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 elemnts equal and its off-diagonal elements of


 opposite sign.

WANTQ

          WANTQ is LOGICAL


          = .TRUE. : accumulate the transformation in the matrix Q;


          = .FALSE.: do not accumulate the transformation.

N

          N is INTEGER


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

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.

LDT

          LDT is INTEGER


          The leading dimension of the array T. LDT >= max(1,N).

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.

LDQ

          LDQ is INTEGER


          The leading dimension of the array Q.


          LDQ >= 1; and if WANTQ is .TRUE., LDQ >= N.

J1

          J1 is INTEGER


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

N1

          N1 is INTEGER


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

N2

          N2 is INTEGER


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

WORK

          WORK is DOUBLE PRECISION array, dimension (N)

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.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

December 2016