dlaexc()

subroutine dlaexc	(	logical	wantq,
		integer	n,
		double precision, dimension( ldt, * )	t,
		integer	ldt,
		double precision, dimension( ldq, * )	q,
		integer	ldq,
		integer	j1,
		integer	n1,
		integer	n2,
		double precision, dimension( * )	work,
		integer	info )

DLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form, by an orthogonal similarity transformation.

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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
!> 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 elements equal and its off-diagonal elements of
!> opposite sign.
!> 

Parameters

[in]	WANTQ	!> WANTQ is LOGICAL !> = .TRUE. : accumulate the transformation in the matrix Q; !> = .FALSE.: do not accumulate the transformation. !>
[in]	N	!> N is INTEGER !> The order of the matrix T. N >= 0. !>
[in,out]	T	!> T is DOUBLE PRECISION 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. !>
[in]	LDT	!> LDT is INTEGER !> The leading dimension of the array T. LDT >= max(1,N). !>
[in,out]	Q	!> Q is 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. !>
[in]	LDQ	!> LDQ is INTEGER !> The leading dimension of the array Q. !> LDQ >= 1; and if WANTQ is .TRUE., LDQ >= N. !>
[in]	J1	!> J1 is INTEGER !> The index of the first row of the first block T11. !>
[in]	N1	!> N1 is INTEGER !> The order of the first block T11. N1 = 0, 1 or 2. !>
[in]	N2	!> N2 is INTEGER !> The order of the second block T22. N2 = 0, 1 or 2. !>
[out]	WORK	!> WORK is DOUBLE PRECISION array, dimension (N) !>
[out]	INFO	!> INFO is 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. !>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.