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+ SUBROUTINE ZUNG2L( M, N, K, A, LDA, TAU, WORK, INFO )
+*
+* -- LAPACK routine (version 3.1) --
+* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
+* November 2006
+*
+* .. Scalar Arguments ..
+ INTEGER INFO, K, LDA, M, N
+* ..
+* .. Array Arguments ..
+ COMPLEX*16 A( LDA, * ), TAU( * ), WORK( * )
+* ..
+*
+* Purpose
+* =======
+*
+* ZUNG2L generates an m by n complex matrix Q with orthonormal columns,
+* which is defined as the last n columns of a product of k elementary
+* reflectors of order m
+*
+* Q = H(k) . . . H(2) H(1)
+*
+* as returned by ZGEQLF.
+*
+* 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. M >= N >= 0.
+*
+* K (input) INTEGER
+* The number of elementary reflectors whose product defines the
+* matrix Q. N >= K >= 0.
+*
+* A (input/output) COMPLEX*16 array, dimension (LDA,N)
+* On entry, the (n-k+i)-th column must contain the vector which
+* defines the elementary reflector H(i), for i = 1,2,...,k, as
+* returned by ZGEQLF in the last k columns 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*16 array, dimension (K)
+* TAU(i) must contain the scalar factor of the elementary
+* reflector H(i), as returned by ZGEQLF.
+*
+* WORK (workspace) COMPLEX*16 array, dimension (N)
+*
+* INFO (output) INTEGER
+* = 0: successful exit
+* < 0: if INFO = -i, the i-th argument has an illegal value
+*
+* =====================================================================
+*
+* .. Parameters ..
+ COMPLEX*16 ONE, ZERO
+ PARAMETER ( ONE = ( 1.0D+0, 0.0D+0 ),
+ $ ZERO = ( 0.0D+0, 0.0D+0 ) )
+* ..
+* .. Local Scalars ..
+ INTEGER I, II, J, L
+* ..
+* .. External Subroutines ..
+ EXTERNAL XERBLA, ZLARF, ZSCAL
+* ..
+* .. Intrinsic Functions ..
+ INTRINSIC MAX
+* ..
+* .. Executable Statements ..
+*
+* Test the input arguments
+*
+ INFO = 0
+ IF( M.LT.0 ) THEN
+ INFO = -1
+ ELSE IF( N.LT.0 .OR. N.GT.M ) THEN
+ INFO = -2
+ ELSE IF( K.LT.0 .OR. K.GT.N ) THEN
+ INFO = -3
+ ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
+ INFO = -5
+ END IF
+ IF( INFO.NE.0 ) THEN
+ CALL XERBLA( 'ZUNG2L', -INFO )
+ RETURN
+ END IF
+*
+* Quick return if possible
+*
+ IF( N.LE.0 )
+ $ RETURN
+*
+* Initialise columns 1:n-k to columns of the unit matrix
+*
+ DO 20 J = 1, N - K
+ DO 10 L = 1, M
+ A( L, J ) = ZERO
+ 10 CONTINUE
+ A( M-N+J, J ) = ONE
+ 20 CONTINUE
+*
+ DO 40 I = 1, K
+ II = N - K + I
+*
+* Apply H(i) to A(1:m-k+i,1:n-k+i) from the left
+*
+ A( M-N+II, II ) = ONE
+ CALL ZLARF( 'Left', M-N+II, II-1, A( 1, II ), 1, TAU( I ), A,
+ $ LDA, WORK )
+ CALL ZSCAL( M-N+II-1, -TAU( I ), A( 1, II ), 1 )
+ A( M-N+II, II ) = ONE - TAU( I )
+*
+* Set A(m-k+i+1:m,n-k+i) to zero
+*
+ DO 30 L = M - N + II + 1, M
+ A( L, II ) = ZERO
+ 30 CONTINUE
+ 40 CONTINUE
+ RETURN
+*
+* End of ZUNG2L
+*
+ END