DSTEIN(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 (N1)

The (n1) subdiagonal elements of the tridiagonal matrix
T, in elements 1 to N1.
 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 splitoff 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 ith 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 ith 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.