DLAEXC(1) LAPACK auxiliary routine (version 3.2) DLAEXC(1)NAME
DLAEXC - swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in
an upper quasi-triangular matrix T by an orthogonal similarity trans‐
formation
SYNOPSIS
SUBROUTINE DLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO )
LOGICAL WANTQ
INTEGER INFO, J1, LDQ, LDT, N, N1, N2
DOUBLE PRECISION Q( LDQ, * ), T( LDT, * ), WORK( * )
PURPOSE
DLAEXC swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in an
upper quasi-triangular matrix T by an orthogonal similarity transforma‐
tion. T must be in Schur canonical form, that is, block upper triangu‐
lar 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 oppo‐
site 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) DOUBLE PRECISION array, dimension (LDT,N)
On entry, the upper quasi-triangular matrix T, in Schur canoni‐
cal form. On exit, the updated matrix T, again in Schur canon‐
ical form.
LDT (input) INTEGER
The leading dimension of the array T. LDT >= max(1,N).
Q (input/output) DOUBLE PRECISION array, dimension (LDQ,N)
On entry, if WANTQ is .TRUE., the orthogonal matrix 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) DOUBLE PRECISION 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 unchanged.
LAPACK auxiliary routine (versioNovember 2008 DLAEXC(1)
