```      SUBROUTINE ZGGQRF( N, M, P, A, LDA, TAUA, B, LDB, TAUB, 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, LDB, LWORK, M, N, P
*     ..
*     .. Array Arguments ..
COMPLEX*16         A( LDA, * ), B( LDB, * ), TAUA( * ), TAUB( * ),
\$                   WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  ZGGQRF computes a generalized QR factorization of an N-by-M matrix A
*  and an N-by-P matrix B:
*
*              A = Q*R,        B = Q*T*Z,
*
*  where Q is an N-by-N unitary matrix, Z is a P-by-P unitary matrix,
*  and R and T assume one of the forms:
*
*  if N >= M,  R = ( R11 ) M  ,   or if N < M,  R = ( R11  R12 ) N,
*                  (  0  ) N-M                         N   M-N
*                     M
*
*  where R11 is upper triangular, and
*
*  if N <= P,  T = ( 0  T12 ) N,   or if N > P,  T = ( T11 ) N-P,
*                   P-N  N                           ( T21 ) P
*                                                       P
*
*  where T12 or T21 is upper triangular.
*
*  In particular, if B is square and nonsingular, the GQR factorization
*  of A and B implicitly gives the QR factorization of inv(B)*A:
*
*               inv(B)*A = Z'*(inv(T)*R)
*
*  where inv(B) denotes the inverse of the matrix B, and Z' denotes the
*  conjugate transpose of matrix Z.
*
*  Arguments
*  =========
*
*  N       (input) INTEGER
*          The number of rows of the matrices A and B. N >= 0.
*
*  M       (input) INTEGER
*          The number of columns of the matrix A.  M >= 0.
*
*  P       (input) INTEGER
*          The number of columns of the matrix B.  P >= 0.
*
*  A       (input/output) COMPLEX*16 array, dimension (LDA,M)
*          On entry, the N-by-M matrix A.
*          On exit, the elements on and above the diagonal of the array
*          contain the min(N,M)-by-M upper trapezoidal matrix R (R is
*          upper triangular if N >= M); the elements below the diagonal,
*          with the array TAUA, represent the unitary matrix Q as a
*          product of min(N,M) elementary reflectors (see Further
*          Details).
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A. LDA >= max(1,N).
*
*  TAUA    (output) COMPLEX*16 array, dimension (min(N,M))
*          The scalar factors of the elementary reflectors which
*          represent the unitary matrix Q (see Further Details).
*
*  B       (input/output) COMPLEX*16 array, dimension (LDB,P)
*          On entry, the N-by-P matrix B.
*          On exit, if N <= P, the upper triangle of the subarray
*          B(1:N,P-N+1:P) contains the N-by-N upper triangular matrix T;
*          if N > P, the elements on and above the (N-P)-th subdiagonal
*          contain the N-by-P upper trapezoidal matrix T; the remaining
*          elements, with the array TAUB, represent the unitary
*          matrix Z as a product of elementary reflectors (see Further
*          Details).
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B. LDB >= max(1,N).
*
*  TAUB    (output) COMPLEX*16 array, dimension (min(N,P))
*          The scalar factors of the elementary reflectors which
*          represent the unitary matrix Z (see Further Details).
*
*  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,M,P).
*          For optimum performance LWORK >= max(N,M,P)*max(NB1,NB2,NB3),
*          where NB1 is the optimal blocksize for the QR factorization
*          of an N-by-M matrix, NB2 is the optimal blocksize for the
*          RQ factorization of an N-by-P matrix, and NB3 is the optimal
*          blocksize for a call of ZUNMQR.
*
*          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
*  ===============
*
*  The matrix Q is represented as a product of elementary reflectors
*
*     Q = H(1) H(2) . . . H(k), where k = min(n,m).
*
*  Each H(i) has the form
*
*     H(i) = I - taua * v * v'
*
*  where taua is a complex scalar, and v is a complex vector with
*  v(1:i-1) = 0 and v(i) = 1; v(i+1:n) is stored on exit in A(i+1:n,i),
*  and taua in TAUA(i).
*  To form Q explicitly, use LAPACK subroutine ZUNGQR.
*  To use Q to update another matrix, use LAPACK subroutine ZUNMQR.
*
*  The matrix Z is represented as a product of elementary reflectors
*
*     Z = H(1) H(2) . . . H(k), where k = min(n,p).
*
*  Each H(i) has the form
*
*     H(i) = I - taub * v * v'
*
*  where taub is a complex scalar, and v is a complex vector with
*  v(p-k+i+1:p) = 0 and v(p-k+i) = 1; v(1:p-k+i-1) is stored on exit in
*  B(n-k+i,1:p-k+i-1), and taub in TAUB(i).
*  To form Z explicitly, use LAPACK subroutine ZUNGRQ.
*  To use Z to update another matrix, use LAPACK subroutine ZUNMRQ.
*
*  =====================================================================
*
*     .. Local Scalars ..
LOGICAL            LQUERY
INTEGER            LOPT, LWKOPT, NB, NB1, NB2, NB3
*     ..
*     .. External Subroutines ..
EXTERNAL           XERBLA, ZGEQRF, ZGERQF, ZUNMQR
*     ..
*     .. External Functions ..
INTEGER            ILAENV
EXTERNAL           ILAENV
*     ..
*     .. Intrinsic Functions ..
INTRINSIC          INT, MAX, MIN
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters
*
INFO = 0
NB1 = ILAENV( 1, 'ZGEQRF', ' ', N, M, -1, -1 )
NB2 = ILAENV( 1, 'ZGERQF', ' ', N, P, -1, -1 )
NB3 = ILAENV( 1, 'ZUNMQR', ' ', N, M, P, -1 )
NB = MAX( NB1, NB2, NB3 )
LWKOPT = MAX( N, M, P )*NB
WORK( 1 ) = LWKOPT
LQUERY = ( LWORK.EQ.-1 )
IF( N.LT.0 ) THEN
INFO = -1
ELSE IF( M.LT.0 ) THEN
INFO = -2
ELSE IF( P.LT.0 ) THEN
INFO = -3
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -5
ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
INFO = -8
ELSE IF( LWORK.LT.MAX( 1, N, M, P ) .AND. .NOT.LQUERY ) THEN
INFO = -11
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'ZGGQRF', -INFO )
RETURN
ELSE IF( LQUERY ) THEN
RETURN
END IF
*
*     QR factorization of N-by-M matrix A: A = Q*R
*
CALL ZGEQRF( N, M, A, LDA, TAUA, WORK, LWORK, INFO )
LOPT = WORK( 1 )
*
*     Update B := Q'*B.
*
CALL ZUNMQR( 'Left', 'Conjugate Transpose', N, P, MIN( N, M ), A,
\$             LDA, TAUA, B, LDB, WORK, LWORK, INFO )
LOPT = MAX( LOPT, INT( WORK( 1 ) ) )
*
*     RQ factorization of N-by-P matrix B: B = T*Z.
*
CALL ZGERQF( N, P, B, LDB, TAUB, WORK, LWORK, INFO )
WORK( 1 ) = MAX( LOPT, INT( WORK( 1 ) ) )
*
RETURN
*
*     End of ZGGQRF
*
END

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