DORGHR - generate a real orthogonal matrix Q which is
defined as the product of IHI-ILO elementary reflectors of
order N, as returned by DGEHRD
SYNOPSIS
SUBROUTINE DORGHR( N, ILO, IHI, A, LDA, TAU, WORK, LWORK,
INFO )
INTEGER IHI, ILO, INFO, LDA, LWORK, N
DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK(
* )
PURPOSE
DORGHR generates a real orthogonal matrix Q which is
defined as the product of IHI-ILO elementary reflectors of
order N, as returned by DGEHRD: Q = H(ilo) H(ilo+1) . . .
H(ihi-1).
ARGUMENTS
N (input) INTEGER
The order of the matrix Q. N >= 0.
ILO (input) INTEGER
IHI (input) INTEGER ILO and IHI must have the
same values as in the previous call of DGEHRD. Q
is equal to the unit matrix except in the subma
trix Q(ilo+1:ihi,ilo+1:ihi). 1 <= ILO <= IHI <=
N, if N > 0; ILO=1 and IHI=0, if N=0.
A (input/output) DOUBLE PRECISION array, dimension
(LDA,N)
On entry, the vectors which define the elementary
reflectors, as returned by DGEHRD. On exit, the
N-by-N orthogonal matrix Q.
LDA (input) INTEGER
The leading dimension of the array A. LDA >=
max(1,N).
TAU (input) DOUBLE PRECISION array, dimension (N-1)
TAU(i) must contain the scalar factor of the ele
mentary reflector H(i), as returned by DGEHRD.
WORK (workspace/output) DOUBLE PRECISION array,
dimension (LWORK)
On exit, if INFO = 0, WORK(1) returns the optimal
LWORK.
LWORK (input) INTEGER
The dimension of the array WORK. LWORK >= IHI-ILO.
For optimum performance LWORK >= (IHI-ILO)*NB,
If LWORK = -1, then a workspace query is assumed;
the routine only calculates the optimal size of
the WORK array, returns this value as the first
entry of the WORK array, and no error message
related to LWORK is issued by XERBLA.
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an ille
gal value
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