Provided by: liblapack-doc_3.3.1-1_all bug

NAME

       LAPACK-3   -   computes  the  eigenvectors  of  a  real  symmetric  tridiagonal  matrix  T
       corresponding to specified eigenvalues, using inverse iteration

SYNOPSIS

       SUBROUTINE DSTEIN( N, D, E, M, W, IBLOCK, ISPLIT, Z, LDZ, WORK, IWORK, IFAIL, INFO )

           INTEGER        INFO, LDZ, M, N

           INTEGER        IBLOCK( * ), IFAIL( * ), ISPLIT( * ), IWORK( * )

           DOUBLE         PRECISION D( * ), E( * ), W( * ), WORK( * ), Z( LDZ, * )

PURPOSE

       DSTEIN 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).

ARGUMENTS

        N       (input) INTEGER
                The order of the matrix.  N >= 0.

        D       (input) DOUBLE PRECISION array, dimension (N)
                The n diagonal elements of the tridiagonal matrix T.

        E       (input) DOUBLE PRECISION array, dimension (N-1)
                The (n-1) subdiagonal elements of the tridiagonal matrix
                T, in elements 1 to N-1.

        M       (input) INTEGER
                The number of eigenvectors to be found.  0 <= M <= N.

        W       (input) DOUBLE PRECISION 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 DSTEBZ with ORDER = 'B' is expected here. )

        IBLOCK  (input) 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 DSTEBZ is expected here. )

        ISPLIT  (input) 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 DSTEBZ is expected here. )

        Z       (output) DOUBLE PRECISION 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.

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

        WORK    (workspace) DOUBLE PRECISION array, dimension (5*N)

        IWORK   (workspace) INTEGER array, dimension (N)

        IFAIL   (output) 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    (output) 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.

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.

 LAPACK routine (version 3.2)               April 2011                            DSTEIN(3lapack)