CXML
        	    
LAPACK version 3.0
sstevd(3)

PURPOSE
  SSTEVD - compute all eigenvalues and, optionally, eigenvectors of a real
  symmetric tridiagonal matrix

SYNTAX
  SUBROUTINE SSTEVD( JOBZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK, LIWORK, INFO
		     )

      CHARACTER	     JOBZ

      INTEGER	     INFO, LDZ, LIWORK, LWORK, N

      INTEGER	     IWORK( * )

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

DESCRIPTION
  SSTEVD computes all eigenvalues and, optionally, eigenvectors of a real
  symmetric tridiagonal matrix. If eigenvectors are desired, it uses a divide
  and conquer algorithm.

  The divide and conquer algorithm makes very mild assumptions about floating
  point arithmetic. It will work on machines with a guard digit in
  add/subtract, or on those binary machines without guard digits which
  subtract like the Cray X-MP, Cray Y-MP, Cray C-90, or Cray-2. It could
  conceivably fail on hexadecimal or decimal machines without guard digits,
  but we know of none.

ARGUMENTS
  JOBZ	  (input) CHARACTER*1
	  = 'N':  Compute eigenvalues only;
	  = 'V':  Compute eigenvalues and eigenvectors.

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

  D	  (input/output) REAL array, dimension (N)
	  On entry, the n diagonal elements of the tridiagonal matrix A.  On
	  exit, if INFO = 0, the eigenvalues in ascending order.

  E	  (input/output) REAL array, dimension (N)
	  On entry, the (n-1) subdiagonal elements of the tridiagonal matrix
	  A, stored in elements 1 to N-1 of E; E(N) need not be set, but is
	  used by the routine.	On exit, the contents of E are destroyed.

  Z	  (output) REAL array, dimension (LDZ, N)
	  If JOBZ = 'V', then if INFO = 0, Z contains the orthonormal
	  eigenvectors of the matrix A, with the i-th column of Z holding the
	  eigenvector associated with D(i).  If JOBZ = 'N', then Z is not
	  referenced.

  LDZ	  (input) INTEGER
	  The leading dimension of the array Z.	 LDZ >= 1, and if JOBZ = 'V',
	  LDZ >= max(1,N).

  WORK	  (workspace/output) REAL array,
	  dimension (LWORK) On exit, if INFO = 0, WORK(1) returns the optimal
	  LWORK.

  LWORK	  (input) INTEGER
	  The dimension of the array WORK.  If JOBZ  = 'N' or N <= 1 then
	  LWORK must be at least 1.  If JOBZ  = 'V' and N > 1 then LWORK must
	  be at least ( 1 + 4*N + N**2 ).

	  If LWORK = -1, then a workspace query is assumed; the routine only
	  calculates the optimal size of the WORK array, returns this value
	  as the first entry of the WORK array, and no error message related
	  to LWORK is issued by XERBLA.

  IWORK	  (workspace/output) INTEGER array, dimension (LIWORK)
	  On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.

  LIWORK  (input) INTEGER
	  The dimension of the array IWORK.  If JOBZ  = 'N' or N <= 1 then
	  LIWORK must be at least 1.  If JOBZ  = 'V' and N > 1 then LIWORK
	  must be at least 3+5*N.

	  If LIWORK = -1, then a workspace query is assumed; the routine only
	  calculates the optimal size of the IWORK array, returns this value
	  as the first entry of the IWORK array, and no error message related
	  to LIWORK is issued by XERBLA.

  INFO	  (output) INTEGER
	  = 0:	successful exit
	  < 0:	if INFO = -i, the i-th argument had an illegal value
	  > 0:	if INFO = i, the algorithm failed to converge; i off-diagonal
	  elements of E did not converge to zero.