0Ythe  canonical  polar  space  isometric  to  the  given polar space [3XP[103X or the
  classical polar space with underlying form [3Xform[103X.[133X
  
  [1X7.2-8 StandardPolarSpace[101X
  
  [29X[2XStandardPolarSpace[102X( [3Xform[103X ) [32X operation
  [29X[2XStandardPolarSpace[102X( [3XP[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya classical polar space[133X
  
  [33X[0;0Ythe  polar  space  induced by a standard form and similar to the given polar
  space [3XP[103X or the classical polar space with underlying form [3Xform[103X.[133X
  
  
  [1X7.3 [33X[0;0YBasic operations for finite classical polar spaces[133X[101X
  
  [1X7.3-1 UnderlyingVectorSpace[101X
  
  [29X[2XUnderlyingVectorSpace[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya vector space[133X
  
  [33X[0;0YThe  polar  space  [3Xps[103X  is  the  geometry  associated  with a sesquilinear or
  quadratic form [22Xf[122X. The vector space on which [22Xf[122X is acting is returned.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,4);[127X[104X
    [4X[28XQ-(5, 4)[128X[104X
    [4X[25Xgap>[125X [27Xvs := UnderlyingVectorSpace(ps);[127X[104X
    [4X[28X( GF(2^2)^6 )[128X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,81);[127X[104X
    [4X[28XW(3, 81)[128X[104X
    [4X[25Xgap>[125X [27Xvs := UnderlyingVectorSpace(ps);[127X[104X
    [4X[28X( GF(3^4)^4 )[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-2 AmbientSpace[101X
  
  [29X[2XAmbientSpace[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe ambient projective space[133X
  
  [33X[0;0YWhen  [3Xps[103X  is  a  polar  space, this operation returns the ambient projective
  space,  i.e. the underlying projective space of the sequilinear or quadratic
  form that defines [3Xps[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,4);[127X[104X
    [4X[28XQ-(5, 4)[128X[104X
    [4X[25Xgap>[125X [27XAmbientSpace(ps);[127X[104X
    [4X[28XProjectiveSpace(5, 4)[128X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,81);[127X[104X
    [4X[28XW(3, 81)[128X[104X
    [4X[25Xgap>[125X [27XAmbientSpace(ps);[127X[104X
    [4X[28XProjectiveSpace(3, 81)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-3 ProjectiveDimension[101X
  
  [29X[2XProjectiveDimension[102X( [3Xps[103X ) [32X operation
  [29X[2XDimension[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe dimension of the ambient projective space of [3Xps[103X[133X
  
  [33X[0;0YWhen  [3Xps[103X is a polar space, an ambient projective space [22XP[122X is uniquely defined
  and  can  be  asked  using  [11XAmbientSpace[111X.  This  operation  and  its synomym
  [11XDimension[111X returns the dimension of [22XP[122X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,4);[127X[104X
    [4X[28XQ-(5, 4)[128X[104X
    [4X[25Xgap>[125X [27XProjectiveDimension(ps);[127X[104X
    [4X[28X5[128X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,81);[127X[104X
    [4X[28XW(3, 81)[128X[104X
    [4X[25Xgap>[125X [27XProjectiveDimension(ps);[127X[104X
    [4X[28X3[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-4 Rank[101X
  
  [29X[2XRank[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe rank of [3Xps[103X[133X
  
  [33X[0;0YWhen  [3Xps[103X  is  a  polar  space, its rank, i.e. the number of different types,
  equals the Witt index of the defining sesquilinear or quadratic form.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,4);[127X[104X
    [4X[28XQ-(5, 4)[128X[104X
    [4X[25Xgap>[125X [27XRank(ps);[127X[104X
    [4X[28X2[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,4);[127X[104X
    [4X[28XQ+(5, 4)[128X[104X
    [4X[25Xgap>[125X [27XRank(ps);[127X[104X
    [4X[28X3[128X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(7,81);[127X[104X
    [4X[28XW(7, 81)[128X[104X
    [4X[25Xgap>[125X [27XRank(ps);[127X[104X
    [4X[28X4[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-5 BaseField[101X
  
  [29X[2XBaseField[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe base field of the polar space [3Xps[103X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,7);[127X[104X
    [4X[28XQ+(5, 7)[128X[104X
    [4X[25Xgap>[125X [27XBaseField(ps);[127X[104X
    [4X[28XGF(7)[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(2,256);[127X[104X
    [4X[28XH(2, 16^2)[128X[104X
    [4X[25Xgap>[125X [27XBaseField(ps);[127X[104X
    [4X[28XGF(2^8)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-6 IsHyperbolicQuadric[101X
  
  [29X[2XIsHyperbolicQuadric[102X( [3Xps[103X ) [32X property
  [6XReturns:[106X  [33X[0;10Ytrue or false[133X
  
  [33X[0;0Yreturns true if and only if [3Xps[103X is a hyperbolic quadric.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(6,GF(5));[127X[104X
    [4X[28X< mutable compressed matrix 6x6 over GF(5) >[128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(5));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X5,GF(5)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsHyperbolicQuadric(ps);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(6,GF(7));[127X[104X
    [4X[28X< mutable compressed matrix 6x6 over GF(7) >[128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(7));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X5,GF(7)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsHyperbolicQuadric(ps);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-7 IsEllipticQuadric[101X
  
  [29X[2XIsEllipticQuadric[102X( [3Xps[103X ) [32X property
  [6XReturns:[106X  [33X[0;10Ytrue or false[133X
  
  [33X[0;0Yreturns true if and only if [3Xps[103X is an elliptic quadric.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(6,GF(5));[127X[104X
    [4X[28X< mutable compressed matrix 6x6 over GF(5) >[128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(5));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X5,GF(5)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsEllipticQuadric(ps);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(6,GF(7));[127X[104X
    [4X[28X< mutable compressed matrix 6x6 over GF(7) >[128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(7));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X5,GF(7)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsEllipticQuadric(ps);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.3-8 IsParabolicQuadric[101X
  
  [29X[2XIsParabolicQuadric[102X( [3Xps[103X ) [32X property
  [6XReturns:[106X  [33X[0;10Ytrue or false[133X
  
  [33X[0;0Yreturns true if and only if [3Xps[103X is a parabolic quadric.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xmat := IdentityMat(5,GF(9));[127X[104X
    [4X[28X[ [ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3) ], [128X[104X
    [4X[28X  [ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ], [128X[104X
    [4X[28X  [ 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ], [128X[104X
    [4X[28X  [ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3) ], [128X[104X
    [4X[28X  [ 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ] ][128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(mat,GF(9));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(4,GF(3^2)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsParabolicQuadric(ps);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[1,0,0,0,0],[0,0,1,0,0],[0,0,0,0,0],[0,0,0,0,1],[0,0,0,0,0]]*Z(2)^0;[127X[104X
    [4X[28X[ [ Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0 ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2) ] ][128X[104X
    [4X[25Xgap>[125X [27Xform := QuadraticFormByMatrix(mat,GF(8));[127X[104X
    [4X[28X< quadratic form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace(4,GF(2^3)): x_1^2+x_2*x_3+x_4*x_5=0 >[128X[104X
    [4X[25Xgap>[125X [27XIsParabolicQuadric(ps);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.4 [33X[0;0YSubspaces of finite classical polar spaces[133X[101X
  
  [33X[0;0YThe  elements  of  a finite classical polar space [22XP[122X are the subspaces of the
  ambient  projective  space  that  are totally isotropic with relation to the
  sesquilinear  or  quadratic  form  that defines [22XP[122X. Constructing subspaces of
  finite  classical  polar  spaces  is  done  as in the projective space case,
  except  that  additional checks are implemented in the methods to check that
  the  subspace  of the vector space is totally isotropic. The empty subspace,
  also  called  the trivial subspace, which has dimension -1, corresponds with
  the  zero  dimensional  vector  space  of the underlying vector space of the
  ambient  projective space of [22XP[122X, and is of course totally isotropic. As such,
  is  is  considered  as  a  subspace of a finite classical polar space in the
  mathematical  sense,  but  not  as an element of the incidence geometry, and
  hence     do     in     [5XFinInG[105X     not     belong     to     the    category
  [10XIsSubspaceOfClassicalPolarSpace[110X.[133X
  
  [1X7.4-1 VectorSpaceToElement[101X
  
  [29X[2XVectorSpaceToElement[102X( [3Xps[103X, [3Xv[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan element of the polar space [3Xgeo[103X[133X
  
  [33X[0;0YLet [3Xps[103X be a polar space, and [3Xv[103X is either a row vector (for points) or an [22Xm[122Xx[22Xn[122X
  matrix  (for  an  [22X(m-1)[122X-subspace  of a polar space with an [22X(n-1)[122X-dimensional
  ambient projective space. In the case that [3Xv[103X is a matrix, the rows represent
  basis  vectors  for  the  subspace.  An  exceptional  case  is  when  [3Xv[103X is a
  zero-vector,  whereby  the  trivial subspace is returned. It is checked that
  the  subspace  defined  by  [22Xv[122X is totally isotropic with relation to the form
  defining [3Xps[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27Xv := [1,0,1,0]*Z(4)^0;[127X[104X
    [4X[28X[ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2) ][128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(ps,v);[127X[104X
    [4X[28X<a point in W(3, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[1,1,0,1],[0,0,1,0]]*Z(4)^0;[127X[104X
    [4X[28X[ [ Z(2)^0, Z(2)^0, 0*Z(2), Z(2)^0 ], [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2) ] ][128X[104X
    [4X[25Xgap>[125X [27Xline := VectorSpaceToElement(ps,mat);[127X[104X
    [4X[28XError, <x> does not generate an element of <geom> called from[128X[104X
    [4X[28X<function "unknown">( <arguments> )[128X[104X
    [4X[28X called from read-eval loop at line 12 of *stdin*[128X[104X
    [4X[28Xyou can 'quit;' to quit to outer loop, or[128X[104X
    [4X[28Xyou can 'return;' to continue[128X[104X
    [4X[26Xbrk>[126X [27Xquit;[127X[104X
    [4X[25Xgap>[125X [27Xmat := [[1,1,0,0],[0,0,1,0]]*Z(4)^0;[127X[104X
    [4X[28X[ [ Z(2)^0, Z(2)^0, 0*Z(2), 0*Z(2) ], [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2) ] ][128X[104X
    [4X[25Xgap>[125X [27Xline := VectorSpaceToElement(ps,mat);[127X[104X
    [4X[28X<a line in W(3, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(ps,[0,0,0,0]*Z(4)^0);[127X[104X
    [4X[28X< empty subspace >[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.4-2 EmptySubspace[101X
  
  [29X[2XEmptySubspace[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe trivial subspace in the projective [3Xps[103X[133X
  
  [33X[0;0YThe  object  returned  by  this  operation  is contained in every projective
  subspace  of  the  projective  space [3Xps[103X, but is not an element of [3Xps[103X. Hence,
  testing incidence results in an error message.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(10,49);[127X[104X
    [4X[28XH(10, 7^2)[128X[104X
    [4X[25Xgap>[125X [27Xe := EmptySubspace(ps);[127X[104X
    [4X[28X< empty subspace >[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.4-3 ProjectiveDimension[101X
  
  [29X[2XProjectiveDimension[102X( [3Xsub[103X ) [32X operation
  [29X[2XDimension[102X( [3Xsub[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe  projective  dimension  of  a  subspace  of a polar space. The
            operation   [11XProjectiveDimension[111X   is   also   applicable   on  the
            EmptySubspace[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(7,8);[127X[104X
    [4X[28XQ-(7, 8)[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[0,0,1,0,0,0,0,0],[0,0,0,0,1,0,0,0]]*Z(8)^0;[127X[104X
    [4X[28X[ [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2) ] ][128X[104X
    [4X[25Xgap>[125X [27Xline := VectorSpaceToElement(ps,mat);[127X[104X
    [4X[28X<a line in Q-(7, 8)>[128X[104X
    [4X[25Xgap>[125X [27XProjectiveDimension(line);[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27XDimension(line);[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27Xe := EmptySubspace(ps);[127X[104X
    [4X[28X< empty subspace >[128X[104X
    [4X[25Xgap>[125X [27XProjectiveDimension(e);[127X[104X
    [4X[28X-1[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.4-4 ElementsOfIncidenceStructure[101X
  
  [29X[2XElementsOfIncidenceStructure[102X( [3Xps[103X, [3Xj[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe collection of elements of the projective space [3Xps[103X of type [3Xj[103X[133X
  
  [33X[0;0YFor  the  projective  space [3Xps[103X of dimension [22Xd[122X and the type [3Xj[103X, [22X1 ≤ j ≤ d[122X this
  operation returns the collection of [22Xj-1[122X dimensional subspaces.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(8,13^2);[127X[104X
    [4X[28XH(8, 13^2)[128X[104X
    [4X[25Xgap>[125X [27Xplanes := ElementsOfIncidenceStructure(ps,3);[127X[104X
    [4X[28X<planes of H(8, 13^2)>[128X[104X
    [4X[25Xgap>[125X [27Xsolids := ElementsOfIncidenceStructure(ps,4);[127X[104X
    [4X[28X<solids of H(8, 13^2)>[128X[104X
    [4X[25Xgap>[125X [27XElementsOfIncidenceStructure(ps,5);[127X[104X
    [4X[28XError, <geo> has no elements of type <j> called from[128X[104X
    [4X[28X<function "unknown">( <arguments> )[128X[104X
    [4X[28X called from read-eval loop at line 11 of *stdin*[128X[104X
    [4X[28Xyou can 'quit;' to quit to outer loop, or[128X[104X
    [4X[28Xyou can 'return;' to continue[128X[104X
    [4X[26Xbrk>[126X [27Xquit;[127X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.4-5 AmbientSpace[101X
  
  [29X[2XAmbientSpace[102X( [3Xel[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yreturns the ambient space of an element [3Xel[103X of a polar space[133X
  
  [33X[0;0YThis  operation  is  also  applicable  on  the  trivial  subspace. For a Lie
  geometry, the ambient space of an element is defined as the ambient space of
  the Lie geometry, i.e. a projective space.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(3,7^2);[127X[104X
    [4X[28XH(3, 7^2)[128X[104X
    [4X[25Xgap>[125X [27Xline := VectorSpaceToElement(ps,[[Z(7)^0,0*Z(7),Z(7^2)^34,Z(7^2)^44],[127X[104X
    [4X[25X>[125X [27X[0*Z(7),Z(7)^0,Z(7^2)^2,Z(7^2)^4]]);[127X[104X
    [4X[28X<a line in H(3, 7^2)>[128X[104X
    [4X[25Xgap>[125X [27XAmbientSpace(line);[127X[104X
    [4X[28XProjectiveSpace(3, 49)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.4-6 Coordinates[101X
  
  [29X[2XCoordinates[102X( [3Xp[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe homogeneous coordinates of the point [3Xp[103X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(6,5);[127X[104X
    [4X[28XQ(6, 5)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(ps,[0,1,0,0,0,0,0]*Z(5)^0);[127X[104X
    [4X[28X<a point in Q(6, 5)>[128X[104X
    [4X[25Xgap>[125X [27XCoordinates(p);[127X[104X
    [4X[28X[ 0*Z(5), Z(5)^0, 0*Z(5), 0*Z(5), 0*Z(5), 0*Z(5), 0*Z(5) ][128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.5 [33X[0;0YBasic operations for polar spaces and subspaces of projective spaces[133X[101X
  
  
  [1X7.5-1 [33X[0;0YIncidence and containment[133X[101X
  
  [29X[2XIsIncident[102X( [3Xel1[103X, [3Xel2[103X ) [32X operation
  [29X[2X\*[102X( [3Xel1[103X, [3Xel2[103X ) [32X operation
  [29X[2X\in[102X( [3Xel1[103X, [3Xel2[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ytrue or false[133X
  
  [33X[0;0YRecall  that  for  projective  spaces, incidence is symmetrized containment,
  where  the  empty  subspace  and  the whole projective space are excluded as
  arguments  for  this operation, since they are not considered as elements of
  the geometry, but both the empty subspace and the whole projective space are
  allowed as arguments for [11X\in[111X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(7,7);[127X[104X
    [4X[28XQ+(7, 7)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(ps,[1,0,1,0,0,0,0,0]*Z(7)^0);[127X[104X
    [4X[28X<a point in Q+(7, 7)>[128X[104X
    [4X[25Xgap>[125X [27Xl := VectorSpaceToElement(ps,[[1,0,1,0,0,0,0,0],[0,-1,0,1,0,0,0,0]]*Z(7)^0);[127X[104X
    [4X[28X<a line in Q+(7, 7)>[128X[104X
    [4X[25Xgap>[125X [27Xp * l;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xl * p;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsIncident(p,l);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xp in l;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xl in p;[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xe := EmptySubspace(ps);[127X[104X
    [4X[28X< empty subspace >[128X[104X
    [4X[25Xgap>[125X [27Xe * l;[127X[104X
    [4X[28XError, no method found! For debugging hints type ?Recovery from NoMethodFound[128X[104X
    [4X[28XError, no 1st choice method found for `*' on 2 arguments called from[128X[104X
    [4X[28X<function "HANDLE_METHOD_NOT_FOUND">( <arguments> )[128X[104X
    [4X[28X called from read-eval loop at line 17 of *stdin*[128X[104X
    [4X[28Xyou can 'quit;' to quit to outer loop, or[128X[104X
    [4X[28Xyou can 'return;' to continue[128X[104X
    [4X[26Xbrk>[126X [27Xquit;[127X[104X
    [4X[25Xgap>[125X [27Xe in l;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xl in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-2 Span[101X
  
  [29X[2XSpan[102X( [3Xu[103X, [3Xv[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan element[133X
  
  [33X[0;0Y[3Xu[103X  and  [3Xv[103X are elements of a projective or polar space. This function returns
  the join of the two elements, that is, the span of the two subspaces.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,2);[127X[104X
    [4X[28XQ+(5, 2)[128X[104X
    [4X[25Xgap>[125X [27Xp := Random(Planes(ps));[127X[104X
    [4X[28X<a plane in Q+(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xq := Random(Planes(ps));[127X[104X
    [4X[28X<a plane in Q+(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xs := Span(p,q);[127X[104X
    [4X[28X<a proj. 4-space in ProjectiveSpace(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xs = Span([p,q]);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xt := Span(EmptySubspace(ps),p);[127X[104X
    [4X[28X<a plane in Q+(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xt = p;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-3 Meet[101X
  
  [29X[2XMeet[102X( [3Xu[103X, [3Xv[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan element[133X
  
  [33X[0;0Y[3Xu[103X  and  [3Xv[103X are elements of a projective or polar space. This function returns
  the meet of the two elements. If two elements do not meet, then Meet returns
  [10XEmptySubspace[110X,  which in [5XFinInG[105X, is an element with projective dimension -1.
  (Note  that  the  poset of subspaces of a polar space is a meet-semilattice,
  but not closed under taking spans).[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,3);[127X[104X
    [4X[28XQ+(5, 3)[128X[104X
    [4X[25Xgap>[125X [27Xpi := Random( Planes(ps) );[127X[104X
    [4X[28X<a plane in Q+(5, 3)>[128X[104X
    [4X[25Xgap>[125X [27Xtau := Random( Planes(ps) );[127X[104X
    [4X[28X<a plane in Q+(5, 3)>[128X[104X
    [4X[25Xgap>[125X [27XMeet(pi,tau);[127X[104X
    [4X[28X<a point in Q+(5, 3)>[128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  [33X[0;0YNote:  the  above example will return different answers depending on the two
  planes chosen at random.[133X
  
  [1X7.5-4 IsCollinear[101X
  
  [29X[2XIsCollinear[102X( [3Xps[103X, [3Xu[103X, [3Xv[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YBoolean[133X
  
  [33X[0;0Y[3Xu[103X and [3Xv[103X are points of the ambient space of the polar space [3Xps[103X. This function
  returns  True if [3Xu[103X and [3Xv[103X are collinear in [3Xps[103X. Note that belonging to [3Xps[103X is a
  necessary condition for [3Xu[103X and [3Xv[103X to be collinear.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,9);[127X[104X
    [4X[28XQ(4, 9)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(PG(4,9),[0,1,0,0,0]*Z(9)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 9)>[128X[104X
    [4X[25Xgap>[125X [27Xq := VectorSpaceToElement(PG(4,9),[0,0,1,0,0]*Z(9)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 9)>[128X[104X
    [4X[25Xgap>[125X [27Xr := VectorSpaceToElement(PG(4,9),[0,0,0,1,0]*Z(9)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 9)>[128X[104X
    [4X[25Xgap>[125X [27Xp in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xq in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xr in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsCollinear(ps,p,q);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XIsCollinear(ps,p,r);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsCollinear(ps,q,r);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,4);[127X[104X
    [4X[28XQ(4, 4)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(PG(4,4),[1,0,0,0,0]*Z(2)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xq := VectorSpaceToElement(PG(4,4),[0,1,0,0,0]*Z(2)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xr := VectorSpaceToElement(PG(4,4),[0,0,0,1,0]*Z(2)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(4, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xp in ps;[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27Xq in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xr in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27XIsCollinear(ps,p,q);[127X[104X
    [4X[28Xfalse[128X[104X
    [4X[25Xgap>[125X [27XIsCollinear(ps,q,r);[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-5 PolarityOfProjectiveSpace[101X
  
  [29X[2XPolarityOfProjectiveSpace[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya polarity of a projective space[133X
  
  [33X[0;0Y[3Xps[103X  must  be a polar space. This operation returns, when possible a polarity
  of  the  ambient  projective  space  of [3Xps[103X. It is well known that except for
  orthogonal  polar  spaces in even characteristic, a classical polar space is
  in fact the geometry of absolute points of a polarity of a projective space,
  and  that  no  polarity  can  be  associated  to  parabolic quadrics in even
  characteristic. Polarities of projective spaces are discussed in more detail
  in Chapter [14X6[114X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(5,9);[127X[104X
    [4X[28XW(5, 9)[128X[104X
    [4X[25Xgap>[125X [27Xphi := PolarityOfProjectiveSpace(ps);[127X[104X
    [4X[28X<polarity of PG(5, GF(3^2)) >[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,4);[127X[104X
    [4X[28XQ-(3, 4)[128X[104X
    [4X[25Xgap>[125X [27Xphi := PolarityOfProjectiveSpace(ps);[127X[104X
    [4X[28X<polarity of PG(3, GF(2^2)) >[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,4);[127X[104X
    [4X[28XQ(4, 4)[128X[104X
    [4X[25Xgap>[125X [27Xphi := PolarityOfProjectiveSpace(ps);[127X[104X
    [4X[28XError, no polarity of the ambient projective space can be associated to <ps> called from[128X[104X
    [4X[28X<function "unknown">( <arguments> )[128X[104X
    [4X[28X called from read-eval loop at line 13 of *stdin*[128X[104X
    [4X[28Xyou can 'quit;' to quit to outer loop, or[128X[104X
    [4X[28Xyou can 'return;' to continue[128X[104X
    [4X[26Xbrk>[126X [27Xquit;[127X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-6 TypeOfSubspace[101X
  
  [29X[2XTypeOfSubspace[102X( [3Xps[103X, [3Xv[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ya string[133X
  
  [33X[0;0YThis  operation  is  a convenient way to find out the intersection type of a
  projective  subspace  with a polar space. The argument [3Xps[103X is a nondegenerate
  polar  space,  and  the  argument  [3Xv[103X is a subspace of the ambient projective
  space.  The  operation  returns  a  string  in  accordance  with the type of
  subspace:   [21Xdegenerate[121X,   [21Xsymplectic[121X,  [21Xhermitian[121X,  [21Xelliptic[121X,  [21Xhyperbolic[121X  or
  [21Xparabolic[121X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xh1 := HermitianPolarSpace(2, 3^2);[127X[104X
    [4X[28XH(2, 3^2)[128X[104X
    [4X[25Xgap>[125X [27Xh2 := HermitianPolarSpace(3, 3^2);[127X[104X
    [4X[28XH(3, 3^2)[128X[104X
    [4X[25Xgap>[125X [27Xpg := AmbientSpace( h2 );    [127X[104X
    [4X[28XProjectiveSpace(3, 9)[128X[104X
    [4X[25Xgap>[125X [27Xpi := VectorSpaceToElement( pg, [[1,0,0,0],[0,1,0,0],[0,0,1,0]] * Z(9)^0 );[127X[104X
    [4X[28X<a plane in ProjectiveSpace(3, 9)>[128X[104X
    [4X[25Xgap>[125X [27XTypeOfSubspace(h2, pi);[127X[104X
    [4X[28X"hermitian"[128X[104X
    [4X[25Xgap>[125X [27Xpi := VectorSpaceToElement( pg, [[1,0,0,0],[0,1,0,0],[0,0,1,Z(9)]] * Z(9)^0 );[127X[104X
    [4X[28X<a plane in ProjectiveSpace(3, 9)>[128X[104X
    [4X[25Xgap>[125X [27XTypeOfSubspace(h2, pi);[127X[104X
    [4X[28X"degenerate"[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-7 TangentSpace[101X
  
  [29X[2XTangentSpace[102X( [3Xel[103X ) [32X operation
  [29X[2XTangentSpace[102X( [3Xps[103X, [3Xel[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YA subspace of a projective space[133X
  
  [33X[0;0YLet  [3Xel[103X  be an element of a classical polar space. The first version returns
  the  tangent  space  at  [3Xel[103X to this polar space. Let [3Xel[103X be a subspace of the
  ambient  space  of  the polar space [3Xps[103X. The second version checks whether [3Xel[103X
  belongs  to  [3Xps[103X  and  returns  the  tangent  space at [3Xel[103X to [3Xps[103X. Some obvious
  properties are demonstrated in the example.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(3,4^2);[127X[104X
    [4X[28XH(3, 4^2)[128X[104X
    [4X[25Xgap>[125X [27Xp := Random(Points(ps));[127X[104X
    [4X[28X<a point in H(3, 4^2)>[128X[104X
    [4X[25Xgap>[125X [27Xplane := TangentSpace(p);[127X[104X
    [4X[28X<a plane in ProjectiveSpace(3, 16)>[128X[104X
    [4X[25Xgap>[125X [27XTypeOfSubspace(ps,plane);[127X[104X
    [4X[28X"degenerate"[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(6,4);[127X[104X
    [4X[28XQ(6, 4)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(PG(6,4),[0,1,0,0,0,0,0]*Z(4)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(6, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xhyp := TangentSpace(ps,p);[127X[104X
    [4X[28X<a proj. 5-space in ProjectiveSpace(6, 4)>[128X[104X
    [4X[25Xgap>[125X [27XNucleusOfParabolicQuadric(ps) in hyp;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(5,2);[127X[104X
    [4X[28XQ-(5, 2)[128X[104X
    [4X[25Xgap>[125X [27Xline := Random(Lines(ps));[127X[104X
    [4X[28X<a line in Q-(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27XTangentSpace(line);[127X[104X
    [4X[28X<a solid in ProjectiveSpace(5, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(5,4);[127X[104X
    [4X[28XH(5, 2^2)[128X[104X
    [4X[25Xgap>[125X [27Xplane := Random(Planes(ps));[127X[104X
    [4X[28X<a plane in H(5, 2^2)>[128X[104X
    [4X[25Xgap>[125X [27Xtan := TangentSpace(plane);[127X[104X
    [4X[28X<a plane in ProjectiveSpace(5, 4)>[128X[104X
    [4X[25Xgap>[125X [27Xtan in ps;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[25Xgap>[125X [27Xtan = plane;[127X[104X
    [4X[28Xtrue[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.5-8 Pole[101X
  
  [29X[2XPole[102X( [3Xps[103X, [3Xel[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10YA subspace of a projective space[133X
  
  [33X[0;0YLet  [3Xel[103X  be  a  subspace  of  the  ambient space of the polar space [3Xps[103X. This
  operation returns the pole of [3Xel[103X with relation to the polar space [3Xps[103X.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xconic := ParabolicQuadric(2,13);[127X[104X
    [4X[28XQ(2, 13)[128X[104X
    [4X[25Xgap>[125X [27Xp := VectorSpaceToElement(PG(2,13),[1,0,0]*Z(13)^0);[127X[104X
    [4X[28X<a point in ProjectiveSpace(2, 13)>[128X[104X
    [4X[25Xgap>[125X [27Xpole := Pole(conic,p);[127X[104X
    [4X[28X<a line in ProjectiveSpace(2, 13)>[128X[104X
    [4X[25Xgap>[125X [27XTypeOfSubspace(conic,pole);[127X[104X
    [4X[28X"hyperbolic"[128X[104X
    [4X[25Xgap>[125X [27Xtangent_points := Filtered(Points(pole),x->x in conic);[127X[104X
    [4X[28X[ <a point in ProjectiveSpace(2, 13)>, <a point in ProjectiveSpace(2, 13)> ][128X[104X
    [4X[25Xgap>[125X [27Xtangent_lines_on_p := List(tangent_points,x->Span(x,p));[127X[104X
    [4X[28X[ <a line in ProjectiveSpace(2, 13)>, <a line in ProjectiveSpace(2, 13)> ][128X[104X
    [4X[25Xgap>[125X [27XList(tangent_lines_on_p,x->Number(Points(x),y->y in conic));[127X[104X
    [4X[28X[ 1, 1 ][128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.6 [33X[0;0YShadow of elements[133X[101X
  
  [33X[0;0YThe  functionality in this section is comparable to the shadow functionality
  for  elements  of projective spaces, which are described in Section [14X4.3[114X. The
  generic  description  of  shadows of elements of incidence structures can be
  found in Section [14X3.4[114X.[133X
  
  [1X7.6-1 ShadowOfElement[101X
  
  [29X[2XShadowOfElement[102X( [3Xps[103X, [3Xel[103X, [3Xi[103X ) [32X operation
  [29X[2XShadowOfElement[102X( [3Xps[103X, [3Xel[103X, [3Xstr[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe  elements  of  type  [3Xi[103X  incident  with  [3Xel[103X. The second variant
            determines  the  type  [3Xi[103X  from  the  position  of  [3Xstr[103X in the list
            returned by [11XTypesOfElementsOfIncidenceStructurePlural[111X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xid := IdentityMat(8,GF(7));[127X[104X
    [4X[28X< mutable compressed matrix 8x8 over GF(7) >[128X[104X
    [4X[25Xgap>[125X [27Xform := BilinearFormByMatrix(id,GF(7));[127X[104X
    [4X[28X< bilinear form >[128X[104X
    [4X[25Xgap>[125X [27Xps := PolarSpace(form);[127X[104X
    [4X[28X<polar space in ProjectiveSpace([128X[104X
    [4X[28X7,GF(7)): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0 >[128X[104X
    [4X[25Xgap>[125X [27XRank(ps);[127X[104X
    [4X[28X4[128X[104X
    [4X[25Xgap>[125X [27Xps;[127X[104X
    [4X[28XQ+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0[128X[104X
    [4X[25Xgap>[125X [27Xmat := [[1,0,0,0,3,2,0,0],[0,1,0,0,0,0,3,2],[0,0,1,0,5,3,0,0]]*Z(7)^0;[127X[104X
    [4X[28X[ [ Z(7)^0, 0*Z(7), 0*Z(7), 0*Z(7), Z(7), Z(7)^2, 0*Z(7), 0*Z(7) ], [128X[104X
    [4X[28X  [ 0*Z(7), Z(7)^0, 0*Z(7), 0*Z(7), 0*Z(7), 0*Z(7), Z(7), Z(7)^2 ], [128X[104X
    [4X[28X  [ 0*Z(7), 0*Z(7), Z(7)^0, 0*Z(7), Z(7)^5, Z(7), 0*Z(7), 0*Z(7) ] ][128X[104X
    [4X[25Xgap>[125X [27Xplane := VectorSpaceToElement(ps,mat);[127X[104X
    [4X[28X<a plane in Q+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0>[128X[104X
    [4X[25Xgap>[125X [27Xtime;[127X[104X
    [4X[28X1[128X[104X
    [4X[25Xgap>[125X [27Xshadow := ShadowOfElement(ps,plane,4);[127X[104X
    [4X[28X<shadow solids in Q+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0>[128X[104X
    [4X[25Xgap>[125X [27XList(shadow);[127X[104X
    [4X[28X[ <a solid in Q+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0>, [128X[104X
    [4X[28X  <a solid in Q+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0> ][128X[104X
    [4X[25Xgap>[125X [27Xshadow := ShadowOfElement(ps,plane,2);[127X[104X
    [4X[28X<shadow lines in Q+(7, 7): x_1^2+x_2^2+x_3^2+x_4^2+x_5^2+x_6^2+x_7^2+x_8^2=0>[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.6-2 ElementsIncidentWithElementOfIncidenceStructure[101X
  
  [29X[2XElementsIncidentWithElementOfIncidenceStructure[102X( [3Xel[103X, [3Xi[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe  elements of type [3Xi[103X incident with [3Xel[103X, in other words, the type
            [3Xi[103X shadow of the element [3Xel[103X[133X
  
  [33X[0;0YInternally,  the  function [11XFlagOfIncidenceStructure[111X is used to create a flag
  from [3Xlist[103X. This function also performs the checking.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(11,2);[127X[104X
    [4X[28XQ+(11, 2)[128X[104X
    [4X[25Xgap>[125X [27Xvec := [[Z(2)^0,0*Z(2),0*Z(2),0*Z(2),0*Z(2),Z(2)^0,0*Z(2),Z(2)^0,Z(2)^0,[127X[104X
    [4X[25X>[125X [27X0*Z(2),0*Z(2),0*Z(2)],[127X[104X
    [4X[25X>[125X [27X[0*Z(2),Z(2)^0,0*Z(2),0*Z(2),0*Z(2),Z(2)^0,Z(2)^0,Z(2)^0,Z(2)^0,[127X[104X
    [4X[25X>[125X [27X0*Z(2),Z(2)^0,Z(2)^0],[127X[104X
    [4X[25X>[125X [27X[0*Z(2),0*Z(2),Z(2)^0,0*Z(2),0*Z(2),Z(2)^0,0*Z(2),Z(2)^0,0*Z(2),[127X[104X
    [4X[25X>[125X [27X0*Z(2),0*Z(2),Z(2)^0],[127X[104X
    [4X[25X>[125X [27X[0*Z(2),0*Z(2),0*Z(2),Z(2)^0,0*Z(2),Z(2)^0,0*Z(2),Z(2)^0,0*Z(2),[127X[104X
    [4X[25X>[125X [27X0*Z(2),Z(2)^0,0*Z(2)],[127X[104X
    [4X[25X>[125X [27X[0*Z(2),0*Z(2),0*Z(2),0*Z(2),Z(2)^0,Z(2)^0,0*Z(2),0*Z(2),0*Z(2),[127X[104X
    [4X[25X>[125X [27XZ(2)^0,Z(2)^0,Z(2)^0]];[127X[104X
    [4X[28X[ [ Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, Z(2)^0, [128X[104X
    [4X[28X      0*Z(2), 0*Z(2), 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0, [128X[104X
    [4X[28X      0*Z(2), Z(2)^0, Z(2)^0 ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), [128X[104X
    [4X[28X      0*Z(2), 0*Z(2), Z(2)^0 ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), [128X[104X
    [4X[28X      0*Z(2), Z(2)^0, 0*Z(2) ], [128X[104X
    [4X[28X  [ 0*Z(2), 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, Z(2)^0, 0*Z(2), 0*Z(2), 0*Z(2), [128X[104X
    [4X[28X      Z(2)^0, Z(2)^0, Z(2)^0 ] ][128X[104X
    [4X[25Xgap>[125X [27Xsubspace := VectorSpaceToElement(ps,vec);[127X[104X
    [4X[28X<a proj. 4-space in Q+(11, 2)>[128X[104X
    [4X[25Xgap>[125X [27Xels := ElementsIncidentWithElementOfIncidenceStructure(subspace,6);[127X[104X
    [4X[28X<shadow proj. subspaces of dim. 5 in Q+(11, 2)>[128X[104X
    [4X[25Xgap>[125X [27XList(els);[127X[104X
    [4X[28X[ <a proj. 5-space in Q+(11, 2)>, <a proj. 5-space in Q+(11, 2)> ][128X[104X
    [4X[28X[128X[104X
  [4X[32X[104X
  
  
  [1X7.7 [33X[0;0YProjective Orthogonal/Unitary/Symplectic groups in [5XFinInG[105X[101X[1X[133X[101X
  
  [33X[0;0YThe  classical  groups  (apart from the general lines group), are the matrix
  groups  that [13Xrespect[113X, in a certain way, a sesquilinear or quadratic form. We
  formally  recall  the  definitions  used  in  [5XFinInG[105X.  These definitions are
  exactly the same as in [5XForms[105X.[133X
  
  [33X[0;0YLet [22X(V,f)[122X and [22X(W,g)[122X be two formed vector spaces over the same field [22XF[122X, where
  both  [22Xf[122X  and [22Xg[122X are sesquilinear forms. Suppose that [22Xϕ[122X is a linear map from [22XV[122X
  to  [22XW[122X.  The  map  [22Xϕ[122X is an [13Xisometry[113X from the formed space [22X(V,f)[122X to the formed
  space [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v,w) = f'(ϕ(v), ϕ(w))[122X.[133X
  
  [33X[0;0YThe  map  [22Xϕ[122X  is a [13Xsimilarity[113X from the formed space [22X(V,f)[122X to the formed space
  [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v,w) = λ f'(ϕ(v), ϕ(w))[122X[133X
  
  [33X[0;0Yfor  some  non-zero  [22Xλ ∈ F[122X. Finally, the map [22Xϕ[122X is a [13Xsemi-similarity[113X from the
  formed space [22X(V,f)[122X to the formed space [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v,w) = λ f'(ϕ(v), ϕ(w))^α[122X[133X
  
  [33X[0;0Yfor some non-zero [22Xλ ∈ F[122X and a field automorphism [22Xα[122X of [22XF[122X.[133X
  
  [33X[0;0YLet [22X(V,f)[122X and [22X(W,g)[122X be two formed vector spaces over the same field [22XF[122X, where
  both  [22Xf[122X  and [22Xg[122X are quadratic forms. Suppose that [22Xϕ[122X is a linear map from [22XV[122X to
  [22XW[122X.  The map [22Xϕ[122X is an [13Xisometry[113X from the formed space [22X(V,f)[122X to the formed space
  [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v) = f'(ϕ(v)).[122X[133X
  
  [33X[0;0YThe  map  [22Xϕ[122X  is  a  [13Xsimilarity[113X from the formed space [22X(V,f)[122X to a formed space
  [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v) = λ f'(ϕ(v))[122X[133X
  
  [33X[0;0Yfor  some  non-zero  [22Xλ ∈ F[122X. Finally, the map [22Xϕ[122X is a [13Xsemi-similarity[113X from the
  formed space [22X(V,f)[122X to the formed space [22X(W,g)[122X if for all [22Xv,w[122X in [22XV[122X we have[133X
  
  [33X[0;0Y[22Xf(v)=λ f'(ϕ(v))^α[122X[133X
  
  [33X[0;0Yfor some non-zero [22Xλ ∈ F[122X and a field automorphism [22Xα[122X of [22XF[122X.[133X
  
  [33X[0;0YCollineations  of classical polar spaces are induced by semi-similarities of
  the  underlying  formed  vector  space,  and  vice  versa,  analoguously  by
  factoring  out  scalar matrices. The only exceptions are the two-dimensional
  unitary groups where the the full semi-similarity group can contain elements
  of  its  centre that are not scalars. In [5XFinInG[105X, the subgroups corresponding
  with  similarities  and isometries are also implemented, including a [13Xspecial[113X
  variant,  corresponding  with  the matrices having determinant one. We use a
  consistent terminology, where isometries, similarities, respectively, of the
  polar  space, correspond with isometries, similarities, respectively, of the
  underlying  formed  vector  space.  Special  isometries of a polar space are
  induced  by  isometries  of  the formed vector space that have a matrix with
  determinant one. If [22XP[122X is a polar space with special isometry group, isometry
  group,  similarity  group,  collineation  group,  respectively,  [22XS[122X, [22XI[122X, [22XG[122X, [22XΓ[122X,
  respectively,  then  clearly [22XS ≤ I ≤ G ≤ Γ[122X . Equalities can occur in certain
  cases, and will, as we will see in the following overview.[133X
  
      ┌──────────────────┬────────────┬────────────┬─────────────┬────────────┬────────────┐
      │ (sub)group       │ symplectic │ hyperbolic │ elliptic    │ parabolic  │ hermitian  │ 
      │ special isometry │ [22XPSp(d,q)[122X   │ [22XPSO(1,d,q)[122X │ [22XPSO(-1,d,q)[122X │ [22XPSO(0,d,q)[122X │ [22XPSU(d,q^2)[122X │ 
      │ isometry         │ [22XPSp(d,q)[122X   │ [22XPGO(1,d,q)[122X │ [22XPGO(-1,d,q)[122X │ [22XPGO(0,d,q)[122X │ [22XPGU(d,q^2)[122X │ 
      │ similarity       │ [22XPGSp(d,q)[122X  │ [22XP∆O+(d,q)[122X  │ [22XP∆O-(d,q)[122X   │ [22XPGO(0,d,q)[122X │ [22XPGU(d,q^2)[122X │ 
      │ collineation     │ [22XPΓSp(d,q)[122X  │ [22XPΓO+(d,q)[122X  │ [22XPΓO-(d,q)[122X   │ [22XPΓO(d,q)[122X   │ [22XPΓU(d,q^2)[122X │ 
      └──────────────────┴────────────┴────────────┴─────────────┴────────────┴────────────┘
  
       [1XTable:[101X projective finite classical groups
  
  
  [1X7.7-1 SpecialIsometryGroup[101X
  
  [29X[2XSpecialIsometryGroup[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe special isometry group of the polar space [3Xps[103X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XSpecialIsometryGroup(ps);[127X[104X
    [4X[28XPSp(4,4)[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,8);[127X[104X
    [4X[28XQ+(5, 8)[128X[104X
    [4X[25Xgap>[125X [27XSpecialIsometryGroup(ps);[127X[104X
    [4X[28XPSO(1,6,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,27);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[25Xgap>[125X [27XSpecialIsometryGroup(ps);[127X[104X
    [4X[28XPSO(-1,4,27)[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,8);[127X[104X
    [4X[28XQ(4, 8)[128X[104X
    [4X[25Xgap>[125X [27XSpecialIsometryGroup(ps);[127X[104X
    [4X[28XPSO(0,5,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(4,9);[127X[104X
    [4X[28XH(4, 3^2)[128X[104X
    [4X[25Xgap>[125X [27XSpecialIsometryGroup(ps);[127X[104X
    [4X[28XPSU(5,3^2)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.7-2 IsometryGroup[101X
  
  [29X[2XIsometryGroup[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe isometry group of the polar space [3Xps[103X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XIsometryGroup(ps);[127X[104X
    [4X[28XPSp(4,4)[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,8);[127X[104X
    [4X[28XQ+(5, 8)[128X[104X
    [4X[25Xgap>[125X [27XIsometryGroup(ps);[127X[104X
    [4X[28XPGO(1,6,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,27);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[25Xgap>[125X [27XIsometryGroup(ps);[127X[104X
    [4X[28XPGO(-1,4,27)[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,8);[127X[104X
    [4X[28XQ(4, 8)[128X[104X
    [4X[25Xgap>[125X [27XIsometryGroup(ps);[127X[104X
    [4X[28XPGO(0,5,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(4,9);[127X[104X
    [4X[28XH(4, 3^2)[128X[104X
    [4X[25Xgap>[125X [27XIsometryGroup(ps);[127X[104X
    [4X[28XPGU(5,3^2)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.7-3 SimilarityGroup[101X
  
  [29X[2XSimilarityGroup[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe similarity group of the polar space [3Xps[103X[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XSimilarityGroup(ps);[127X[104X
    [4X[28XPGSp(4,4)[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,8);[127X[104X
    [4X[28XQ+(5, 8)[128X[104X
    [4X[25Xgap>[125X [27XSimilarityGroup(ps);[127X[104X
    [4X[28XPDeltaO+(6,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,27);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[25Xgap>[125X [27XSimilarityGroup(ps);[127X[104X
    [4X[28XPDeltaO-(4,27)[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,8);[127X[104X
    [4X[28XQ(4, 8)[128X[104X
    [4X[25Xgap>[125X [27XSimilarityGroup(ps);[127X[104X
    [4X[28XPGO(0,5,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(4,9);[127X[104X
    [4X[28XH(4, 3^2)[128X[104X
    [4X[25Xgap>[125X [27XSimilarityGroup(ps);[127X[104X
    [4X[28XPGU(5,3^2)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.7-4 CollineationGroup[101X
  
  [29X[2XCollineationGroup[102X( [3Xps[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Ythe collineation group of the polar space [3Xps[103X[133X
  
  [33X[0;0YIn  most  cases,  the  full projective semisimilarity group is returned. For
  two-dimensional unitary groups, the centre may contain elements that are not
  scalars.  In  this  case,  we  return  a central extension of the projective
  semisimilarity  group.  If  the  basefield  of  [3Xps[103X  is [22XGF(q^2)[122X, [22Xq[122X prime, the
  similarity group is returned.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := SymplecticSpace(3,4);[127X[104X
    [4X[28XW(3, 4)[128X[104X
    [4X[25Xgap>[125X [27XCollineationGroup(ps);[127X[104X
    [4X[28XPGammaSp(4,4)[128X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,8);[127X[104X
    [4X[28XQ+(5, 8)[128X[104X
    [4X[25Xgap>[125X [27XCollineationGroup(ps);[127X[104X
    [4X[28XPGammaO+(6,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := EllipticQuadric(3,27);[127X[104X
    [4X[28XQ-(3, 27)[128X[104X
    [4X[25Xgap>[125X [27XCollineationGroup(ps);[127X[104X
    [4X[28XPGammaO-(4,27)[128X[104X
    [4X[25Xgap>[125X [27Xps := ParabolicQuadric(4,8);[127X[104X
    [4X[28XQ(4, 8)[128X[104X
    [4X[25Xgap>[125X [27XCollineationGroup(ps);[127X[104X
    [4X[28XPGammaO(5,8)[128X[104X
    [4X[25Xgap>[125X [27Xps := HermitianPolarSpace(4,9);[127X[104X
    [4X[28XH(4, 3^2)[128X[104X
    [4X[25Xgap>[125X [27XCollineationGroup(ps);[127X[104X
    [4X[28XPGammaU(5,3^2)[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.8 [33X[0;0YEnumerating subspaces of polar spaces[133X[101X
  
  
  [1X7.8-1 [33X[0;0YEnumerators for polar spaces[133X[101X
  
  [33X[0;0YAn enumerator for a collection of subspaces of a given type of a polar space
  is  provided in [5XFinInG[105X. If [3XC[103X is such a collection, then [11XList(C)[111X will use the
  enumerator to compute a list with all the elements of [3XC[103X.[133X
  
  [1X7.8-2 Enumerator[101X
  
  [29X[2XEnumerator[102X( [3XC[103X ) [32X operation
  [29X[2XList[102X( [3XC[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan enumerator for the collection [3XC[103X and a list with all elements of
            [3XC[103X[133X
  
  [33X[0;0YThe argument [3XC[103X is a collection of subspaces of a polar space.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27XEnumerator(Points(ParabolicQuadric(6,3)));[127X[104X
    [4X[28XEnumeratorOfSubspacesOfClassicalPolarSpace( <points of Q(6, 3)> )[128X[104X
    [4X[25Xgap>[125X [27XEnumerator(Lines(HermitianPolarSpace(4,4)));[127X[104X
    [4X[28XEnumeratorOfSubspacesOfClassicalPolarSpace( <lines of H(4, 2^2)> )[128X[104X
    [4X[25Xgap>[125X [27Xplanes := List(Planes(HermitianPolarSpace(5,4)));;[127X[104X
    [4X[25Xgap>[125X [27Xtime;[127X[104X
    [4X[28X11515[128X[104X
    [4X[25Xgap>[125X [27XLength(planes);[127X[104X
    [4X[28X891[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  
  [1X7.8-3 [33X[0;0YIterators for polar spaces[133X[101X
  
  [33X[0;0YFor  all  polar  spaces an iterator is constructed using [11XIteratorList(enum)[111X,
  where [3Xenum[103X is an appropriate enumerator.[133X
  
  [1X7.8-4 Iterator[101X
  
  [29X[2XIterator[102X( [3Xelements[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan iterator[133X
  
  [33X[0;0Y[3XC[103X is a collection of subspaces of a polar space.[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xiter := Iterator(Lines(ParabolicQuadric(4,2)));[127X[104X
    [4X[28X<iterator>[128X[104X
    [4X[25Xgap>[125X [27XNextIterator(iter);[127X[104X
    [4X[28X<a line in Q(4, 2)>[128X[104X
    [4X[25Xgap>[125X [27XNextIterator(iter);[127X[104X
    [4X[28X<a line in Q(4, 2)>[128X[104X
    [4X[25Xgap>[125X [27XNextIterator(iter);[127X[104X
    [4X[28X<a line in Q(4, 2)>[128X[104X
    [4X[25Xgap>[125X [27XNextIterator(iter);[127X[104X
    [4X[28X<a line in Q(4, 2)>[128X[104X
    [4X[25Xgap>[125X [27XNextIterator(iter);[127X[104X
    [4X[28X<a line in Q(4, 2)>[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
  [1X7.8-5 AsList[101X
  
  [29X[2XAsList[102X( [3Xsubspaces[103X ) [32X operation
  [6XReturns:[106X  [33X[0;10Yan Orb object or list[133X
  
  [4X[32X  Example  [32X[104X
    [4X[25Xgap>[125X [27Xps := HyperbolicQuadric(5,3);[127X[104X
    [4X[28XQ+(5, 3)[128X[104X
    [4X[25Xgap>[125X [27Xlines := AsList(Lines(ps));[127X[104X
    [4X[28X<closed orbit, 520 points>[128X[104X
    [4X[28X [128X[104X
  [4X[32X[104X
  
