SRC/cbdsqr.f(3)

NAME
SRC/cbdsqr.f

SYNOPSIS

  Functions/Subroutines
subroutine cbdsqr (uplo, n, ncvt, nru, ncc, d, e, vt, ldvt, u, ldu, c, ldc, rwork, info)
CBDSQR

Function/Subroutine Documentation

  subroutine cbdsqr (character uplo, integer n, integer ncvt, integer nru, integer ncc, real, dimension( * ) d, real, dimension( * ) e, complex, dimension( ldvt, * ) vt, integer ldvt, complex, dimension( ldu, * ) u, integer ldu, complex, dimension( ldc, * ) c, integer ldc, real, dimension( * ) rwork, integer info)

CBDSQR

Purpose:

    !>
    !> CBDSQR computes the singular values and, optionally, the right and/or
    !> left singular vectors from the singular value decomposition (SVD) of
    !> a real N-by-N (upper or lower) bidiagonal matrix B using the implicit
    !> zero-shift QR algorithm.  The SVD of B has the form
    !>
    !>    B = Q * S * P**H
    !>
    !> where S is the diagonal matrix of singular values, Q is an orthogonal
    !> matrix of left singular vectors, and P is an orthogonal matrix of
    !> right singular vectors.  If left singular vectors are requested, this
    !> subroutine actually returns U*Q instead of Q, and, if right singular
    !> vectors are requested, this subroutine returns P**H*VT instead of
    !> P**H, for given complex input matrices U and VT.  When U and VT are
    !> the unitary matrices that reduce a general matrix A to bidiagonal
    !> form: A = U*B*VT, as computed by CGEBRD, then
    !>
    !>    A = (U*Q) * S * (P**H*VT)
    !>
    !> is the SVD of A.  Optionally, the subroutine may also compute Q**H*C
    !> for a given complex input matrix C.
    !>
    !> See  by J. Demmel and W. Kahan,
    !> LAPACK Working Note #3 (or SIAM J. Sci. Statist. Comput. vol. 11,
    !> no. 5, pp. 873-912, Sept 1990) and
    !>  by
    !> B. Parlett and V. Fernando, Technical Report CPAM-554, Mathematics
    !> Department, University of California at Berkeley, July 1992
    !> for a detailed description of the algorithm.
    !>

Parameters
    UPLO

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

    N

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

    NCVT

    !>          NCVT is INTEGER
    !>          The number of columns of the matrix VT. NCVT >= 0.
    !>

    NRU

    !>          NRU is INTEGER
    !>          The number of rows of the matrix U. NRU >= 0.
    !>

    NCC

    !>          NCC is INTEGER
    !>          The number of columns of the matrix C. NCC >= 0.
    !>

    D

    !>          D is REAL array, dimension (N)
    !>          On entry, the n diagonal elements of the bidiagonal matrix B.
    !>          On exit, if INFO=0, the singular values of B in decreasing
    !>          order.
    !>

    E

    !>          E is REAL array, dimension (N-1)
    !>          On entry, the N-1 offdiagonal elements of the bidiagonal
    !>          matrix B.
    !>          On exit, if INFO = 0, E is destroyed; if INFO > 0, D and E
    !>          will contain the diagonal and superdiagonal elements of a
    !>          bidiagonal matrix orthogonally equivalent to the one given
    !>          as input.
    !>

    VT

    !>          VT is COMPLEX array, dimension (LDVT, NCVT)
    !>          On entry, an N-by-NCVT matrix VT.
    !>          On exit, VT is overwritten by P**H * VT.
    !>          Not referenced if NCVT = 0.
    !>

    LDVT

    !>          LDVT is INTEGER
    !>          The leading dimension of the array VT.
    !>          LDVT >= max(1,N) if NCVT > 0; LDVT >= 1 if NCVT = 0.
    !>

    U

    !>          U is COMPLEX array, dimension (LDU, N)
    !>          On entry, an NRU-by-N matrix U.
    !>          On exit, U is overwritten by U * Q.
    !>          Not referenced if NRU = 0.
    !>

    LDU

    !>          LDU is INTEGER
    !>          The leading dimension of the array U.  LDU >= max(1,NRU).
    !>

    C

    !>          C is COMPLEX array, dimension (LDC, NCC)
    !>          On entry, an N-by-NCC matrix C.
    !>          On exit, C is overwritten by Q**H * C.
    !>          Not referenced if NCC = 0.
    !>

    LDC

    !>          LDC is INTEGER
    !>          The leading dimension of the array C.
    !>          LDC >= max(1,N) if NCC > 0; LDC >=1 if NCC = 0.
    !>

    RWORK

    !>          RWORK is REAL array, dimension (LRWORK)
    !>          LRWORK = 4*N, if NCVT = NRU = NCC = 0, and
    !>          LRWORK = 4*(N-1), otherwise
    !>

    INFO

    !>          INFO is INTEGER
    !>          = 0:  successful exit
    !>          < 0:  If INFO = -i, the i-th argument had an illegal value
    !>          > 0:  the algorithm did not converge; D and E contain the
    !>                elements of a bidiagonal matrix which is orthogonally
    !>                similar to the input matrix B;  if INFO = i, i
    !>                elements of E have not converged to zero.
    !>

Internal Parameters:

    !>  TOLMUL  REAL, default = max(10,min(100,EPS**(-1/8)))
    !>          TOLMUL controls the convergence criterion of the QR loop.
    !>          If it is positive, TOLMUL*EPS is the desired relative
    !>             precision in the computed singular values.
    !>          If it is negative, abs(TOLMUL*EPS*sigma_max) is the
    !>             desired absolute accuracy in the computed singular
    !>             values (corresponds to relative accuracy
    !>             abs(TOLMUL*EPS) in the largest singular value.
    !>          abs(TOLMUL) should be between 1 and 1/EPS, and preferably
    !>             between 10 (for fast convergence) and .1/EPS
    !>             (for there to be some accuracy in the results).
    !>          Default is to lose at either one eighth or 2 of the
    !>             available decimal digits in each computed singular value
    !>             (whichever is smaller).
    !>
    !>  MAXITR  INTEGER, default = 6
    !>          MAXITR controls the maximum number of passes of the
    !>          algorithm through its inner loop. The algorithms stops
    !>          (and so fails to converge) if the number of passes
    !>          through the inner loop exceeds MAXITR*N**2.
    !>
    !>

Note:

    !>  Bug report from Cezary Dendek.
    !>  On November 3rd 2023, the INTEGER variable MAXIT = MAXITR*N**2 is
    !>  removed since it can overflow pretty easily (for N larger or equal
    !>  than 18,919). We instead use MAXITDIVN = MAXITR*N.
    !>

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Definition at line 233 of file cbdsqr.f.

Author

Generated automatically by Doxygen for LAPACK from the source code.