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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