```      SUBROUTINE DTZRZF( M, 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 ..
INTEGER            INFO, LDA, LWORK, M, N
*     ..
*     .. Array Arguments ..
DOUBLE PRECISION   A( LDA, * ), TAU( * ), WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  DTZRZF reduces the M-by-N ( M<=N ) real upper trapezoidal matrix A
*  to upper triangular form by means of orthogonal transformations.
*
*  The upper trapezoidal matrix A is factored as
*
*     A = ( R  0 ) * Z,
*
*  where Z is an N-by-N orthogonal matrix and R is an M-by-M upper
*  triangular matrix.
*
*  Arguments
*  =========
*
*  M       (input) INTEGER
*          The number of rows of the matrix A.  M >= 0.
*
*  N       (input) INTEGER
*          The number of columns of the matrix A.  N >= M.
*
*  A       (input/output) DOUBLE PRECISION array, dimension (LDA,N)
*          On entry, the leading M-by-N upper trapezoidal part of the
*          array A must contain the matrix to be factorized.
*          On exit, the leading M-by-M upper triangular part of A
*          contains the upper triangular matrix R, and elements M+1 to
*          N of the first M rows of A, with the array TAU, represent the
*          orthogonal matrix Z as a product of M elementary reflectors.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,M).
*
*  TAU     (output) DOUBLE PRECISION array, dimension (M)
*          The scalar factors of the elementary reflectors.
*
*  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,M).
*          For optimum performance LWORK >= M*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
*
*  Further Details
*  ===============
*
*  Based on contributions by
*    A. Petitet, Computer Science Dept., Univ. of Tenn., Knoxville, USA
*
*  The factorization is obtained by Householder's method.  The kth
*  transformation matrix, Z( k ), which is used to introduce zeros into
*  the ( m - k + 1 )th row of A, is given in the form
*
*     Z( k ) = ( I     0   ),
*              ( 0  T( k ) )
*
*  where
*
*     T( k ) = I - tau*u( k )*u( k )',   u( k ) = (   1    ),
*                                                 (   0    )
*                                                 ( z( k ) )
*
*  tau is a scalar and z( k ) is an ( n - m ) element vector.
*  tau and z( k ) are chosen to annihilate the elements of the kth row
*  of X.
*
*  The scalar tau is returned in the kth element of TAU and the vector
*  u( k ) in the kth row of A, such that the elements of z( k ) are
*  in  a( k, m + 1 ), ..., a( k, n ). The elements of R are returned in
*  the upper triangular part of A.
*
*  Z is given by
*
*     Z =  Z( 1 ) * Z( 2 ) * ... * Z( m ).
*
*  =====================================================================
*
*     .. Parameters ..
DOUBLE PRECISION   ZERO
PARAMETER          ( ZERO = 0.0D+0 )
*     ..
*     .. Local Scalars ..
LOGICAL            LQUERY
INTEGER            I, IB, IWS, KI, KK, LDWORK, LWKOPT, M1, MU, NB,
\$                   NBMIN, NX
*     ..
*     .. External Subroutines ..
EXTERNAL           DLARZB, DLARZT, DLATRZ, XERBLA
*     ..
*     .. Intrinsic Functions ..
INTRINSIC          MAX, MIN
*     ..
*     .. External Functions ..
INTEGER            ILAENV
EXTERNAL           ILAENV
*     ..
*     .. Executable Statements ..
*
*     Test the input arguments
*
INFO = 0
LQUERY = ( LWORK.EQ.-1 )
IF( M.LT.0 ) THEN
INFO = -1
ELSE IF( N.LT.M ) THEN
INFO = -2
ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
INFO = -4
END IF
*
IF( INFO.EQ.0 ) THEN
IF( M.EQ.0 .OR. M.EQ.N ) THEN
LWKOPT = 1
ELSE
*
*           Determine the block size.
*
NB = ILAENV( 1, 'DGERQF', ' ', M, N, -1, -1 )
LWKOPT = M*NB
END IF
WORK( 1 ) = LWKOPT
*
IF( LWORK.LT.MAX( 1, M ) .AND. .NOT.LQUERY ) THEN
INFO = -7
END IF
END IF
*
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'DTZRZF', -INFO )
RETURN
ELSE IF( LQUERY ) THEN
RETURN
END IF
*
*     Quick return if possible
*
IF( M.EQ.0 ) THEN
RETURN
ELSE IF( M.EQ.N ) THEN
DO 10 I = 1, N
TAU( I ) = ZERO
10    CONTINUE
RETURN
END IF
*
NBMIN = 2
NX = 1
IWS = M
IF( NB.GT.1 .AND. NB.LT.M ) THEN
*
*        Determine when to cross over from blocked to unblocked code.
*
NX = MAX( 0, ILAENV( 3, 'DGERQF', ' ', M, N, -1, -1 ) )
IF( NX.LT.M ) THEN
*
*           Determine if workspace is large enough for blocked code.
*
LDWORK = M
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, 'DGERQF', ' ', M, N, -1,
\$                 -1 ) )
END IF
END IF
END IF
*
IF( NB.GE.NBMIN .AND. NB.LT.M .AND. NX.LT.M ) THEN
*
*        Use blocked code initially.
*        The last kk rows are handled by the block method.
*
M1 = MIN( M+1, N )
KI = ( ( M-NX-1 ) / NB )*NB
KK = MIN( M, KI+NB )
*
DO 20 I = M - KK + KI + 1, M - KK + 1, -NB
IB = MIN( M-I+1, NB )
*
*           Compute the TZ factorization of the current block
*           A(i:i+ib-1,i:n)
*
CALL DLATRZ( IB, N-I+1, N-M, A( I, I ), LDA, TAU( I ),
\$                   WORK )
IF( I.GT.1 ) THEN
*
*              Form the triangular factor of the block reflector
*              H = H(i+ib-1) . . . H(i+1) H(i)
*
CALL DLARZT( 'Backward', 'Rowwise', N-M, IB, A( I, M1 ),
\$                      LDA, TAU( I ), WORK, LDWORK )
*
*              Apply H to A(1:i-1,i:n) from the right
*
CALL DLARZB( 'Right', 'No transpose', 'Backward',
\$                      'Rowwise', I-1, N-I+1, IB, N-M, A( I, M1 ),
\$                      LDA, WORK, LDWORK, A( 1, I ), LDA,
\$                      WORK( IB+1 ), LDWORK )
END IF
20    CONTINUE
MU = I + NB - 1
ELSE
MU = M
END IF
*
*     Use unblocked code to factor the last or only block
*
IF( MU.GT.0 )
\$   CALL DLATRZ( MU, N, N-M, A, LDA, TAU, WORK )
*
WORK( 1 ) = LWKOPT
*
RETURN
*
*     End of DTZRZF
*
END

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