SRC/dsteqr.f(3)

NAME
SRC/dsteqr.f

SYNOPSIS

  Functions/Subroutines
subroutine dsteqr (compz, n, d, e, z, ldz, work, info)
DSTEQR

Function/Subroutine Documentation

  subroutine dsteqr (character compz, integer n, double precision, dimension( * ) d, double precision, dimension( * ) e, double precision, dimension( ldz, * ) z, integer ldz, double precision, dimension( * ) work, integer info)

DSTEQR

Purpose:

    !>
    !> DSTEQR computes all eigenvalues and, optionally, eigenvectors of a
    !> symmetric tridiagonal matrix using the implicit QL or QR method.
    !> The eigenvectors of a full or band symmetric matrix can also be found
    !> if DSYTRD or DSPTRD or DSBTRD has been used to reduce this matrix to
    !> tridiagonal form.
    !>

Parameters
    COMPZ

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

    N

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

    D

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

    Z

    !>          Z is DOUBLE PRECISION 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, and if
    !>          eigenvectors are desired, then  LDZ >= max(1,N).
    !>

    WORK

    !>          WORK is DOUBLE PRECISION array, dimension (max(1,2*N-2))
    !>          If COMPZ = 'N', then WORK is not referenced.
    !>

    INFO

    !>          INFO is INTEGER
    !>          = 0:  successful exit
    !>          < 0:  if INFO = -i, the i-th argument had an illegal value
    !>          > 0:  the algorithm has failed to find all the eigenvalues in
    !>                a total of 30*N iterations; if INFO = i, then i
    !>                elements of E have not converged to zero; on exit, D
    !>                and E contain the elements of a symmetric tridiagonal
    !>                matrix which is orthogonally similar to the original
    !>                matrix.
    !>

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Definition at line 130 of file dsteqr.f.

Author

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