SRC/sbdsvdx.f(3)

NAME
SRC/sbdsvdx.f

SYNOPSIS

  Functions/Subroutines
subroutine sbdsvdx (uplo, jobz, range, n, d, e, vl, vu, il, iu, ns, s, z, ldz, work, iwork, info)
SBDSVDX

Function/Subroutine Documentation

  subroutine sbdsvdx (character uplo, character jobz, character range, integer n, real, dimension( * ) d, real, dimension( * ) e, real vl, real vu, integer il, integer iu, integer ns, real, dimension( * ) s, real, dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer, dimension( * ) iwork, integer info)

SBDSVDX

Purpose:

      SBDSVDX computes the singular value decomposition (SVD) of a real
      N-by-N (upper or lower) bidiagonal matrix B, B = U * S * VT,
      where S is a diagonal matrix with non-negative diagonal elements
      (the singular values of B), and U and VT are orthogonal matrices
      of left and right singular vectors, respectively.
      Given an upper bidiagonal B with diagonal D = [ d_1 d_2 ... d_N ]
      and superdiagonal E = [ e_1 e_2 ... e_N-1 ], SBDSVDX computes the
      singular value decomposition of B through the eigenvalues and
      eigenvectors of the N*2-by-N*2 tridiagonal matrix
            |  0  d_1                |
            | d_1  0  e_1            |
      TGK = |     e_1  0  d_2        |
            |         d_2  .   .     |
            |              .   .   . |
      If (s,u,v) is a singular triplet of B with ||u|| = ||v|| = 1, then
      (+/-s,q), ||q|| = 1, are eigenpairs of TGK, with q = P * ( u' +/-v' ) /
      sqrt(2) = ( v_1 u_1 v_2 u_2 ... v_n u_n ) / sqrt(2), and
      P = [ e_{n+1} e_{1} e_{n+2} e_{2} ... ].
      Given a TGK matrix, one can either a) compute -s,-v and change signs
      so that the singular values (and corresponding vectors) are already in
      descending order (as in SGESVD/SGESDD) or b) compute s,v and reorder
      the values (and corresponding vectors). SBDSVDX implements a) by
      calling SSTEVX (bisection plus inverse iteration, to be replaced
      with a version of the Multiple Relative Robust Representation
      algorithm. (See P. Willems and B. Lang, A framework for the MR^3
      algorithm: theory and implementation, SIAM J. Sci. Comput.,
      35:740-766, 2013.)

Parameters
    UPLO

              UPLO is CHARACTER*1
              = 'U':  B is upper bidiagonal;
              = 'L':  B is lower bidiagonal.

    JOBZ

              JOBZ is CHARACTER*1
              = 'N':  Compute singular values only;
              = 'V':  Compute singular values and singular vectors.

    RANGE

              RANGE is CHARACTER*1
              = 'A': all singular values will be found.
              = 'V': all singular values in the half-open interval [VL,VU)
                     will be found.
              = 'I': the IL-th through IU-th singular values will be found.

    N

              N is INTEGER
              The order of the bidiagonal matrix.  N >= 0.

    D

              D is REAL array, dimension (N)
              The n diagonal elements of the bidiagonal matrix B.

    E

              E is REAL array, dimension (max(1,N-1))
              The (n-1) superdiagonal elements of the bidiagonal matrix
              B in elements 1 to N-1.

    VL

             VL is REAL
              If RANGE='V', the lower bound of the interval to
              be searched for singular values. VU > VL.
              Not referenced if RANGE = 'A' or 'I'.

    VU

             VU is REAL
              If RANGE='V', the upper bound of the interval to
              be searched for singular values. VU > VL.
              Not referenced if RANGE = 'A' or 'I'.

    IL

              IL is INTEGER
              If RANGE='I', the index of the
              smallest singular value to be returned.
              1 <= IL <= IU <= min(M,N), if min(M,N) > 0.
              Not referenced if RANGE = 'A' or 'V'.

    IU

              IU is INTEGER
              If RANGE='I', the index of the
              largest singular value to be returned.
              1 <= IL <= IU <= min(M,N), if min(M,N) > 0.
              Not referenced if RANGE = 'A' or 'V'.

    NS

              NS is INTEGER
              The total number of singular values found.  0 <= NS <= N.
              If RANGE = 'A', NS = N, and if RANGE = 'I', NS = IU-IL+1.

    S

              S is REAL array, dimension (N)
              The first NS elements contain the selected singular values in
              ascending order.

    Z

              Z is REAL array, dimension (2*N,K)
              If JOBZ = 'V', then if INFO = 0 the first NS columns of Z
              contain the singular vectors of the matrix B corresponding to
              the selected singular values, with U in rows 1 to N and V
              in rows N+1 to N*2, i.e.
              Z = [ U ]
                  [ V ]
              If JOBZ = 'N', then Z is not referenced.
              Note: The user must ensure that at least K = NS+1 columns are
              supplied in the array Z; if RANGE = 'V', the exact value of
              NS is not known in advance and an upper bound must be used.

    LDZ

              LDZ is INTEGER
              The leading dimension of the array Z. LDZ >= 1, and if
              JOBZ = 'V', LDZ >= max(2,N*2).

    WORK

              WORK is REAL array, dimension (14*N)

    IWORK

              IWORK is INTEGER array, dimension (12*N)
              If JOBZ = 'V', then if INFO = 0, the first NS elements of
              IWORK are zero. If INFO > 0, then IWORK contains the indices
              of the eigenvectors that failed to converge in DSTEVX.

    INFO

              INFO is INTEGER
              = 0:  successful exit
              < 0:  if INFO = -i, the i-th argument had an illegal value
              > 0:  if INFO = i, then i eigenvectors failed to converge
                       in SSTEVX. The indices of the eigenvectors
                       (as returned by SSTEVX) are stored in the
                       array IWORK.
                    if INFO = N*2 + 1, an internal error occurred.

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Definition at line 224 of file sbdsvdx.f.

Author

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