dorgtr(3)

NAME

DORGTR - generate a real orthogonal matrix Q which is de

fined 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 de

fined 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 re

flectors from DSYTRD; = 'L': Lower triangle of A contains elemen

tary 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 orthogo

nal 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, dimen

sion (LWORK)

On exit, if INFO = 0, WORK(1) returns the optimal

LWORK.

LWORK (input) INTEGER

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

LAPACK version 3.0 15 June 2000

DORGTR(3)