```      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

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