SRC/sstedc.f(3)

NAME
SRC/sstedc.f

SYNOPSIS

  Functions/Subroutines
subroutine sstedc (compz, n, d, e, z, ldz, work, lwork, iwork, liwork, info)
SSTEDC

Function/Subroutine Documentation

  subroutine sstedc (character compz, integer n, real, dimension( * ) d, real, dimension( * ) e, real, dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer lwork, integer, dimension( * ) iwork, integer liwork, integer info)

SSTEDC

Purpose:

    !>
    !> SSTEDC computes all eigenvalues and, optionally, eigenvectors of a
    !> symmetric tridiagonal matrix using the divide and conquer method.
    !> The eigenvectors of a full or band real symmetric matrix can also be
    !> found if SSYTRD or SSPTRD or SSBTRD has been used to reduce this
    !> matrix to tridiagonal form.
    !>
    !>

Parameters
    COMPZ

    !>          COMPZ is CHARACTER*1
    !>          = 'N':  Compute eigenvalues only.
    !>          = 'I':  Compute eigenvectors of tridiagonal matrix also.
    !>          = 'V':  Compute eigenvectors of original dense symmetric
    !>                  matrix also.  On entry, Z contains the orthogonal
    !>                  matrix used to reduce the original matrix to
    !>                  tridiagonal form.
    !>

    N

    !>          N is INTEGER
    !>          The dimension of the symmetric tridiagonal matrix.  N >= 0.
    !>

    D

    !>          D is REAL array, dimension (N)
    !>          On entry, the diagonal elements of the tridiagonal matrix.
    !>          On exit, if INFO = 0, the eigenvalues in ascending order.
    !>

    E

    !>          E is REAL array, dimension (N-1)
    !>          On entry, the subdiagonal elements of the tridiagonal matrix.
    !>          On exit, E has been destroyed.
    !>

    Z

    !>          Z is REAL array, dimension (LDZ,N)
    !>          On entry, if COMPZ = 'V', then Z contains the orthogonal
    !>          matrix used in the reduction to tridiagonal form.
    !>          On exit, if INFO = 0, then if COMPZ = 'V', Z contains the
    !>          orthonormal eigenvectors of the original symmetric matrix,
    !>          and if COMPZ = 'I', Z contains the orthonormal eigenvectors
    !>          of the symmetric tridiagonal matrix.
    !>          If  COMPZ = 'N', then Z is not referenced.
    !>

    LDZ

    !>          LDZ is INTEGER
    !>          The leading dimension of the array Z.  LDZ >= 1.
    !>          If eigenvectors are desired, then LDZ >= max(1,N).
    !>

    WORK

    !>          WORK is REAL array, dimension (MAX(1,LWORK))
    !>          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
    !>

    LWORK

    !>          LWORK is INTEGER
    !>          The dimension of the array WORK.
    !>          If COMPZ = 'N' or N <= 1 then LWORK must be at least 1.
    !>          If COMPZ = 'V' and N > 1 then LWORK must be at least
    !>                         ( 1 + 3*N + 2*N*lg N + 4*N**2 ),
    !>                         where lg( N ) = smallest integer k such
    !>                         that 2**k >= N.
    !>          If COMPZ = 'I' and N > 1 then LWORK must be at least
    !>                         ( 1 + 4*N + N**2 ).
    !>          Note that for COMPZ = 'I' or 'V', then if N is less than or
    !>          equal to the minimum divide size, usually 25, then LWORK need
    !>          only be max(1,2*(N-1)).
    !>
    !>          If LWORK = -1, then a workspace query is assumed; the routine
    !>          only calculates the optimal size of the WORK array, returns
    !>          this value as the first entry of the WORK array, and no error
    !>          message related to LWORK is issued by XERBLA.
    !>

    IWORK

    !>          IWORK is INTEGER array, dimension (MAX(1,LIWORK))
    !>          On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
    !>

    LIWORK

    !>          LIWORK is INTEGER
    !>          The dimension of the array IWORK.
    !>          If COMPZ = 'N' or N <= 1 then LIWORK must be at least 1.
    !>          If COMPZ = 'V' and N > 1 then LIWORK must be at least
    !>                         ( 6 + 6*N + 5*N*lg N ).
    !>          If COMPZ = 'I' and N > 1 then LIWORK must be at least
    !>                         ( 3 + 5*N ).
    !>          Note that for COMPZ = 'I' or 'V', then if N is less than or
    !>          equal to the minimum divide size, usually 25, then LIWORK
    !>          need only be 1.
    !>
    !>          If LIWORK = -1, then a workspace query is assumed; the
    !>          routine only calculates the optimal size of the IWORK array,
    !>          returns this value as the first entry of the IWORK array, and
    !>          no error message related to LIWORK is issued by XERBLA.
    !>

    INFO

    !>          INFO is INTEGER
    !>          = 0:  successful exit.
    !>          < 0:  if INFO = -i, the i-th argument had an illegal value.
    !>          > 0:  The algorithm failed to compute an eigenvalue while
    !>                working on the submatrix lying in rows and columns
    !>                INFO/(N+1) through mod(INFO,N+1).
    !>

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Contributors:
    Jeff Rutter, Computer Science Division, University of California at Berkeley, USA
     Modified by Francoise Tisseur, University of Tennessee

Definition at line 180 of file sstedc.f.

Author

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