NAG FL Interface
f08psf (zhseqr)

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1 Purpose

f08psf computes all the eigenvalues and, optionally, the Schur factorization of a complex Hessenberg matrix or a complex general matrix which has been reduced to Hessenberg form.

2 Specification

Fortran Interface
Subroutine f08psf ( job, compz, n, ilo, ihi, h, ldh, w, z, ldz, work, lwork, info)
Integer, Intent (In) :: n, ilo, ihi, ldh, ldz, lwork
Integer, Intent (Out) :: info
Complex (Kind=nag_wp), Intent (Inout) :: h(ldh,*), w(*), z(ldz,*)
Complex (Kind=nag_wp), Intent (Out) :: work(max(1,lwork))
Character (1), Intent (In) :: job, compz
C Header Interface
#include <nag.h>
void  f08psf_ (const char *job, const char *compz, const Integer *n, const Integer *ilo, const Integer *ihi, Complex h[], const Integer *ldh, Complex w[], Complex z[], const Integer *ldz, Complex work[], const Integer *lwork, Integer *info, const Charlen length_job, const Charlen length_compz)
The routine may be called by the names f08psf, nagf_lapackeig_zhseqr or its LAPACK name zhseqr.

3 Description

f08psf computes all the eigenvalues and, optionally, the Schur factorization of a complex upper Hessenberg matrix H:
H = ZTZH ,  
where T is an upper triangular matrix (the Schur form of H), and Z is the unitary matrix whose columns are the Schur vectors zi. The diagonal elements of T are the eigenvalues of H.
The routine may also be used to compute the Schur factorization of a complex general matrix A which has been reduced to upper Hessenberg form H:
A = QHQH, where ​Q​ is unitary, = (QZ)T(QZ)H.  
In this case, after f08nsf has been called to reduce A to Hessenberg form, f08ntf must be called to form Q explicitly; Q is then passed to f08psf, which must be called with compz='V'.
The routine can also take advantage of a previous call to f08nvf which may have balanced the original matrix before reducing it to Hessenberg form, so that the Hessenberg matrix H has the structure:
( H11 H12 H13 H22 H23 H33 )  
where H11 and H33 are upper triangular. If so, only the central diagonal block H22 (in rows and columns ilo to ihi) needs to be further reduced to Schur form (the blocks H12 and H23 are also affected). Therefore, the values of ilo and ihi can be supplied to f08psf directly. Also, f08nwf must be called after this routine to permute the Schur vectors of the balanced matrix to those of the original matrix. If f08nvf has not been called however, then ilo must be set to 1 and ihi to n. Note that if the Schur factorization of A is required, f08nvf must not be called with job='S' or 'B', because the balancing transformation is not unitary.
f08psf uses a multishift form of the upper Hessenberg QR algorithm, due to Bai and Demmel (1989). The Schur vectors are normalized so that zi2=1, but are determined only to within a complex factor of absolute value 1.

4 References

Bai Z and Demmel J W (1989) On a block implementation of Hessenberg multishift QR iteration Internat. J. High Speed Comput. 1 97–112
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

5 Arguments

1: job Character(1) Input
On entry: indicates whether eigenvalues only or the Schur form T is required.
job='E'
Eigenvalues only are required.
job='S'
The Schur form T is required.
Constraint: job='E' or 'S'.
2: compz Character(1) Input
On entry: indicates whether the Schur vectors are to be computed.
compz='N'
No Schur vectors are computed (and the array z is not referenced).
compz='V'
The Schur vectors of A are computed (and the array z must contain the matrix Q on entry).
compz='I'
The Schur vectors of H are computed (and the array z is initialized by the routine).
Constraint: compz='N', 'V' or 'I'.
3: n Integer Input
On entry: n, the order of the matrix H.
Constraint: n0.
4: ilo Integer Input
5: ihi Integer Input
On entry: if the matrix A has been balanced by f08nvf, ilo and ihi must contain the values returned by that routine. Otherwise, ilo must be set to 1 and ihi to n.
Constraint: ilo1 and min(ilo,n) ihin .
6: h(ldh,*) Complex (Kind=nag_wp) array Input/Output
Note: the second dimension of the array h must be at least max(1,n).
On entry: the n×n upper Hessenberg matrix H, as returned by f08nsf.
On exit: if job='E', the array contains no useful information.
If job='S', h is overwritten by the upper triangular matrix T from the Schur decomposition (the Schur form) unless info>0.
7: ldh Integer Input
On entry: the first dimension of the array h as declared in the (sub)program from which f08psf is called.
Constraint: ldhmax(1,n).
8: w(*) Complex (Kind=nag_wp) array Output
Note: the dimension of the array w must be at least max(1,n).
On exit: the computed eigenvalues, unless info>0 (in which case see Section 6). The eigenvalues are stored in the same order as on the diagonal of the Schur form T (if computed).
9: z(ldz,*) Complex (Kind=nag_wp) array Input/Output
Note: the second dimension of the array z must be at least max(1,n) if compz='V' or 'I' and at least 1 if compz='N'.
On entry: if compz='V', z must contain the unitary matrix Q from the reduction to Hessenberg form.
If compz='I', z need not be set.
On exit: if compz='V' or 'I', z contains the unitary matrix of the required Schur vectors, unless info>0.
If compz='N', z is not referenced.
10: ldz Integer Input
On entry: the first dimension of the array z as declared in the (sub)program from which f08psf is called.
Constraints:
  • if compz='V' or 'I', ldz max(1,n) ;
  • if compz='N', ldz1.
11: work(max(1,lwork)) Complex (Kind=nag_wp) array Workspace
On exit: if info=0, the real part of work(1) contains the minimum value of lwork required for optimal performance.
12: lwork Integer Input
On entry: the dimension of the array work as declared in the (sub)program from which f08psf is called, unless lwork=−1, in which case a workspace query is assumed and the routine only calculates the minimum dimension of work.
Constraint: lworkmax(1,n) or lwork=−1.
13: info Integer Output
On exit: info=0 unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

-999<info<0
If info=-i, argument i had an illegal value. An explanatory message is output, and execution of the program is terminated.
info=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.
An explanatory message is output, and execution of the program is terminated.
info>0
The algorithm has failed to find all the eigenvalues after a total of 30(ihi-ilo+1) iterations.

7 Accuracy

The computed Schur factorization is the exact factorization of a nearby matrix (H+E), where
E2 = O(ε) H2 ,  
and ε is the machine precision.
If λi is an exact eigenvalue, and λ~i is the corresponding computed value, then
|λ~i-λi| c (n) ε H2 si ,  
where c(n) is a modestly increasing function of n, and si is the reciprocal condition number of λi. The condition numbers si may be computed by calling f08qyf.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
f08psf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f08psf 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 real floating-point operations depends on how rapidly the algorithm converges, but is typically about:
The real analogue of this routine is f08pef.

10 Example

This example computes all the eigenvalues and the Schur factorization of the upper Hessenberg matrix H, where
H = ( -3.9700-5.0400i -1.1318-2.5693i -4.6027-0.1426i -1.4249+1.7330i -5.4797+0.0000i 1.8585-1.5502i 4.4145-0.7638i -0.4805-1.1976i 0.0000+0.0000i 6.2673+0.0000i -0.4504-0.0290i -1.3467+1.6579i 0.0000+0.0000i 0.0000+0.0000i -3.5000+0.0000i 2.5619-3.3708i ) .  
See also f08ntf, which illustrates the use of this routine to compute the Schur factorization of a general matrix.

10.1 Program Text

Program Text (f08psfe.f90)

10.2 Program Data

Program Data (f08psfe.d)

10.3 Program Results

Program Results (f08psfe.r)