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NAME DSTEV - compute all eigenvalues and, optionally, eigenvec- tors of a real symmetric tridiagonal matrix A SYNOPSIS SUBROUTINE DSTEV( JOBZ, N, D, E, Z, LDZ, WORK, INFO ) CHARACTER JOBZ INTEGER INFO, LDZ, N DOUBLE PRECISION D( * ), E( * ), WORK( * ), Z( LDZ, * ) PURPOSE DSTEV computes all eigenvalues and, optionally, eigenvectors of a real symmetric tridiagonal matrix A. 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) DOUBLE PRECISION 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/workspace) DOUBLE PRECISION array, dimension (N) On entry, the (n-1) subdiagonal elements of the tri- diagonal 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) DOUBLE PRECISION 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). (max(1,2*N-2)) WORK (workspace) DOUBLE PRECISION array, dimension If JOBZ = 'N', WORK is not referenced. 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.