F08WSF (ZGGHRD) (PDF version)
F08 Chapter Contents
F08 Chapter Introduction
NAG Library Manual

NAG Library Routine Document

F08WSF (ZGGHRD)

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

 Contents

    1  Purpose
    7  Accuracy
    10  Example

1  Purpose

F08WSF (ZGGHRD) reduces a pair of complex matrices A,B, where B is upper triangular, to the generalized upper Hessenberg form using unitary transformations.

2  Specification

SUBROUTINE F08WSF ( COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q, LDQ, Z, LDZ, INFO)
INTEGER  N, ILO, IHI, LDA, LDB, LDQ, LDZ, INFO
COMPLEX (KIND=nag_wp)  A(LDA,*), B(LDB,*), Q(LDQ,*), Z(LDZ,*)
CHARACTER(1)  COMPQ, COMPZ
The routine may be called by its LAPACK name zgghrd.

3  Description

F08WSF (ZGGHRD) is usually the third step in the solution of the complex generalized eigenvalue problem
Ax=λBx.  
The (optional) first step balances the two matrices using F08WVF (ZGGBAL). In the second step, matrix B is reduced to upper triangular form using the QR factorization routine F08ASF (ZGEQRF) and this unitary transformation Q is applied to matrix A by calling F08AUF (ZUNMQR).
F08WSF (ZGGHRD) reduces a pair of complex matrices A,B, where B is triangular, to the generalized upper Hessenberg form using unitary transformations. This two-sided transformation is of the form
QHAZ=H QHBZ=T  
where H is an upper Hessenberg matrix, T is an upper triangular matrix and Q and Z are unitary matrices determined as products of Givens rotations. They may either be formed explicitly, or they may be postmultiplied into input matrices Q1 and Z1, so that
Q1AZ1H=Q1QHZ1ZH, Q1BZ1H=Q1QTZ1ZH.  

4  References

Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore
Moler C B and Stewart G W (1973) An algorithm for generalized matrix eigenproblems SIAM J. Numer. Anal. 10 241–256

5  Arguments

1:     COMPQ – CHARACTER(1)Input
On entry: specifies the form of the computed unitary matrix Q.
COMPQ='N'
Do not compute Q.
COMPQ='I'
The unitary matrix Q is returned.
COMPQ='V'
Q must contain a unitary matrix Q1, and the product Q1Q is returned.
Constraint: COMPQ='N', 'I' or 'V'.
2:     COMPZ – CHARACTER(1)Input
On entry: specifies the form of the computed unitary matrix Z.
COMPZ='N'
Do not compute Z.
COMPZ='V'
Z must contain a unitary matrix Z1, and the product Z1Z is returned.
COMPZ='I'
The unitary matrix Z is returned.
Constraint: COMPZ='N', 'V' or 'I'.
3:     N – INTEGERInput
On entry: n, the order of the matrices A and B.
Constraint: N0.
4:     ILO – INTEGERInput
5:     IHI – INTEGERInput
On entry: ilo and ihi as determined by a previous call to F08WVF (ZGGBAL). Otherwise, they should be set to 1 and n, respectively.
Constraints:
  • if N>0, 1 ILO IHI N ;
  • if N=0, ILO=1 and IHI=0.
6:     ALDA* – COMPLEX (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array A must be at least max1,N.
On entry: the matrix A of the matrix pair A,B. Usually, this is the matrix A returned by F08AUF (ZUNMQR).
On exit: A is overwritten by the upper Hessenberg matrix H.
7:     LDA – INTEGERInput
On entry: the first dimension of the array A as declared in the (sub)program from which F08WSF (ZGGHRD) is called.
Constraint: LDAmax1,N.
8:     BLDB* – COMPLEX (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array B must be at least max1,N.
On entry: the upper triangular matrix B of the matrix pair A,B. Usually, this is the matrix B returned by the QR factorization routine F08ASF (ZGEQRF).
On exit: B is overwritten by the upper triangular matrix T.
9:     LDB – INTEGERInput
On entry: the first dimension of the array B as declared in the (sub)program from which F08WSF (ZGGHRD) is called.
Constraint: LDBmax1,N.
10:   QLDQ* – COMPLEX (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array Q must be at least max1,N if COMPQ='I' or 'V' and at least 1 if COMPQ='N'.
On entry: if COMPQ='V', Q must contain a unitary matrix Q1.
If COMPQ='N', Q is not referenced.
On exit: if COMPQ='I', Q contains the unitary matrix Q.
Iif COMPQ='V', Q is overwritten by Q1Q.
11:   LDQ – INTEGERInput
On entry: the first dimension of the array Q as declared in the (sub)program from which F08WSF (ZGGHRD) is called.
Constraints:
  • if COMPQ='I' or 'V', LDQ max1,N ;
  • if COMPQ='N', LDQ1.
12:   ZLDZ* – COMPLEX (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array Z must be at least max1,N if COMPZ='V' or 'I' and at least 1 if COMPZ='N'.
On entry: if COMPZ='V', Z must contain a unitary matrix Z1.
If COMPZ='N', Z is not referenced.
On exit: if COMPZ='I', Z contains the unitary matrix Z.
If COMPZ='V', Z is overwritten by Z1Z.
13:   LDZ – INTEGERInput
On entry: the first dimension of the array Z as declared in the (sub)program from which F08WSF (ZGGHRD) is called.
Constraints:
  • if COMPZ='V' or 'I', LDZ max1,N ;
  • if COMPZ='N', LDZ1.
14:   INFO – INTEGEROutput
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.

7  Accuracy

The reduction to the generalized Hessenberg form is implemented using unitary transformations which are backward stable.

8  Parallelism and Performance

F08WSF (ZGGHRD) is not threaded in any implementation.

9  Further Comments

This routine is usually followed by F08XSF (ZHGEQZ) which implements the QZ algorithm for computing generalized eigenvalues of a reduced pair of matrices.
The real analogue of this routine is F08WEF (DGGHRD).

10  Example

See Section 10 in F08XSF (ZHGEQZ) and F08YXF (ZTGEVC).

F08WSF (ZGGHRD) (PDF version)
F08 Chapter Contents
F08 Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2016