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# cgghrd

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NAME
CGGHRD - reduce a pair of complex matrices (A,B) to general-
ized upper Hessenberg form using unitary similarity
transformations, where A is a (generally non-symmetric)
square 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 general-
ized upper Hessenberg form using unitary similarity
transformations, where A is a (generally non-symmetric)
square matrix and B is upper triangular.  More precisely,
CGGHRD simultaneously decomposes  A into  Q H Z* and  B
into  Q T Z* , where H is upper Hessenberg, T is upper tri-
angular, Q and Z are unitary, and * means conjugate tran-
spose.

If COMPQ and COMPZ are 'V' or 'I', then the unitary
transformations used to reduce (A,B) are accumulated into
the arrays Q and Z s.t.:

Q(in) A(in) Z(in)* = Q(out) A(out) Z(out)*
Q(in) B(in) Z(in)* = Q(out) B(out) Z(out)*

CGGHRD implements an unblocked form of the method, there
being no blocked form known at this time.

ARGUMENTS
COMPQ   (input) CHARACTER*1
= 'N': do not compute Q;
= 'V': accumulate the row transformations into Q;
= 'I': overwrite the array Q with the row transfor-
mations.

COMPZ   (input) CHARACTER*1
= 'N': do not compute Z;
= 'V': accumulate the column transformations into Z;
= 'I': overwrite the array Z with the column
transformations.

N       (input) INTEGER

The number of rows and columns in the matrices A, B,
Q, and Z.  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 >= 1 and IHI <= N.

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 subdiago-
nal of A are overwritten with the 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
entries 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', then Q will not be referenced.  If
COMPQ='V', then the conjugate transpose of the
Givens transformations which are applied to A and B
on the left will be applied to the array Q on the
right.  If COMPQ='I', then Q will first be overwrit-
ten with an identity matrix, and then the Givens
transformations will be applied to Q as in the case
COMPQ='V'.

LDQ     (input) INTEGER
The leading dimension of the array Q.  LDQ >= 1.  If
COMPQ='V' or 'I', then LDQ >= N.

Z       (input/output) COMPLEX array, dimension (LDZ, N)
If COMPZ='N', then Z will not be referenced.  If
COMPZ='V', then the Givens transformations which are
applied to A and B on the right will be applied to
the array Z on the right.  If COMPZ='I', then Z will
first be overwritten with an identity matrix, and
then the Givens transformations will be applied to Z
as in the case COMPZ='V'.

LDZ     (input) INTEGER
The leading dimension of the array Z.  LDZ >= 1.  If

COMPZ='V' or 'I', then LDZ >= N.

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).
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