SRC/cstein.f(3)

NAME
SRC/cstein.f

SYNOPSIS

  Functions/Subroutines
subroutine cstein (n, d, e, m, w, iblock, isplit, z, ldz, work, iwork, ifail, info)
CSTEIN

Function/Subroutine Documentation

  subroutine cstein (integer n, real, dimension( * ) d, real, dimension( * ) e, integer m, real, dimension( * ) w, integer, dimension( * ) iblock, integer, dimension( * ) isplit, complex, dimension( ldz, * ) z, integer ldz, real, dimension( * ) work, integer, dimension( * ) iwork, integer, dimension( * ) ifail, integer info)

CSTEIN

Purpose:

     CSTEIN computes the eigenvectors of a real symmetric tridiagonal
     matrix T corresponding to specified eigenvalues, using inverse
     iteration.
     The maximum number of iterations allowed for each eigenvector is
     specified by an internal parameter MAXITS (currently set to 5).
     Although the eigenvectors are real, they are stored in a complex
     array, which may be passed to CUNMTR or CUPMTR for back
     transformation to the eigenvectors of a complex Hermitian matrix
     which was reduced to tridiagonal form.

Parameters
    N

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

    D

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

    E

              E is REAL array, dimension (N-1)
              The (n-1) subdiagonal elements of the tridiagonal matrix
              T, stored in elements 1 to N-1.

    M

              M is INTEGER
              The number of eigenvectors to be found.  0 <= M <= N.

    W

              W is REAL array, dimension (N)
              The first M elements of W contain the eigenvalues for
              which eigenvectors are to be computed.  The eigenvalues
              should be grouped by split-off block and ordered from
              smallest to largest within the block.  ( The output array
              W from SSTEBZ with ORDER = 'B' is expected here. )

    IBLOCK

              IBLOCK is INTEGER array, dimension (N)
              The submatrix indices associated with the corresponding
              eigenvalues in W; IBLOCK(i)=1 if eigenvalue W(i) belongs to
              the first submatrix from the top, =2 if W(i) belongs to
              the second submatrix, etc.  ( The output array IBLOCK
              from SSTEBZ is expected here. )

    ISPLIT

              ISPLIT is INTEGER array, dimension (N)
              The splitting points, at which T breaks up into submatrices.
              The first submatrix consists of rows/columns 1 to
              ISPLIT( 1 ), the second of rows/columns ISPLIT( 1 )+1
              through ISPLIT( 2 ), etc.
              ( The output array ISPLIT from SSTEBZ is expected here. )

    Z

              Z is COMPLEX array, dimension (LDZ, M)
              The computed eigenvectors.  The eigenvector associated
              with the eigenvalue W(i) is stored in the i-th column of
              Z.  Any vector which fails to converge is set to its current
              iterate after MAXITS iterations.
              The imaginary parts of the eigenvectors are set to zero.

    LDZ

              LDZ is INTEGER
              The leading dimension of the array Z.  LDZ >= max(1,N).

    WORK

              WORK is REAL array, dimension (5*N)

    IWORK

              IWORK is INTEGER array, dimension (N)

    IFAIL

              IFAIL is INTEGER array, dimension (M)
              On normal exit, all elements of IFAIL are zero.
              If one or more eigenvectors fail to converge after
              MAXITS iterations, then their indices are stored in
              array IFAIL.

    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 MAXITS iterations.  Their indices are stored in
                   array IFAIL.

Internal Parameters:

      MAXITS  INTEGER, default = 5
              The maximum number of iterations performed.
      EXTRA   INTEGER, default = 2
              The number of iterations performed after norm growth
              criterion is satisfied, should be at least 1.

Author
    Univ. of Tennessee

    Univ. of California Berkeley

    Univ. of Colorado Denver

    NAG Ltd.

Definition at line 180 of file cstein.f.

Author

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