sgees (l)  Linux Manuals
sgees: computes for an NbyN real nonsymmetric matrix A, the eigenvalues, the real Schur form T, and, optionally, the matrix of Schur vectors Z
NAME
SGEES  computes for an NbyN real nonsymmetric matrix A, the eigenvalues, the real Schur form T, and, optionally, the matrix of Schur vectors ZSYNOPSIS
 SUBROUTINE SGEES(
 JOBVS, SORT, SELECT, N, A, LDA, SDIM, WR, WI, VS, LDVS, WORK, LWORK, BWORK, INFO )
 CHARACTER JOBVS, SORT
 INTEGER INFO, LDA, LDVS, LWORK, N, SDIM
 LOGICAL BWORK( * )
 REAL A( LDA, * ), VS( LDVS, * ), WI( * ), WORK( * ), WR( * )
 LOGICAL SELECT
 EXTERNAL SELECT
PURPOSE
SGEES computes for an NbyN real nonsymmetric matrix A, the eigenvalues, the real Schur form T, and, optionally, the matrix of Schur vectors Z. This gives the Schur factorization A = Z*T*(Z**T). Optionally, it also orders the eigenvalues on the diagonal of the real Schur form so that selected eigenvalues are at the top left. The leading columns of Z then form an orthonormal basis for the invariant subspace corresponding to the selected eigenvalues. A matrix is in real Schur form if it is upper quasitriangular with 1by1 and 2by2 blocks. 2by2 blocks will be standardized in the formwhere b*c < 0. The eigenvalues of such a block are a + sqrt(bc).
ARGUMENTS
 JOBVS (input) CHARACTER*1

= aqNaq: Schur vectors are not computed;
= aqVaq: Schur vectors are computed.  SORT (input) CHARACTER*1

Specifies whether or not to order the eigenvalues on the
diagonal of the Schur form.
= aqNaq: Eigenvalues are not ordered;
= aqSaq: Eigenvalues are ordered (see SELECT).  SELECT (external procedure) LOGICAL FUNCTION of two REAL arguments
 SELECT must be declared EXTERNAL in the calling subroutine. If SORT = aqSaq, SELECT is used to select eigenvalues to sort to the top left of the Schur form. If SORT = aqNaq, SELECT is not referenced. An eigenvalue WR(j)+sqrt(1)*WI(j) is selected if SELECT(WR(j),WI(j)) is true; i.e., if either one of a complex conjugate pair of eigenvalues is selected, then both complex eigenvalues are selected. Note that a selected complex eigenvalue may no longer satisfy SELECT(WR(j),WI(j)) = .TRUE. after ordering, since ordering may change the value of complex eigenvalues (especially if the eigenvalue is illconditioned); in this case INFO is set to N+2 (see INFO below).
 N (input) INTEGER
 The order of the matrix A. N >= 0.
 A (input/output) REAL array, dimension (LDA,N)
 On entry, the NbyN matrix A. On exit, A has been overwritten by its real Schur form T.
 LDA (input) INTEGER
 The leading dimension of the array A. LDA >= max(1,N).
 SDIM (output) INTEGER
 If SORT = aqNaq, SDIM = 0. If SORT = aqSaq, SDIM = number of eigenvalues (after sorting) for which SELECT is true. (Complex conjugate pairs for which SELECT is true for either eigenvalue count as 2.)
 WR (output) REAL array, dimension (N)
 WI (output) REAL array, dimension (N) WR and WI contain the real and imaginary parts, respectively, of the computed eigenvalues in the same order that they appear on the diagonal of the output Schur form T. Complex conjugate pairs of eigenvalues will appear consecutively with the eigenvalue having the positive imaginary part first.
 VS (output) REAL array, dimension (LDVS,N)
 If JOBVS = aqVaq, VS contains the orthogonal matrix Z of Schur vectors. If JOBVS = aqNaq, VS is not referenced.
 LDVS (input) INTEGER
 The leading dimension of the array VS. LDVS >= 1; if JOBVS = aqVaq, LDVS >= N.
 WORK (workspace/output) REAL array, dimension (MAX(1,LWORK))
 On exit, if INFO = 0, WORK(1) contains the optimal LWORK.
 LWORK (input) INTEGER
 The dimension of the array WORK. LWORK >= max(1,3*N). For good performance, LWORK must generally be larger. If LWORK = 1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.
 BWORK (workspace) LOGICAL array, dimension (N)
 Not referenced if SORT = aqNaq.
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value.
> 0: if INFO = i, and i is
<= N: the QR algorithm failed to compute all the
eigenvalues; elements 1:ILO1 and i+1:N of WR and WI contain those eigenvalues which have converged; if JOBVS = aqVaq, VS contains the matrix which reduces A to its partially converged Schur form. = N+1: the eigenvalues could not be reordered because some eigenvalues were too close to separate (the problem is very illconditioned); = N+2: after reordering, roundoff changed values of some complex eigenvalues so that leading eigenvalues in the Schur form no longer satisfy SELECT=.TRUE. This could also be caused by underflow due to scaling.