DORGTR - generate a real orthogonal matrix Q which is
defined as the product of n-1 elementary reflectors of
order N, as returned by DSYTRD
SYNOPSIS
SUBROUTINE DORGTR( UPLO, N, A, LDA, TAU, WORK, LWORK, INFO
)
CHARACTER UPLO
INTEGER INFO, LDA, LWORK, N
DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK(
* )
PURPOSE
DORGTR generates a real orthogonal matrix Q which is
defined as the product of n-1 elementary reflectors of
order N, as returned by DSYTRD: if UPLO = 'U', Q = H(n-1)
. . . H(2) H(1),
if UPLO = 'L', Q = H(1) H(2) . . . H(n-1).
ARGUMENTS
UPLO (input) CHARACTER*1
= 'U': Upper triangle of A contains elementary
reflectors from DSYTRD; = 'L': Lower triangle of A
contains elementary reflectors from DSYTRD.
N (input) INTEGER
The order of the matrix Q. N >= 0.
A (input/output) DOUBLE PRECISION array, dimension
(LDA,N)
On entry, the vectors which define the elementary
reflectors, as returned by DSYTRD. 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 DSYTRD.
WORK (workspace/output) DOUBLE PRECISION array,
dimension (LWORK)
On exit, if INFO = 0, WORK(1) returns the optimal
LWORK.
The dimension of the array WORK. LWORK >=
max(1,N-1). For optimum performance LWORK >=
(N-1)*NB, where NB is the optimal blocksize.
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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