dsyev(3)

NAME

DSYEV - compute all eigenvalues and, optionally, eigenvec

tors of a real symmetric matrix A

SYNOPSIS

SUBROUTINE  DSYEV(  JOBZ, UPLO, N, A, LDA, W, WORK, LWORK,
INFO )
    CHARACTER     JOBZ, UPLO
    INTEGER       INFO, LDA, LWORK, N
    DOUBLE        PRECISION A( LDA, * ), W( * ), WORK( * )

PURPOSE

DSYEV computes all eigenvalues and, optionally, eigenvec

tors of a real symmetric matrix A.

ARGUMENTS

JOBZ (input) CHARACTER*1

= 'N': Compute eigenvalues only;
= 'V': Compute eigenvalues and eigenvectors.

UPLO (input) CHARACTER*1

= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.

N (input) INTEGER

The order of the matrix A. N >= 0.

A (input/output) DOUBLE PRECISION array, dimension

(LDA, N)

On entry, the symmetric matrix A. If UPLO = 'U',

the leading N-by-N upper triangular part of A contains the upper

triangular part of the matrix A. If UPLO = 'L', the leading N

by-N lower triangular part of A contains the lower triangular

part of the matrix A. On exit, if JOBZ = 'V', then if INFO = 0,

A contains the orthonormal eigenvectors of the matrix A. If JOBZ

= 'N', then on exit the lower triangle (if UPLO='L') or the upper

triangle (if UPLO='U') of A, including the diagonal, is de

stroyed.

LDA (input) INTEGER

The leading dimension of the array A. LDA >=

max(1,N).

W (output) DOUBLE PRECISION array, dimension (N)

If INFO = 0, the eigenvalues in ascending order.

WORK (workspace/output) DOUBLE PRECISION array, dimen

sion (LWORK)

On exit, if INFO = 0, WORK(1) returns the optimal

LWORK.

LWORK (input) INTEGER

The length of the array WORK. LWORK >=

max(1,3*N-1). For optimal efficiency, LWORK >= (NB+2)*N, where

NB is the blocksize for DSYTRD returned by ILAENV.

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.

INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = -i, the i-th argument had an ille

gal value
> 0: if INFO = i, the algorithm failed to con

verge; i off-diagonal elements of an intermediate tridiagonal

form did not converge to zero.

LAPACK version 3.0 15 June 2000

DSYEV(3)