CGGQRF(3)      LAPACK routine of NEC Numeric Library Collection      CGGQRF(3)



NAME
       CGGQRF

SYNOPSIS
       SUBROUTINE CGGQRF (N, M, P, A, LDA, TAUA, B, LDB, TAUB, WORK, LWORK,
           INFO)



PURPOSE
            CGGQRF 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**H * (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)
                     N is INTEGER
                     The number of rows of the matrices A and B. N >= 0.

           M         (input)
                     M is INTEGER
                     The number of columns of the matrix A.  M >= 0.

           P         (input)
                     P is INTEGER
                     The number of columns of the matrix B.  P >= 0.

           A         (input/output)
                     A is COMPLEX 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)
                     LDA is INTEGER
                     The leading dimension of the array A. LDA >= max(1,N).

           TAUA      (output)
                     TAUA is COMPLEX array, dimension (min(N,M))
                     The scalar factors of the elementary reflectors which
                     represent the unitary matrix Q (see Further Details).

           B         (input/output)
                     B is COMPLEX 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)
                     LDB is INTEGER
                     The leading dimension of the array B. LDB >= max(1,N).

           TAUB      (output)
                     TAUB is COMPLEX array, dimension (min(N,P))
                     The scalar factors of the elementary reflectors which
                     represent the unitary matrix Z (see Further Details).

           WORK      (output)
                     WORK is COMPLEX array, dimension (MAX(1,LWORK))
                     On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

           LWORK     (input)
                     LWORK is 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 CUNMQR.

                     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)
                     INFO is 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**H

             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 CUNGQR.
             To use Q to update another matrix, use LAPACK subroutine CUNMQR.

             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**H

             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 CUNGRQ.
             To use Z to update another matrix, use LAPACK subroutine CUNMRQ.



LAPACK routine                  31 October 2017                      CGGQRF(3)