slaexc(3)

NAME

SLAEXC - swap adjacent diagonal blocks T11 and T22 of or

der 1 or 2 in an upper quasi-triangular matrix T by an orthogonal

similarity transformation

SYNOPSIS

SUBROUTINE SLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1,  N1,  N2,
WORK, INFO )
    LOGICAL        WANTQ
    INTEGER        INFO, J1, LDQ, LDT, N, N1, N2
    REAL           Q( LDQ, * ), T( LDT, * ), WORK( * )

PURPOSE

SLAEXC swaps adjacent diagonal blocks T11 and T22 of order

1 or 2 in an upper quasi-triangular matrix T by an orthogonal

similarity transformation. T must be in Schur canonical form,

that is, block upper triangular with 1-by-1 and 2-by-2 diagonal

blocks; each 2-by-2 diagonal block has its diagonal elemnts equal

and its off-diagonal elements of opposite sign.

ARGUMENTS

WANTQ (input) LOGICAL

= .TRUE. : accumulate the transformation in the

matrix Q;
= .FALSE.: do not accumulate the transformation.

N (input) INTEGER

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

T (input/output) REAL array, dimension (LDT,N)

On entry, the upper quasi-triangular matrix T, in

Schur canonical form. On exit, the updated matrix T, again in

Schur canonical form.

LDT (input) INTEGER

The leading dimension of the array T. LDT >=

max(1,N).

Q (input/output) REAL array, dimension (LDQ,N)

On entry, if WANTQ is .TRUE., the orthogonal ma

trix Q. On exit, if WANTQ is .TRUE., the updated matrix Q. If

WANTQ is .FALSE., Q is not referenced.

LDQ (input) INTEGER

The leading dimension of the array Q. LDQ >= 1;

and if WANTQ is .TRUE., LDQ >= N.

J1 (input) INTEGER

The index of the first row of the first block T11.

N1 (input) INTEGER

The order of the first block T11. N1 = 0, 1 or 2.

N2 (input) INTEGER

The order of the second block T22. N2 = 0, 1 or 2.

WORK (workspace) REAL array, dimension (N)

INFO (output) INTEGER

= 0: successful exit
= 1: the transformed matrix T would be too far

from Schur form; the blocks are not swapped and T and Q are un

changed.

LAPACK version 3.0 15 June 2000

SLAEXC(3)