dgghrd.f(3) LAPACK dgghrd.f(3)
subroutine dgghrd (COMPQ, COMPZ, N, ILO, IHI, A, LDA, B, LDB, Q, LDQ, Z, LDZ, INFO)
subroutine dgghrd (characterCOMPQ, characterCOMPZ, integerN, integerILO, integerIHI, double
precision, dimension( lda, * )A, integerLDA, double precision, dimension( ldb, * )B,
integerLDB, double precision, dimension( ldq, * )Q, integerLDQ, double precision,
dimension( ldz, * )Z, integerLDZ, integerINFO)
DGGHRD reduces a pair of real matrices (A,B) to generalized upper
Hessenberg form using orthogonal transformations, where A is a
general matrix and B is upper triangular. The form of the
generalized eigenvalue problem is
A*x = lambda*B*x,
and B is typically made upper triangular by computing its QR
factorization and moving the orthogonal matrix Q to the left side
of the equation.
This subroutine simultaneously reduces A to a Hessenberg matrix H:
Q**T*A*Z = H
and transforms B to another upper triangular matrix T:
Q**T*B*Z = T
in order to reduce the problem to its standard form
H*y = lambda*T*y
where y = Z**T*x.
The orthogonal 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**T = (Q1*Q) * H * (Z1*Z)**T
Q1 * B * Z1**T = (Q1*Q) * T * (Z1*Z)**T
If Q1 is the orthogonal matrix from the QR factorization of B in the
original equation A*x = lambda*B*x, then DGGHRD reduces the original
problem to generalized Hessenberg form.
COMPQ is CHARACTER*1
= 'N': do not compute Q;
= 'I': Q is initialized to the unit matrix, and the
orthogonal matrix Q is returned;
= 'V': Q must contain an orthogonal matrix Q1 on entry,
and the product Q1*Q is returned.
COMPZ is CHARACTER*1
= 'N': do not compute Z;
= 'I': Z is initialized to the unit matrix, and the
orthogonal matrix Z is returned;
= 'V': Z must contain an orthogonal matrix Z1 on entry,
and the product Z1*Z is returned.
N is INTEGER
The order of the matrices A and B. N >= 0.
ILO is INTEGER
IHI is INTEGER
ILO and IHI mark the rows and columns of A which are to be
reduced. 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 DGGBAL; 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 is DOUBLE PRECISION 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 is INTEGER
The leading dimension of the array A. LDA >= max(1,N).
B is DOUBLE PRECISION array, dimension (LDB, N)
On entry, the N-by-N upper triangular matrix B.
On exit, the upper triangular matrix T = Q**T B Z. The
elements below the diagonal are set to zero.
LDB is INTEGER
The leading dimension of the array B. LDB >= max(1,N).
Q is DOUBLE PRECISION array, dimension (LDQ, N)
On entry, if COMPQ = 'V', the orthogonal matrix Q1,
typically from the QR factorization of B.
On exit, if COMPQ='I', the orthogonal matrix Q, and if
COMPQ = 'V', the product Q1*Q.
Not referenced if COMPQ='N'.
LDQ is INTEGER
The leading dimension of the array Q.
LDQ >= N if COMPQ='V' or 'I'; LDQ >= 1 otherwise.
Z is DOUBLE PRECISION array, dimension (LDZ, N)
On entry, if COMPZ = 'V', the orthogonal matrix Z1.
On exit, if COMPZ='I', the orthogonal matrix Z, and if
COMPZ = 'V', the product Z1*Z.
Not referenced if COMPZ='N'.
LDZ is INTEGER
The leading dimension of the array Z.
LDZ >= N if COMPZ='V' or 'I'; LDZ >= 1 otherwise.
INFO is INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value.
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
This routine reduces A to Hessenberg and B to triangular form by
an unblocked reduction, as described in <em>Matrix_Computations</em>,
by Golub and Van Loan (Johns Hopkins Press.)
Definition at line 207 of file dgghrd.f.
Generated automatically by Doxygen for LAPACK from the source code.
Version 3.4.2 Tue Sep 25 2012 dgghrd.f(3)