# sstedc (l) - Linux Man Pages

## NAME

SSTEDC - computes all eigenvalues and, optionally, eigenvectors of a symmetric tridiagonal matrix using the divide and conquer method

## SYNOPSIS

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

CHARACTER COMPZ

INTEGER INFO, LDZ, LIWORK, LWORK, N

INTEGER IWORK( * )

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

## PURPOSE

SSTEDC 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 symmetric matrix can also be found if SSYTRD or SSPTRD or SSBTRD 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 conceivably fail on hexadecimal or decimal machines without guard digits, but we know of none. See SLAED3 for details.

## ARGUMENTS

COMPZ (input) CHARACTER*1
= aqNaq: Compute eigenvalues only.
= aqIaq: Compute eigenvectors of tridiagonal matrix also.
= aqVaq: 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) REAL 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) REAL array, dimension (N-1)
On entry, the subdiagonal elements of the tridiagonal matrix. On exit, E has been destroyed.
Z (input/output) REAL array, dimension (LDZ,N)
On entry, if COMPZ = aqVaq, then Z contains the orthogonal matrix used in the reduction to tridiagonal form. On exit, if INFO = 0, then if COMPZ = aqVaq, Z contains the orthonormal eigenvectors of the original symmetric matrix, and if COMPZ = aqIaq, Z contains the orthonormal eigenvectors of the symmetric tridiagonal matrix. If COMPZ = aqNaq, 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) REAL array, dimension (MAX(1,LWORK))
On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
LWORK (input) INTEGER
The dimension of the array WORK. If COMPZ = aqNaq or N <= 1 then LWORK must be at least 1. If COMPZ = aqVaq 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 = aqIaq and N > 1 then LWORK must be at least ( 1 + 4*N + N**2 ). Note that for COMPZ = aqIaq or aqVaq, then if N is less than or equal to the minimum divide size, usually 25, then LWORK need only be max(1,2*(N-1)). 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 (MAX(1,LIWORK))
On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK.
LIWORK (input) INTEGER
The dimension of the array IWORK. If COMPZ = aqNaq or N <= 1 then LIWORK must be at least 1. If COMPZ = aqVaq and N > 1 then LIWORK must be at least ( 6 + 6*N + 5*N*lg N ). If COMPZ = aqIaq and N > 1 then LIWORK must be at least ( 3 + 5*N ). Note that for COMPZ = aqIaq or aqVaq, then if N is less than or equal to the minimum divide size, usually 25, then LIWORK need only be 1. 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.