          SUBROUTINE ZUNGQR( M, N, K, 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 ..
          INTEGER            INFO, K, LDA, LWORK, M, N
    *     ..
    *     .. Array Arguments ..
          COMPLEX*16         A( LDA, * ), TAU( * ), WORK( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  ZUNGQR generates an M-by-N complex matrix Q with orthonormal columns,
    *  which is defined as the first N columns of a product of K elementary
    *  reflectors of order M
    *
    *        Q  =  H(1) H(2) . . . H(k)
    *
    *  as returned by ZGEQRF.
    *
    *  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 i-th column must contain the vector which
    *          defines the elementary reflector H(i), for i = 1,2,...,k, as
    *          returned by ZGEQRF in the first 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 ZGEQRF.
    *
    *  WORK    (workspace/output) COMPLEX*16 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).
    *          For optimum performance LWORK >= N*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 has an illegal value
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          COMPLEX*16         ZERO
          PARAMETER          ( ZERO = ( 0.0D+0, 0.0D+0 ) )
    *     ..
    *     .. Local Scalars ..
          LOGICAL            LQUERY
          INTEGER            I, IB, IINFO, IWS, J, KI, KK, L, LDWORK,
         $                   LWKOPT, NB, NBMIN, NX
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           XERBLA, ZLARFB, ZLARFT, ZUNG2R
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          MAX, MIN
    *     ..
    *     .. External Functions ..
          INTEGER            ILAENV
          EXTERNAL           ILAENV
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input arguments
    *
          INFO = 0
          NB = ILAENV( 1, 'ZUNGQR', ' ', M, N, K, -1 )
          LWKOPT = MAX( 1, N )*NB
          WORK( 1 ) = LWKOPT
          LQUERY = ( LWORK.EQ.-1 )
          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
          ELSE IF( LWORK.LT.MAX( 1, N ) .AND. .NOT.LQUERY ) THEN
             INFO = -8
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'ZUNGQR', -INFO )
             RETURN
          ELSE IF( LQUERY ) THEN
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( N.LE.0 ) THEN
             WORK( 1 ) = 1
             RETURN
          END IF
    *
          NBMIN = 2
          NX = 0
          IWS = N
          IF( NB.GT.1 .AND. NB.LT.K ) THEN
    *
    *        Determine when to cross over from blocked to unblocked code.
    *
             NX = MAX( 0, ILAENV( 3, 'ZUNGQR', ' ', M, N, K, -1 ) )
             IF( NX.LT.K ) THEN
    *
    *           Determine if workspace is large enough for blocked code.
    *
                LDWORK = N
                IWS = LDWORK*NB
                IF( LWORK.LT.IWS ) THEN
    *
    *              Not enough workspace to use optimal NB:  reduce NB and
    *              determine the minimum value of NB.
    *
                   NB = LWORK / LDWORK
                   NBMIN = MAX( 2, ILAENV( 2, 'ZUNGQR', ' ', M, N, K, -1 ) )
                END IF
             END IF
          END IF
    *
          IF( NB.GE.NBMIN .AND. NB.LT.K .AND. NX.LT.K ) THEN
    *
    *        Use blocked code after the last block.
    *        The first kk columns are handled by the block method.
    *
             KI = ( ( K-NX-1 ) / NB )*NB
             KK = MIN( K, KI+NB )
    *
    *        Set A(1:kk,kk+1:n) to zero.
    *
             DO 20 J = KK + 1, N
                DO 10 I = 1, KK
                   A( I, J ) = ZERO
       10       CONTINUE
       20    CONTINUE
          ELSE
             KK = 0
          END IF
    *
    *     Use unblocked code for the last or only block.
    *
          IF( KK.LT.N )
         $   CALL ZUNG2R( M-KK, N-KK, K-KK, A( KK+1, KK+1 ), LDA,
         $                TAU( KK+1 ), WORK, IINFO )
    *
          IF( KK.GT.0 ) THEN
    *
    *        Use blocked code
    *
             DO 50 I = KI + 1, 1, -NB
                IB = MIN( NB, K-I+1 )
                IF( I+IB.LE.N ) THEN
    *
    *              Form the triangular factor of the block reflector
    *              H = H(i) H(i+1) . . . H(i+ib-1)
    *
                   CALL ZLARFT( 'Forward', 'Columnwise', M-I+1, IB,
         $                      A( I, I ), LDA, TAU( I ), WORK, LDWORK )
    *
    *              Apply H to A(i:m,i+ib:n) from the left
    *
                   CALL ZLARFB( 'Left', 'No transpose', 'Forward',
         $                      'Columnwise', M-I+1, N-I-IB+1, IB,
         $                      A( I, I ), LDA, WORK, LDWORK, A( I, I+IB ),
         $                      LDA, WORK( IB+1 ), LDWORK )
                END IF
    *
    *           Apply H to rows i:m of current block
    *
                CALL ZUNG2R( M-I+1, IB, IB, A( I, I ), LDA, TAU( I ), WORK,
         $                   IINFO )
    *
    *           Set rows 1:i-1 of current block to zero
    *
                DO 40 J = I, I + IB - 1
                   DO 30 L = 1, I - 1
                      A( L, J ) = ZERO
       30          CONTINUE
       40       CONTINUE
       50    CONTINUE
          END IF
    *
          WORK( 1 ) = IWS
          RETURN
    *
    *     End of ZUNGQR
    *
          END