NAG FL Interface
f08xbf (dggesx)

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1 Purpose

f08xbf computes the generalized eigenvalues, the generalized real Schur form (S,T) and, optionally, the left and/or right generalized Schur vectors for a pair of n×n real nonsymmetric matrices (A,B) .
Estimates of condition numbers for selected generalized eigenvalue clusters and Schur vectors are also computed.

2 Specification

Fortran Interface
Integer, Intent (In) :: n, lda, ldb, ldvsl, ldvsr, lwork, liwork
Integer, Intent (Out) :: sdim, iwork(max(1,liwork)), info
Real (Kind=nag_wp), Intent (Inout) :: a(lda,*), b(ldb,*), vsl(ldvsl,*), vsr(ldvsr,*)
Real (Kind=nag_wp), Intent (Out) :: alphar(n), alphai(n), beta(n), rconde(2), rcondv(2), work(max(1,lwork))
Logical, External :: selctg
Logical, Intent (Inout) :: bwork(*)
Character (1), Intent (In) :: jobvsl, jobvsr, sort, sense
C Header Interface
#include <nag.h>
void  f08xbf_ (const char *jobvsl, const char *jobvsr, const char *sort,
logical (NAG_CALL *selctg)(const double *ar, const double *ai, const double *b),
const char *sense, const Integer *n, double a[], const Integer *lda, double b[], const Integer *ldb, Integer *sdim, double alphar[], double alphai[], double beta[], double vsl[], const Integer *ldvsl, double vsr[], const Integer *ldvsr, double rconde[], double rcondv[], double work[], const Integer *lwork, Integer iwork[], const Integer *liwork, logical bwork[], Integer *info, const Charlen length_jobvsl, const Charlen length_jobvsr, const Charlen length_sort, const Charlen length_sense)
The routine may be called by the names f08xbf, nagf_lapackeig_dggesx or its LAPACK name dggesx.

3 Description

The generalized real Schur factorization of (A,B) is given by
A = QSZT ,   B = QTZT ,  
where Q and Z are orthogonal, T is upper triangular and S is upper quasi-triangular with 1×1 and 2×2 diagonal blocks. The generalized eigenvalues, λ , of (A,B) are computed from the diagonals of T and S and satisfy
Az = λBz ,  
where z is the corresponding generalized eigenvector. λ is actually returned as the pair (α,β) such that
λ = α/β  
since β , or even both α and β can be zero. The columns of Q and Z are the left and right generalized Schur vectors of (A,B) .
Optionally, f08xbf can order the generalized eigenvalues on the diagonals of (S,T) so that selected eigenvalues are at the top left. The leading columns of Q and Z then form an orthonormal basis for the corresponding eigenspaces, the deflating subspaces.
f08xbf computes T to have non-negative diagonal elements, and the 2×2 blocks of S correspond to complex conjugate pairs of generalized eigenvalues. The generalized Schur factorization, before reordering, is computed by the QZ algorithm.
The reciprocals of the condition estimates, the reciprocal values of the left and right projection norms, are returned in rconde(1) and rconde(2) respectively, for the selected generalized eigenvalues, together with reciprocal condition estimates for the corresponding left and right deflating subspaces, in rcondv(1) and rcondv(2) . See Section 4.11 of Anderson et al. (1999) for further information.

4 References

Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999) LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia https://www.netlib.org/lapack/lug
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

5 Arguments

1: jobvsl Character(1) Input
On entry: if jobvsl='N', do not compute the left Schur vectors.
If jobvsl='V', compute the left Schur vectors.
Constraint: jobvsl='N' or 'V'.
2: jobvsr Character(1) Input
On entry: if jobvsr='N', do not compute the right Schur vectors.
If jobvsr='V', compute the right Schur vectors.
Constraint: jobvsr='N' or 'V'.
3: sort Character(1) Input
On entry: specifies whether or not to order the eigenvalues on the diagonal of the generalized Schur form.
sort='N'
Eigenvalues are not ordered.
sort='S'
Eigenvalues are ordered (see selctg).
Constraint: sort='N' or 'S'.
4: selctg Logical Function, supplied by the user. External Procedure
If sort='S', selctg is used to select generalized eigenvalues to be moved to the top left of the generalized Schur form.
If sort='N', selctg is not referenced by f08xbf, and may be called with the dummy function f08xaz.
The specification of selctg is:
Fortran Interface
Function selctg ( ar, ai, b)
Logical :: selctg
Real (Kind=nag_wp), Intent (In) :: ar, ai, b
C Header Interface
Nag_Boolean  selctg (const double *ar, const double *ai, const double *b)
1: ar Real (Kind=nag_wp) Input
2: ai Real (Kind=nag_wp) Input
3: b Real (Kind=nag_wp) Input
On entry: an eigenvalue (ar(j)+-1×ai(j)) / b(j) is selected if selctg(ar(j),ai(j),b(j)) is .TRUE.. If either one of a complex conjugate pair is selected, then both complex generalized eigenvalues are selected.
Note that in the ill-conditioned case, a selected complex generalized eigenvalue may no longer satisfy selctg(ar(j),ai(j),b(j))=.TRUE. after ordering. info=n+2 in this case.
selctg must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which f08xbf is called. Arguments denoted as Input must not be changed by this procedure.
5: sense Character(1) Input
On entry: determines which reciprocal condition numbers are computed.
sense='N'
None are computed.
sense='E'
Computed for average of selected eigenvalues only.
sense='V'
Computed for selected deflating subspaces only.
sense='B'
Computed for both.
If sense='E', 'V' or 'B', sort='S'.
Constraint: sense='N', 'E', 'V' or 'B'.
6: n Integer Input
On entry: n, the order of the matrices A and B.
Constraint: n0.
7: a(lda,*) Real (Kind=nag_wp) array Input/Output
Note: the second dimension of the array a must be at least max(1,n).
On entry: the first of the pair of matrices, A.
On exit: a has been overwritten by its generalized Schur form S.
8: lda Integer Input
On entry: the first dimension of the array a as declared in the (sub)program from which f08xbf is called.
Constraint: ldamax(1,n).
9: b(ldb,*) Real (Kind=nag_wp) array Input/Output
Note: the second dimension of the array b must be at least max(1,n).
On entry: the second of the pair of matrices, B.
On exit: b has been overwritten by its generalized Schur form T.
10: ldb Integer Input
On entry: the first dimension of the array b as declared in the (sub)program from which f08xbf is called.
Constraint: ldbmax(1,n).
11: sdim Integer Output
On exit: if sort='N', sdim=0.
If sort='S', sdim= number of eigenvalues (after sorting) for which selctg is .TRUE.. (Complex conjugate pairs for which selctg is .TRUE. for either eigenvalue count as 2.)
12: alphar(n) Real (Kind=nag_wp) array Output
On exit: see the description of beta.
13: alphai(n) Real (Kind=nag_wp) array Output
On exit: see the description of beta.
14: beta(n) Real (Kind=nag_wp) array Output
On exit: (alphar(j)+alphai(j)×i)/beta(j), for j=1,2,,n, will be the generalized eigenvalues. alphar(j)+alphai(j)×i, and beta(j), for j=1,2,,n, are the diagonals of the complex Schur form (S,T) that would result if the 2×2 diagonal blocks of the real Schur form of (A,B) were further reduced to triangular form using 2×2 complex unitary transformations.
If alphai(j) is zero, then the jth eigenvalue is real; if positive, then the jth and (j+1)st eigenvalues are a complex conjugate pair, with alphai(j+1) negative.
Note:  the quotients alphar(j)/beta(j) and alphai(j)/beta(j) may easily overflow or underflow, and beta(j) may even be zero. Thus, you should avoid naively computing the ratio α/β. However, alphar and alphai will always be less than and usually comparable with A2 in magnitude, and beta will always be less than and usually comparable with B2.
15: vsl(ldvsl,*) Real (Kind=nag_wp) array Output
Note: the second dimension of the array vsl must be at least max(1,n) if jobvsl='V'.
On exit: if jobvsl='V', vsl will contain the left Schur vectors, Q.
If jobvsl='N', vsl is not referenced.
16: ldvsl Integer Input
On entry: the first dimension of the array vsl as declared in the (sub)program from which f08xbf is called.
Constraints:
  • if jobvsl='V', ldvsl max(1,n) ;
  • otherwise ldvsl1.
17: vsr(ldvsr,*) Real (Kind=nag_wp) array Output
Note: the second dimension of the array vsr must be at least max(1,n) if jobvsr='V'.
On exit: if jobvsr='V', vsr will contain the right Schur vectors, Z.
If jobvsr='N', vsr is not referenced.
18: ldvsr Integer Input
On entry: the first dimension of the array vsr as declared in the (sub)program from which f08xbf is called.
Constraints:
  • if jobvsr='V', ldvsr max(1,n) ;
  • otherwise ldvsr1.
19: rconde(2) Real (Kind=nag_wp) array Output
On exit: if sense='E' or 'B', rconde(1) and rconde(2) contain the reciprocal condition numbers for the average of the selected eigenvalues.
If sense='N' or 'V', rconde is not referenced.
20: rcondv(2) Real (Kind=nag_wp) array Output
On exit: if sense='V' or 'B', rcondv(1) and rcondv(2) contain the reciprocal condition numbers for the selected deflating subspaces.
if sense='N' or 'E', rcondv is not referenced.
21: work(max(1,lwork)) Real (Kind=nag_wp) array Workspace
On exit: if info=0, work(1) returns the optimal lwork.
22: lwork Integer Input
On entry: the dimension of the array work as declared in the (sub)program from which f08xbf is called.
If lwork=-1, a workspace query is assumed; the routine only calculates the bound on the optimal size of the work array and the minimum size of the iwork array, returns these values as the first entries of the work and iwork arrays, and no error message related to lwork or liwork is issued.
Constraints:
  • if sense='E', 'V' or 'B', lwork max(8×(n+1)+16,2×n+2×sdim×(n-sdim)) ;
  • otherwise lwork max(1,8×n,6×n+16).
Note: that 2×sdim×(n-sdim)n×n/2. Note also that an error is only returned if lwork<8×(n+1)+16, but if sense='E', 'V' or 'B' this may not be large enough. Consider increasing lwork by nb, where nb is the optimal block size.
23: iwork(max(1,liwork)) Integer array Workspace
On exit: if info=0, iwork(1) returns the minimum liwork.
24: liwork Integer Input
On entry: the dimension of the array iwork as declared in the (sub)program from which f08xbf is called.
If liwork=-1, a workspace query is assumed; the routine only calculates the bound on the optimal size of the work array and the minimum size of the iwork array, returns these values as the first entries of the work and iwork arrays, and no error message related to lwork or liwork is issued.
Constraints:
  • if sense='N' or n=0, liwork1;
  • otherwise liworkn+6.
25: bwork(*) Logical array Workspace
Note: the dimension of the array bwork must be at least 1 if sort='N', and at least max(1,n) otherwise.
If sort='N', bwork is not referenced.
26: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

info<0
If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.
info=1,,n
The QZ iteration failed. No eigenvectors have been calculated but alphar(j), alphai(j) and beta(j) should be correct from element value.
info=n+1
The QZ iteration failed with an unexpected error, please contact NAG.
info=n+2
After reordering, roundoff changed values of some complex eigenvalues so that leading eigenvalues in the generalized Schur form no longer satisfy selctg=.TRUE.. This could also be caused by underflow due to scaling.
info=n+3
The eigenvalues could not be reordered because some eigenvalues were too close to separate (the problem is very ill-conditioned).

7 Accuracy

The computed generalized Schur factorization satisfies
A+E = QS ZT ,   B+F = QT ZT ,  
where
(E,F) F = O(ε) (A,B) F  
and ε is the machine precision. See Section 4.11 of Anderson et al. (1999) for further details.

8 Parallelism and Performance

f08xbf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08xbf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The total number of floating-point operations is proportional to n3.
The complex analogue of this routine is f08xpf.

10 Example

This example finds the generalized Schur factorization of the matrix pair (A,B), where
A = ( 3.9 12.5 -34.5 -0.5 4.3 21.5 -47.5 7.5 4.3 21.5 -43.5 3.5 4.4 26.0 -46.0 6.0 )   and   B= ( 1.0 2.0 -3.0 1.0 1.0 3.0 -5.0 4.0 1.0 3.0 -4.0 3.0 1.0 3.0 -4.0 4.0 ) ,  
such that the real positive eigenvalues of (A,B) correspond to the top left diagonal elements of the generalized Schur form, (S,T). Estimates of the condition numbers for the selected eigenvalue cluster and corresponding deflating subspaces are also returned.
Note that the block size (NB) of 64 assumed in this example is not realistic for such a small problem, but should be suitable for large problems.

10.1 Program Text

Program Text (f08xbfe.f90)

10.2 Program Data

Program Data (f08xbfe.d)

10.3 Program Results

Program Results (f08xbfe.r)