SUBROUTINE DORGTR( UPLO, N, A, LDA, TAU, WORK, LWORK, INFO )
!
! -- LAPACK routine (version 3.1) --
! Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
! November 2006
!
! .. Scalar Arguments ..
CHARACTER UPLO
INTEGER INFO, LDA, LWORK, N
! ..
! .. Array Arguments ..
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 elementary
! reflector H(i), as returned by DSYTRD.
!
! WORK (workspace/output) DOUBLE PRECISION array, dimension (MAX(1,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 illegal value
!
! =====================================================================
!
! .. Parameters ..
DOUBLE PRECISION ZERO, ONE
PARAMETER ( ZERO = 0.0D+0, ONE = 1.0D+0 )
! ..
! .. Local Scalars ..
LOGICAL LQUERY, UPPER
INTEGER I, IINFO, J, LWKOPT, NB
! ..
! .. External Functions ..
! LOGICAL LSAME
! INTEGER ILAENV
! EXTERNAL LSAME, ILAENV
! ..
! .. External Subroutines ..
! EXTERNAL DORGQL, DORGQR, XERBLA
! ..
! .. Intrinsic Functions ..
INTRINSIC MAX
! ..
! .. Executable Statements ..
!
! Test the input arguments
!
INFO = 0
LQUERY = ( LWORK.EQ.-1 )
UPPER = LSAME( UPLO, 'U' )
IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF( LWORK.LT.MAX( 1, N-1 ) .AND. .NOT.LQUERY ) THEN
INFO = -7
END IF
!
IF( INFO.EQ.0 ) THEN
IF( UPPER ) THEN
NB = ILAENV( 1, 'DORGQL', ' ', N-1, N-1, N-1, -1 )
ELSE
NB = ILAENV( 1, 'DORGQR', ' ', N-1, N-1, N-1, -1 )
END IF
LWKOPT = MAX( 1, N-1 )*NB
WORK( 1 ) = LWKOPT
END IF
!
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'DORGTR', -INFO )
RETURN
ELSE IF( LQUERY ) THEN
RETURN
END IF
!
! Quick return if possible
!
IF( N.EQ.0 ) THEN
WORK( 1 ) = 1
RETURN
END IF
!
IF( UPPER ) THEN
!
! Q was determined by a call to DSYTRD with UPLO = 'U'
!
! Shift the vectors which define the elementary reflectors one
! column to the left, and set the last row and column of Q to
! those of the unit matrix
!
DO 20 J = 1, N - 1
DO 10 I = 1, J - 1
A( I, J ) = A( I, J+1 )
10 CONTINUE
A( N, J ) = ZERO
20 CONTINUE
DO 30 I = 1, N - 1
A( I, N ) = ZERO
30 CONTINUE
A( N, N ) = ONE
!
! Generate Q(1:n-1,1:n-1)
!
CALL DORGQL( N-1, N-1, N-1, A, LDA, TAU, WORK, LWORK, IINFO )
!
ELSE
!
! Q was determined by a call to DSYTRD with UPLO = 'L'.
!
! Shift the vectors which define the elementary reflectors one
! column to the right, and set the first row and column of Q to
! those of the unit matrix
!
DO 50 J = N, 2, -1
A( 1, J ) = ZERO
DO 40 I = J + 1, N
A( I, J ) = A( I, J-1 )
40 CONTINUE
50 CONTINUE
A( 1, 1 ) = ONE
DO 60 I = 2, N
A( I, 1 ) = ZERO
60 CONTINUE
IF( N.GT.1 ) THEN
!
! Generate Q(2:n,2:n)
!
CALL DORGQR( N-1, N-1, N-1, A( 2, 2 ), LDA, TAU, WORK, &
LWORK, IINFO )
END IF
END IF
WORK( 1 ) = LWKOPT
RETURN
!
! End of DORGTR
!
END SUBROUTINE DORGTR