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

DSTEDC(l)					)					DSTEDC(l)

NAME
       DSTEDC  - compute all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal
       matrix using the divide and conquer method

SYNOPSIS
       SUBROUTINE DSTEDC( COMPZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK, LIWORK, INFO )

	   CHARACTER	  COMPZ

	   INTEGER	  INFO, LDZ, LIWORK, LWORK, N

	   INTEGER	  IWORK( * )

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

PURPOSE
       DSTEDC computes all eigenvalues and, optionally, eigenvectors of a  symmetric  tridiagonal
       matrix  using  the divide and conquer method. The eigenvectors of a full or band real sym-
       metric matrix can also be found if DSYTRD or DSPTRD or DSBTRD has been used to reduce this
       matrix to tridiagonal form.

       This  code  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 con-
       ceivably fail on hexadecimal or decimal machines without guard  digits,	but  we  know  of
       none.  See DLAED3 for details.

ARGUMENTS
       COMPZ   (input) CHARACTER*1
	       = 'N':  Compute eigenvalues only.
	       = 'I':  Compute eigenvectors of tridiagonal matrix also.
	       = 'V':  Compute eigenvectors of original dense symmetric matrix also.  On entry, Z
	       contains the orthogonal matrix used to reduce the original matrix  to  tridiagonal
	       form.

       N       (input) INTEGER
	       The dimension of the symmetric tridiagonal matrix.  N >= 0.

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

       E       (input/output) DOUBLE PRECISION array, dimension (N-1)
	       On entry, the subdiagonal elements of the tridiagonal matrix.  On exit, E has been
	       destroyed.

       Z       (input/output) DOUBLE PRECISION array, dimension (LDZ,N)
	       On entry, if COMPZ = 'V', then Z contains the orthogonal matrix used in the reduc-
	       tion to tridiagonal form.  On exit, if INFO = 0, then if COMPZ = 'V',  Z  contains
	       the orthonormal eigenvectors of the original symmetric matrix, and if COMPZ = 'I',
	       Z contains the orthonormal eigenvectors of the symmetric tridiagonal  matrix.   If
	       COMPZ = 'N', then Z is not referenced.

       LDZ     (input) INTEGER
	       The  leading  dimension	of  the array Z.  LDZ >= 1.  If eigenvectors are desired,
	       then LDZ >= max(1,N).

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

       LWORK   (input) INTEGER
	       The dimension of the array WORK.  If COMPZ = 'N' or N <= 1 then LWORK must  be  at
	       least  1.  If COMPZ = 'V' and N > 1 then LWORK must be at least ( 1 + 3*N + 2*N*lg
	       N + 3*N**2 ), where lg( N ) = smallest integer k such that 2**k >= N.  If COMPZ	=
	       'I' 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 COMPZ = 'N' or N <= 1 then LIWORK must be at
	       least 1.  If COMPZ = 'V' and N > 1 then LIWORK must be at least ( 6 + 6*N + 5*N*lg
	       N ).  If COMPZ = 'I' 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:  The algorithm failed to compute an eigenvalue while working on the submatrix
	       lying in rows and columns INFO/(N+1) through mod(INFO,N+1).

FURTHER DETAILS
       Based on contributions by
	  Jeff Rutter, Computer Science Division, University of California
	  at Berkeley, USA
       Modified by Francoise Tisseur, University of Tennessee.

LAPACK version 3.0			   15 June 2000 				DSTEDC(l)


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