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SUBROUTINE CUNGR2( M, N, K, A, LDA, TAU, WORK, INFO )
*
* -- LAPACK routine (version 3.2) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* November 2006
*
* .. Scalar Arguments ..
INTEGER INFO, K, LDA, M, N
* ..
* .. Array Arguments ..
COMPLEX A( LDA, * ), TAU( * ), WORK( * )
* ..
*
* Purpose
* =======
*
* CUNGR2 generates an m by n complex matrix Q with orthonormal rows,
* which is defined as the last m rows of a product of k elementary
* reflectors of order n
*
* Q = H(1)' H(2)' . . . H(k)'
*
* as returned by CGERQF.
*
* Arguments
* =========
*
* M (input) INTEGER
* The number of rows of the matrix Q. M >= 0.
*
* N (input) INTEGER
* The number of columns of the matrix Q. N >= M.
*
* K (input) INTEGER
* The number of elementary reflectors whose product defines the
* matrix Q. M >= K >= 0.
*
* A (input/output) COMPLEX array, dimension (LDA,N)
* On entry, the (m-k+i)-th row must contain the vector which
* defines the elementary reflector H(i), for i = 1,2,...,k, as
* returned by CGERQF in the last k rows of its array argument
* A.
* On exit, the m-by-n matrix Q.
*
* LDA (input) INTEGER
* The first dimension of the array A. LDA >= max(1,M).
*
* TAU (input) COMPLEX array, dimension (K)
* TAU(i) must contain the scalar factor of the elementary
* reflector H(i), as returned by CGERQF.
*
* WORK (workspace) COMPLEX array, dimension (M)
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument has an illegal value
*
* =====================================================================
*
* .. Parameters ..
COMPLEX ONE, ZERO
PARAMETER ( ONE = ( 1.0E+0, 0.0E+0 ),
$ ZERO = ( 0.0E+0, 0.0E+0 ) )
* ..
* .. Local Scalars ..
INTEGER I, II, J, L
* ..
* .. External Subroutines ..
EXTERNAL CLACGV, CLARF, CSCAL, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC CONJG, MAX
* ..
* .. Executable Statements ..
*
* Test the input arguments
*
INFO = 0
IF( M.LT.0 ) THEN
INFO = -1
ELSE IF( N.LT.M ) THEN
INFO = -2
ELSE IF( K.LT.0 .OR. K.GT.M ) THEN
INFO = -3
ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
INFO = -5
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'CUNGR2', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( M.LE.0 )
$ RETURN
*
IF( K.LT.M ) THEN
*
* Initialise rows 1:m-k to rows of the unit matrix
*
DO 20 J = 1, N
DO 10 L = 1, M - K
A( L, J ) = ZERO
10 CONTINUE
IF( J.GT.N-M .AND. J.LE.N-K )
$ A( M-N+J, J ) = ONE
20 CONTINUE
END IF
*
DO 40 I = 1, K
II = M - K + I
*
* Apply H(i)' to A(1:m-k+i,1:n-k+i) from the right
*
CALL CLACGV( N-M+II-1, A( II, 1 ), LDA )
A( II, N-M+II ) = ONE
CALL CLARF( 'Right', II-1, N-M+II, A( II, 1 ), LDA,
$ CONJG( TAU( I ) ), A, LDA, WORK )
CALL CSCAL( N-M+II-1, -TAU( I ), A( II, 1 ), LDA )
CALL CLACGV( N-M+II-1, A( II, 1 ), LDA )
A( II, N-M+II ) = ONE - CONJG( TAU( I ) )
*
* Set A(m-k+i,n-k+i+1:n) to zero
*
DO 30 L = N - M + II + 1, N
A( II, L ) = ZERO
30 CONTINUE
40 CONTINUE
RETURN
*
* End of CUNGR2
*
END
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