CXML
        	    
LAPACK version 3.0
sstein(3)

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

SYNTAX
  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, * )

DESCRIPTION
  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.