Singular Manual: betti

  | ### 5.1.4 betti `Syntax:` `betti (` list_expression `)` `betti (` resolution_expression `)` `betti (` list_expression `,` int_expression `)` `betti (` resolution_expression `,` int_expression `)` `Type:` intmat `Purpose:` with 1 argument: computes the graded Betti numbers of a minimal resolution of , if denotes the basering, is a homogeneous submodule of and the argument represents a resolution of . The entry d of the intmat at place (i,j) is the minimal number of generators in degree i+j of the j-th syzygy module (= module of relations) of , i.e. the 0th (resp. 1st) syzygy module of is (resp. ).The argument is considered to be the result of a res/fres/sres/mres/nres/lres command. This implies that a zero is only allowed (and counted) as a generator in the first module. For the computation betti uses only the initial monomials. This could lead to confusing results for a non-homogeneous input. If the optional second argument is non-zero, the Betti numbers will be minimized. `betti` sets the attribute `rowShift`. `Example:`
        ring r=32003,(a,b,c,d),dp;
        ideal j=bc-ad,b3-a2c,c3-bd2,ac2-b2d;
        list T=mres(j,0); // 0 forces a full resolution
        // a minimal set of generators for j:
        print(T[1]);
      ==> bc-ad,
      ==> c3-bd2,
      ==> ac2-b2d,
      ==> b3-a2c
        // second syzygy module of r/j which is the first
        // syzygy module of j (minimal generating set):
        print(T[2]);
      ==> bd,c2,ac,b2,
      ==> -a,-b,0, 0,
      ==> c, d, -b,-a,
      ==> 0, 0, -d,-c
        // the second syzygy module (minimal generating set):
        print(T[3]);
      ==> -b,
      ==> a,
      ==> -c,
      ==> d
        print(T[4]);
      ==> 0
        betti(T);
      ==> 1,0,0,0,
      ==> 0,1,0,0,
      ==> 0,3,4,1
        // most useful for reading off the graded Betti numbers:
        print(betti(T),"betti");
      ==>            0     1     2     3
      ==> ------------------------------
      ==>     0:     1     -     -     -
      ==>     1:     -     1     -     -
      ==>     2:     -     3     4     1
      ==> ------------------------------
      ==> total:     1     4     4     1
      ==>
Hence, the 0th syzygy module of r/j (which is r) has 1 generator in degree 0 (which is 1), the 1st syzygy module `T[1]` (which is j) has 4 generators (one in degree 2 and three in degree 3), the 2nd syzygy module `T[2]` has 4 generators (all in degree 4), the 3rd syzygy module `T[3]` has 1 generator in degree 5, where the generators are the columns of the displayed matrix and degrees are assigned such that the corresponding maps have degree 0: See Syzygies and resolutions; fres; hres; lres; mres; print; res; resolution; sres.

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