F08 Chapter Contents
F08 Chapter Introduction
NAG Library Manual

# NAG Library Routine DocumentF08WEF (DGGHRD)

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.

## 1  Purpose

F08WEF (DGGHRD) reduces a pair of real matrices $\left(A,B\right)$, where $B$ is upper triangular, to the generalized upper Hessenberg form using orthogonal transformations.

## 2  Specification

 SUBROUTINE F08WEF ( COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q, LDQ, Z, LDZ, INFO)
 INTEGER N, ILO, IHI, LDA, LDB, LDQ, LDZ, INFO REAL (KIND=nag_wp) A(LDA,*), B(LDB,*), Q(LDQ,*), Z(LDZ,*) CHARACTER(1) COMPQ, COMPZ
The routine may be called by its LAPACK name dgghrd.

## 3  Description

F08WEF (DGGHRD) is the third step in the solution of the real generalized eigenvalue problem
 $Ax=λBx.$
The (optional) first step balances the two matrices using F08WHF (DGGBAL). In the second step, matrix $B$ is reduced to upper triangular form using the $QR$ factorization routine F08AEF (DGEQRF) and this orthogonal transformation $Q$ is applied to matrix $A$ by calling F08AGF (DORMQR).
F08WEF (DGGHRD) reduces a pair of real matrices $\left(A,B\right)$, where $B$ is upper triangular, to the generalized upper Hessenberg form using orthogonal transformations. This two-sided transformation is of the form
 $QTAZ=H QTBZ=T$
where $H$ is an upper Hessenberg matrix, $T$ is an upper triangular matrix and $Q$ and $Z$ are orthogonal matrices determined as products of Givens rotations. They may either be formed explicitly, or they may be postmultiplied into input matrices ${Q}_{1}$ and ${Z}_{1}$, so that
 $Q1AZ1T=Q1QHZ1ZT, Q1BZ1T=Q1QTZ1ZT.$
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  Parameters

1:     $\mathrm{COMPQ}$ – CHARACTER(1)Input
On entry: specifies the form of the computed orthogonal matrix $Q$.
${\mathbf{COMPQ}}=\text{'N'}$
Do not compute $Q$.
${\mathbf{COMPQ}}=\text{'I'}$
The orthogonal matrix $Q$ is returned.
${\mathbf{COMPQ}}=\text{'V'}$
Q must contain an orthogonal matrix ${Q}_{1}$, and the product ${Q}_{1}Q$ is returned.
Constraint: ${\mathbf{COMPQ}}=\text{'N'}$, $\text{'I'}$ or $\text{'V'}$.
2:     $\mathrm{COMPZ}$ – CHARACTER(1)Input
On entry: specifies the form of the computed orthogonal matrix $Z$.
${\mathbf{COMPZ}}=\text{'N'}$
Do not compute $Z$.
${\mathbf{COMPZ}}=\text{'I'}$
The orthogonal matrix $Z$ is returned.
${\mathbf{COMPZ}}=\text{'V'}$
Z must contain an orthogonal matrix ${Z}_{1}$, and the product ${Z}_{1}Z$ is returned.
Constraint: ${\mathbf{COMPZ}}=\text{'N'}$, $\text{'V'}$ or $\text{'I'}$.
3:     $\mathrm{N}$ – INTEGERInput
On entry: $n$, the order of the matrices $A$ and $B$.
Constraint: ${\mathbf{N}}\ge 0$.
4:     $\mathrm{ILO}$ – INTEGERInput
5:     $\mathrm{IHI}$ – INTEGERInput
On entry: ${i}_{\mathrm{lo}}$ and ${i}_{\mathrm{hi}}$ as determined by a previous call to F08WHF (DGGBAL). Otherwise, they should be set to $1$ and $n$, respectively.
Constraints:
• if ${\mathbf{N}}>0$, $1\le {\mathbf{ILO}}\le {\mathbf{IHI}}\le {\mathbf{N}}$;
• if ${\mathbf{N}}=0$, ${\mathbf{ILO}}=1$ and ${\mathbf{IHI}}=0$.
6:     $\mathrm{A}\left({\mathbf{LDA}},*\right)$ – REAL (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array A must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$.
On entry: the matrix $A$ of the matrix pair $\left(A,B\right)$. Usually, this is the matrix $A$ returned by F08AGF (DORMQR).
On exit: A is overwritten by the upper Hessenberg matrix $H$.
7:     $\mathrm{LDA}$ – INTEGERInput
On entry: the first dimension of the array A as declared in the (sub)program from which F08WEF (DGGHRD) is called.
Constraint: ${\mathbf{LDA}}\ge \mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$.
8:     $\mathrm{B}\left({\mathbf{LDB}},*\right)$ – REAL (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array B must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$.
On entry: the upper triangular matrix $B$ of the matrix pair $\left(A,B\right)$. Usually, this is the matrix $B$ returned by the $QR$ factorization routine F08AEF (DGEQRF).
On exit: B is overwritten by the upper triangular matrix $T$.
9:     $\mathrm{LDB}$ – INTEGERInput
On entry: the first dimension of the array B as declared in the (sub)program from which F08WEF (DGGHRD) is called.
Constraint: ${\mathbf{LDB}}\ge \mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$.
10:   $\mathrm{Q}\left({\mathbf{LDQ}},*\right)$ – REAL (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array Q must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$ if ${\mathbf{COMPQ}}=\text{'I'}$ or $\text{'V'}$ and at least $1$ if ${\mathbf{COMPQ}}=\text{'N'}$.
On entry: if ${\mathbf{COMPQ}}=\text{'V'}$, Q must contain an orthogonal matrix ${Q}_{1}$.
If ${\mathbf{COMPQ}}=\text{'N'}$, Q is not referenced.
On exit: if ${\mathbf{COMPQ}}=\text{'I'}$, Q contains the orthogonal matrix $Q$.
If ${\mathbf{COMPQ}}=\text{'V'}$, Q is overwritten by ${Q}_{1}Q$.
11:   $\mathrm{LDQ}$ – INTEGERInput
On entry: the first dimension of the array Q as declared in the (sub)program from which F08WEF (DGGHRD) is called.
Constraints:
• if ${\mathbf{COMPQ}}=\text{'I'}$ or $\text{'V'}$, ${\mathbf{LDQ}}\ge \mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$;
• if ${\mathbf{COMPQ}}=\text{'N'}$, ${\mathbf{LDQ}}\ge 1$.
12:   $\mathrm{Z}\left({\mathbf{LDZ}},*\right)$ – REAL (KIND=nag_wp) arrayInput/Output
Note: the second dimension of the array Z must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$ if ${\mathbf{COMPZ}}=\text{'V'}$ or $\text{'I'}$ and at least $1$ if ${\mathbf{COMPZ}}=\text{'N'}$.
On entry: if ${\mathbf{COMPZ}}=\text{'V'}$, Z must contain an orthogonal matrix ${Z}_{1}$.
If ${\mathbf{COMPZ}}=\text{'N'}$, Z is not referenced.
On exit: if ${\mathbf{COMPZ}}=\text{'I'}$, Z contains the orthogonal matrix $Z$.
If ${\mathbf{COMPZ}}=\text{'V'}$, Z is overwritten by ${Z}_{1}Z$.
13:   $\mathrm{LDZ}$ – INTEGERInput
On entry: the first dimension of the array Z as declared in the (sub)program from which F08WEF (DGGHRD) is called.
Constraints:
• if ${\mathbf{COMPZ}}=\text{'V'}$ or $\text{'I'}$, ${\mathbf{LDZ}}\ge \mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{N}}\right)$;
• if ${\mathbf{COMPZ}}=\text{'N'}$, ${\mathbf{LDZ}}\ge 1$.
14:   $\mathrm{INFO}$ – INTEGEROutput
On exit: ${\mathbf{INFO}}=0$ unless the routine detects an error (see Section 6).

## 6  Error Indicators and Warnings

${\mathbf{INFO}}<0$
If ${\mathbf{INFO}}=-i$, 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 orthogonal transformations which are backward stable.

## 8  Parallelism and Performance

F08WEF (DGGHRD) is not threaded by NAG in any implementation.
F08WEF (DGGHRD) 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.

This routine is usually followed by F08XEF (DHGEQZ) which implements the $QZ$ algorithm for computing generalized eigenvalues of a reduced pair of matrices.