# claed0 (l) - Linux Man Pages

## NAME

CLAED0 - the divide and conquer method, CLAED0 computes all eigenvalues of a symmetric tridiagonal matrix which is one diagonal block of those from reducing a dense or band Hermitian matrix and corresponding eigenvectors of the dense or band matrix

## SYNOPSIS

SUBROUTINE CLAED0(
QSIZ, N, D, E, Q, LDQ, QSTORE, LDQS, RWORK, IWORK, INFO )

INTEGER INFO, LDQ, LDQS, N, QSIZ

INTEGER IWORK( * )

REAL D( * ), E( * ), RWORK( * )

COMPLEX Q( LDQ, * ), QSTORE( LDQS, * )

## PURPOSE

Using the divide and conquer method, CLAED0 computes all eigenvalues of a symmetric tridiagonal matrix which is one diagonal block of those from reducing a dense or band Hermitian matrix and corresponding eigenvectors of the dense or band matrix.

## ARGUMENTS

QSIZ (input) INTEGER
The dimension of the unitary matrix used to reduce the full matrix to tridiagonal form. QSIZ >= N if ICOMPQ = 1.
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, the eigenvalues in ascending order.
E (input/output) REAL array, dimension (N-1)
On entry, the off-diagonal elements of the tridiagonal matrix. On exit, E has been destroyed.
Q (input/output) COMPLEX array, dimension (LDQ,N)
On entry, Q must contain an QSIZ x N matrix whose columns unitarily orthonormal. It is a part of the unitary matrix that reduces the full dense Hermitian matrix to a (reducible) symmetric tridiagonal matrix.
LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >= max(1,N).
IWORK (workspace) INTEGER array,
the dimension of IWORK must be at least 6 + 6*N + 5*N*lg N ( lg( N ) = smallest integer k such that 2^k >= N )
RWORK (workspace) REAL array,
dimension (1 + 3*N + 2*N*lg N + 3*N**2) ( lg( N ) = smallest integer k such that 2^k >= N ) QSTORE (workspace) COMPLEX array, dimension (LDQS, N) Used to store parts of the eigenvector matrix when the updating matrix multiplies take place.
LDQS (input) INTEGER
The leading dimension of the array QSTORE. LDQS >= max(1,N).
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).