ssteqr()

subroutine ssteqr	(	character	compz,
		integer	n,
		real, dimension( * )	d,
		real, dimension( * )	e,
		real, dimension( ldz, * )	z,
		integer	ldz,
		real, dimension( * )	work,
		integer	info )

SSTEQR

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Purpose:

!>
!> SSTEQR computes all eigenvalues and, optionally, eigenvectors of a
!> symmetric tridiagonal matrix using the implicit QL or QR method.
!> The eigenvectors of a full or band symmetric matrix can also be found
!> if SSYTRD or SSPTRD or SSBTRD has been used to reduce this matrix to
!> tridiagonal form.
!> 

Parameters

[in]	COMPZ	!> COMPZ is CHARACTER*1 !> = 'N': Compute eigenvalues only. !> = 'V': Compute eigenvalues and eigenvectors of the original !> symmetric matrix. On entry, Z must contain the !> orthogonal matrix used to reduce the original matrix !> to tridiagonal form. !> = 'I': Compute eigenvalues and eigenvectors of the !> tridiagonal matrix. Z is initialized to the identity !> matrix. !>
[in]	N	!> N is INTEGER !> The order of the matrix. N >= 0. !>
[in,out]	D	!> D is REAL array, dimension (N) !> On entry, the diagonal elements of the tridiagonal matrix. !> On exit, if INFO = 0, the eigenvalues in ascending order. !>
[in,out]	E	!> E is REAL array, dimension (N-1) !> On entry, the (n-1) subdiagonal elements of the tridiagonal !> matrix. !> On exit, E has been destroyed. !>
[in,out]	Z	!> Z is REAL array, dimension (LDZ, N) !> On entry, if COMPZ = 'V', then Z contains the orthogonal !> matrix used in the reduction 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. !>
[in]	LDZ	!> LDZ is INTEGER !> The leading dimension of the array Z. LDZ >= 1, and if !> eigenvectors are desired, then LDZ >= max(1,N). !>
[out]	WORK	!> WORK is REAL array, dimension (max(1,2*N-2)) !> If COMPZ = 'N', then WORK is not referenced. !>
[out]	INFO	!> INFO is INTEGER !> = 0: successful exit !> < 0: if INFO = -i, the i-th argument had an illegal value !> > 0: the algorithm has failed to find all the eigenvalues in !> a total of 30*N iterations; if INFO = i, then i !> elements of E have not converged to zero; on exit, D !> and E contain the elements of a symmetric tridiagonal !> matrix which is orthogonally similar to the original !> matrix. !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.