CGGHRD(l) — LAPACK routine (version 2.0)

NAME

CGGHRD - reduce a pair of complex matrices (A,B) to generalized upper Hessenberg form using unitary transformations, where A is a general matrix and B is upper triangular

SYNOPSIS

SUBROUTINE CGGHRD(
COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q, LDQ, Z, LDZ, INFO )

CHARACTER COMPQ, COMPZ

INTEGER IHI, ILO, INFO, LDA, LDB, LDQ, LDZ, N

COMPLEX A( LDA, ∗ ), B( LDB, ∗ ), Q( LDQ, ∗ ), Z( LDZ, ∗ )

PURPOSE

CGGHRD reduces a pair of complex matrices (A,B) to generalized upper Hessenberg form using unitary transformations, where A is a general matrix and B is upper triangular: Q’ ∗ A ∗ Z = H and Q’ ∗ B ∗ Z = T, where H is upper Hessenberg, T is upper triangular, and Q and Z are unitary, and ’ means conjugate transpose.

The unitary matrices Q and Z are 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

Q1 ∗ A ∗ Z1’ = (Q1∗Q) ∗ H ∗ (Z1∗Z)’
Q1 ∗ B ∗ Z1’ = (Q1∗Q) ∗ T ∗ (Z1∗Z)’

ARGUMENTS

COMPQ (input) CHARACTER∗1
= ’N’: do not compute Q;
= ’I’: Q is initialized to the unit matrix, and the unitary matrix Q is returned; = ’V’: Q must contain a unitary matrix Q1 on entry, and the product Q1∗Q is returned.

COMPZ (input) CHARACTER∗1
= ’N’: do not compute Q;
= ’I’: Q is initialized to the unit matrix, and the unitary matrix Q is returned; = ’V’: Q must contain a unitary matrix Q1 on entry, and the product Q1∗Q is returned.

N (input) INTEGER
The order of the matrices A and B. N >= 0.

ILO (input) INTEGER
IHI (input) INTEGER It is assumed that A is already upper triangular in rows and columns 1:ILO-1 and IHI+1:N. ILO and IHI are normally set by a previous call to CGGBAL; otherwise they should be set to 1 and N respectively. 1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if N=0.

A (input/output) COMPLEX array, dimension (LDA, N)
On entry, the N-by-N general matrix to be reduced. On exit, the upper triangle and the first subdiagonal of A are overwritten with the upper Hessenberg matrix H, and the rest is set to zero.

LDA (input) INTEGER
The leading dimension of the array A. LDA >= max(1,N).

B (input/output) COMPLEX array, dimension (LDB, N)
On entry, the N-by-N upper triangular matrix B. On exit, the upper triangular matrix T = Q’ B Z. The elements below the diagonal are set to zero.

LDB (input) INTEGER
The leading dimension of the array B. LDB >= max(1,N).

Q (input/output) COMPLEX array, dimension (LDQ, N)
If COMPQ=’N’: Q is not referenced.
If COMPQ=’I’: on entry, Q need not be set, and on exit it contains the unitary matrix Q, where Q’ is the product of the Givens transformations which are applied to A and B on the left. If COMPQ=’V’: on entry, Q must contain a unitary matrix Q1, and on exit this is overwritten by Q1∗Q.

LDQ (input) INTEGER
The leading dimension of the array Q. LDQ >= N if COMPQ=’V’ or ’I’; LDQ >= 1 otherwise.

Z (input/output) COMPLEX array, dimension (LDZ, N)
If COMPZ=’N’: Z is not referenced.
If COMPZ=’I’: on entry, Z need not be set, and on exit it contains the unitary matrix Z, which is the product of the Givens transformations which are applied to A and B on the right. If COMPZ=’V’: on entry, Z must contain a unitary matrix Z1, and on exit this is overwritten by Z1∗Z.

LDZ (input) INTEGER
The leading dimension of the array Z. LDZ >= N if COMPZ=’V’ or ’I’; LDZ >= 1 otherwise.

INFO (output) INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value.

FURTHER DETAILS

This routine reduces A to Hessenberg and B to triangular form by an unblocked reduction, as described in _Matrix_Computations_, by Golub and van Loan (Johns Hopkins Press).