NAG FL Interface
f08xqf (zgges3)

1 Purpose

f08xqf computes the generalized eigenvalues, the generalized Schur form S,T and, optionally, the left and/or right generalized Schur vectors for a pair of n by n complex nonsymmetric matrices A,B .

2 Specification

Fortran Interface
Subroutine f08xqf ( jobvsl, jobvsr, sort, selctg, n, a, lda, b, ldb, sdim, alpha, beta, vsl, ldvsl, vsr, ldvsr, work, lwork, rwork, bwork, info)
Integer, Intent (In) :: n, lda, ldb, ldvsl, ldvsr, lwork
Integer, Intent (Out) :: sdim, info
Real (Kind=nag_wp), Intent (Out) :: rwork(max(1,8*n))
Complex (Kind=nag_wp), Intent (Inout) :: a(lda,*), b(ldb,*), vsl(ldvsl,*), vsr(ldvsr,*)
Complex (Kind=nag_wp), Intent (Out) :: alpha(n), beta(n), work(max(1,lwork))
Logical, External :: selctg
Logical, Intent (Inout) :: bwork(*)
Character (1), Intent (In) :: jobvsl, jobvsr, sort
C Header Interface
#include <nag.h>
void  f08xqf_ (const char *jobvsl, const char *jobvsr, const char *sort,
logical (NAG_CALL *selctg)(const Complex *a, const Complex *b),
const Integer *n, Complex a[], const Integer *lda, Complex b[], const Integer *ldb, Integer *sdim, Complex alpha[], Complex beta[], Complex vsl[], const Integer *ldvsl, Complex vsr[], const Integer *ldvsr, Complex work[], const Integer *lwork, double rwork[], logical bwork[], Integer *info, const Charlen length_jobvsl, const Charlen length_jobvsr, const Charlen length_sort)
The routine may be called by the names f08xqf, nagf_lapackeig_zgges3 or its LAPACK name zgges3.

3 Description

The generalized Schur factorization for a pair of complex matrices A,B is given by
A = QSZH ,   B = QTZH ,  
where Q and Z are unitary, T and S are upper triangular. 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, f08xqf 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.
f08xqf computes T to have real non-negative diagonal entries. The generalized Schur factorization, before reordering, is computed by the QZ algorithm.

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 (2012) Matrix Computations (4th 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 f08xqf, and may be called with the dummy function f08xnz.
The specification of selctg is:
Fortran Interface
Function selctg ( a, b)
Logical :: selctg
Complex (Kind=nag_wp), Intent (In) :: a, b
C Header Interface
Nag_Boolean  selctg_ (const Complex *a, const Complex *b)
1: a Complex (Kind=nag_wp) Input
2: b Complex (Kind=nag_wp) Input
On entry: an eigenvalue aj / bj is selected if selctg aj,bj is .TRUE..
Note that in the ill-conditioned case, a selected generalized eigenvalue may no longer satisfy selctgaj,bj=.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 f08xqf is called. Arguments denoted as Input must not be changed by this procedure.
5: n Integer Input
On entry: n, the order of the matrices A and B.
Constraint: n0.
6: alda* Complex (Kind=nag_wp) array Input/Output
Note: the second dimension of the array a must be at least max1,n.
On entry: the first of the pair of matrices, A.
On exit: a has been overwritten by its generalized Schur form S.
7: lda Integer Input
On entry: the first dimension of the array a as declared in the (sub)program from which f08xqf is called.
Constraint: ldamax1,n.
8: bldb* Complex (Kind=nag_wp) array Input/Output
Note: the second dimension of the array b must be at least max1,n.
On entry: the second of the pair of matrices, B.
On exit: b has been overwritten by its generalized Schur form T.
9: ldb Integer Input
On entry: the first dimension of the array b as declared in the (sub)program from which f08xqf is called.
Constraint: ldbmax1,n.
10: sdim Integer Output
On exit: if sort='N', sdim=0.
If sort='S', sdim= number of eigenvalues (after sorting) for which selctg is .TRUE..
11: alphan Complex (Kind=nag_wp) array Output
On exit: see the description of beta.
12: betan Complex (Kind=nag_wp) array Output
On exit: alphaj/betaj, for j=1,2,,n, will be the generalized eigenvalues. alphaj, for j=1,2,,n and betaj, for j=1,2,,n, are the diagonals of the complex Schur form A,B output by f08xqf. The betaj will be non-negative real.
Note:  the quotients alphaj/betaj may easily overflow or underflow, and betaj may even be zero. Thus, you should avoid naively computing the ratio α/β. However, alpha will always be less than and usually comparable with A2 in magnitude, and beta will always be less than and usually comparable with B2.
13: vslldvsl* Complex (Kind=nag_wp) array Output
Note: the second dimension of the array vsl must be at least max1,n if jobvsl='V', and at least 1 otherwise.
On exit: if jobvsl='V', vsl will contain the left Schur vectors, Q.
If jobvsl='N', vsl is not referenced.
14: ldvsl Integer Input
On entry: the first dimension of the array vsl as declared in the (sub)program from which f08xqf is called.
Constraints:
  • if jobvsl='V', ldvsl max1,n ;
  • otherwise ldvsl1.
15: vsrldvsr* Complex (Kind=nag_wp) array Output
Note: the second dimension of the array vsr must be at least max1,n if jobvsr='V', and at least 1 otherwise.
On exit: if jobvsr='V', vsr will contain the right Schur vectors, Z.
If jobvsr='N', vsr is not referenced.
16: ldvsr Integer Input
On entry: the first dimension of the array vsr as declared in the (sub)program from which f08xqf is called.
Constraints:
  • if jobvsr='V', ldvsr max1,n ;
  • otherwise ldvsr1.
17: workmax1,lwork Complex (Kind=nag_wp) array Workspace
On exit: if info=0, the real part of work1 contains the minimum value of lwork required for optimal performance.
18: lwork Integer Input
On entry: the dimension of the array work as declared in the (sub)program from which f08xqf is called.
If lwork=-1, 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.
Suggested value: for optimal performance, lwork must generally be larger than the minimum, say 2×n+nb×n×6, where nb is the optimal block size for f08wtf.
Constraint: lworkmax1,2×n.
19: rworkmax1,8×n Real (Kind=nag_wp) array Workspace
20: bwork* Logical array Workspace
Note: the dimension of the array bwork must be at least 1 if sort='N', and at least max1,n otherwise.
If sort='N', bwork is not referenced.
21: 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 did not converge and the matrix pair A,B is not in the generalized Schur form. The computed αi and βi should be correct for i=value,,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 ZH ,   B+F = QT ZH ,  
where
E,F F = Oε 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

f08xqf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08xqf 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 real analogue of this routine is f08xcf.

10 Example

This example finds the generalized Schur factorization of the matrix pair A,B, where
A = -21.10-22.50i 53.50-50.50i -34.50+127.50i 7.50+00.50i -0.46-07.78i -3.50-37.50i -15.50+058.50i -10.50-01.50i 4.30-05.50i 39.70-17.10i -68.50+012.50i -7.50-03.50i 5.50+04.40i 14.40+43.30i -32.50-046.00i -19.00-32.50i  
and
B = 1.00-5.00i 1.60+1.20i -3.00+0.00i 0.00-1.00i 0.80-0.60i 3.00-5.00i -4.00+3.00i -2.40-3.20i 1.00+0.00i 2.40+1.80i -4.00-5.00i 0.00-3.00i 0.00+1.00i -1.80+2.40i 0.00-4.00i 4.00-5.00i .  

10.1 Program Text

Program Text (f08xqfe.f90)

10.2 Program Data

Program Data (f08xqfe.d)

10.3 Program Results

Program Results (f08xqfe.r)