SLAEXC(3)      LAPACK routine of NEC Numeric Library Collection      SLAEXC(3)



NAME
       SLAEXC

SYNOPSIS
       SUBROUTINE SLAEXC (WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO)



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)
                     WANTQ is LOGICAL
                     = .TRUE. : accumulate the transformation in the matrix Q;
                     = .FALSE.: do not accumulate the transformation.

           N         (input)
                     N is INTEGER
                     The order of the matrix T. N >= 0.

           T         (input/output)
                     T is 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)
                     LDT is INTEGER
                     The leading dimension of the array T. LDT >= max(1,N).

           Q         (input/output)
                     Q is REAL 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)
                     LDQ is INTEGER
                     The leading dimension of the array Q.
                     LDQ >= 1; and if WANTQ is .TRUE., LDQ >= N.

           J1        (input)
                     J1 is INTEGER
                     The index of the first row of the first block T11.

           N1        (input)
                     N1 is INTEGER
                     The order of the first block T11. N1 = 0, 1 or 2.

           N2        (input)
                     N2 is INTEGER
                     The order of the second block T22. N2 = 0, 1 or 2.

           WORK      (output)
                     WORK is REAL array, dimension (N)

           INFO      (output)
                     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.



LAPACK routine                  31 October 2017                      SLAEXC(3)