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NAME SLAEXC - swap adjacent diagonal blocks T11 and T22 of order 1 or 2 in an upper quasi-triangular matrix T by an orthogo- nal 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 tri- angular 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. 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 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) 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 unchanged.