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NAME ZTGEVC - compute selected left and/or right generalized eigenvectors of a pair of complex upper triangular matrices (A,B) SYNOPSIS SUBROUTINE ZTGEVC( JOB, SIDE, SELECT, N, A, LDA, B, LDB, VL, LDVL, VR, LDVR, MM, M, WORK, RWORK, INFO ) CHARACTER JOB, SIDE INTEGER INFO, LDA, LDB, LDVL, LDVR, M, MM, N LOGICAL SELECT( * ) DOUBLE PRECISION RWORK( N, * ) COMPLEX*16 A( LDA, * ), B( LDB, * ), VL( LDVL, * ), VR( LDVR, * ), WORK( N, * ) PURPOSE ZTGEVC computes selected left and/or right generalized eigenvectors of a pair of complex upper triangular matrices (A,B). The j-th generalized left and right eigenvectors are y and x, resp., such that: H y (A - wB) = 0 (A - wB)x = 0 H Note: the left eigenvector is sometimes defined as the row vector y but ZTGEVC computes the column vector y. ARGUMENTS JOB (input) CHARACTER*1 = 'A': compute All (left/right/left+right) general- ized eigenvectors of (A,B); = 'S': compute Selected (left/right/left+right) generalized eigenvectors of (A,B) -- see the description of the argument SELECT; = 'B' or 'T': compute all (left/right/left+right) generalized eigenvectors of (A,B), and Back Transform them using the initial contents of VL/VR -- see the descriptions of the arguments VL and VR. SIDE (input) CHARACTER*1 Specifies for which side eigenvectors are to be com- puted: = 'R': compute right eigenvectors only; = 'L': compute left eigenvectors only; = 'B': compute both right and left eigenvectors. SELECT (input) LOGICAL array, dimension (N) If JOB='S', then SELECT specifies the (generalized) eigenvectors to be computed. To get the eigenvector corresponding to the j-th eigenvalue, set SELECT(j) to .TRUE.. If JOB='A', 'B', or 'T', SELECT is not referenced, and all eigenvectors are selected. N (input) INTEGER The order of the matrices A and B. N >= 0. A (input) COMPLEX*16 array, dimension (LDA,N) One of the pair of matrices whose generalized eigen- vectors are to be computed. It must be upper tri- angular. LDA (input) INTEGER The leading dimension of array A. LDA >= max(1, N). B (input) COMPLEX*16 array, dimension (LDB,N) The other of the pair of matrices whose generalized eigenvectors are to be computed. It must be upper triangular with real diagonal elements. LDB (input) INTEGER The leading dimension of array B. LDB >= max(1, N). VL (input/output) COMPLEX*16 array, dimension (LDVL,MM) The left eigenvectors (column vectors -- see the note in "Purpose".) If JOB='A', then all left eigen- vectors of (A,B) will be computed and stored in VL. If JOB='S', then only the eigenvectors selected by SELECT will be computed; the first selected eigen- vector will go in column 1, the second in column 2, etc. If JOB='B' or 'T', then all left eigenvectors of (A,B) will be computed and multiplied (on the left) by the matrix found in VL on entry to ZTGEVC. Usually, this will be the Q matrix computed by ZGGHRD and ZHGEQZ, so that on exit, VL will contain the left eigenvectors of the original matrix pair. In any case, each eigenvector will be scaled so the largest component of each vector has abs(real part) + abs(imag. part)=1, *unless* A and B both have a zero in the diagonal entry corresponding to the eigenvector, in which case the eigenvector will be zero. If SIDE = 'R', VL is not referenced. LDVL (input) INTEGER The leading dimension of array VL. LDVL >= 1; if SIDE = 'B' or 'L', LDVL >= N. VR (input/output) COMPLEX*16 array, dimension (LDVR,MM) The right eigenvectors. If JOB='A', then all right eigenvectors of (A,B) will be computed and stored in VR. If JOB='S', then only the eigenvectors selected by SELECT will be computed; the first selected eigenvector will go in column 1, the second in column 2, etc. If JOB='B' or 'T', then all right eigenvectors of (A,B) will be computed and multi- plied (on the left) by the matrix found in VR on entry to ZTGEVC. Usually, this will be the Z matrix computed by ZGGHRD and ZHGEQZ, so that on exit, VR will contain the right eigenvectors of the original matrix pair. In any case, each eigenvector will be scaled so the largest component of each vector has abs(real part) + abs(imag. part)=1, *unless* A and B both have a zero in the diagonal entry correspond- ing to the eigenvector, in which case the eigenvec- tor will be zero. If SIDE = 'L', VR is not refer- enced. LDVR (input) INTEGER The leading dimension of array VR. LDVR >= 1; if SIDE = 'B' or 'R', LDVR >= N. MM (input) INTEGER The number of columns in VL and/or VR. If JOB='A', 'B', or 'T', then MM >= N. If JOB='S', then MM must be at least the number of .TRUE. values in SELECT(1:N). M (output) INTEGER The number of columns in VL and/or VR actually used to store the eigenvectors. WORK (workspace) COMPLEX*16 array, dimension ( 2, N ) RWORK (workspace) DOUBLE PRECISION array, dimension ( 2, N ) INFO (output) INTEGER = 0: successful exit. < 0: if INFO = -i, the i-th argument had an illegal value.