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RedHat 9 (Linux i386) - man page for sstein (redhat section l)

SSTEIN(l)					)					SSTEIN(l)

NAME
       SSTEIN  -  compute the eigenvectors of a real symmetric tridiagonal matrix T corresponding
       to specified eigenvalues, using inverse iteration

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

	   INTEGER	  INFO, LDZ, M, N

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

	   REAL 	  D( * ), E( * ), W( * ), WORK( * ), Z( LDZ, * )

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

       E       (input) REAL array, dimension (N)
	       The (n-1) subdiagonal elements of the tridiagonal matrix T, in elements 1 to  N-1.
	       E(N) need not be set.

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

       W       (input) 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  (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  SSTEBZ
	       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 SSTEBZ is expected
	       here. )

       Z       (output) REAL 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) REAL 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 version 3.0			   15 June 2000 				SSTEIN(l)


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