ZGGRQF(3)

NAME
ZGGRQF - a generalized RQ factorization of an M-by-N matrix A and a P-by-N matrix B

SYNOPSIS

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

INTEGER
    INFO, LDA, LDB, LWORK, M, N, P

COMPLEX*16
    A( LDA, * ), B( LDB, * ), TAUA( * ), TAUB( * ), WORK( * )

PURPOSE
ZGGRQF computes a generalized RQ factorization of an M-by-N matrix A and a P-by-N matrix B:
            A = R*Q, B = Z*T*Q,
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 M <= N, R = ( 0 R12 ) M, or if M > N, R = ( R11 ) M-N,
                 N-M M ( R21 ) N
                        N
where R12 or R21 is upper triangular, and
if P >= N, T = ( T11 ) N , or if P < N, T = ( T11 T12 ) P,
                ( 0 ) P-N P N-P
                   N
where T11 is upper triangular.
In particular, if B is square and nonsingular, the GRQ factorization of A and B implicitly gives the RQ factorization of A*inv(B):
             A*inv(B) = (R*inv(T))*Z'
where inv(B) denotes the inverse of the matrix B, and Z' denotes the conjugate transpose of the matrix Z.

ARGUMENTS

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

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

N       (input) INTEGER
    The number of columns of the matrices A and B. N >= 0.

A       (input/output) COMPLEX*16 array, dimension (LDA,N)
    On entry, the M-by-N matrix A. On exit, if M <= N, the upper triangle of the subarray A(1:M,N-M+1:N) contains the M-by-M upper triangular matrix R; if M > N, the elements on and above the (M-N)-th subdiagonal contain the M-by-N upper trapezoidal matrix R; the remaining elements, with the array TAUA, represent the unitary matrix Q as a product of elementary reflectors (see Further Details).

LDA     (input) INTEGER
    The leading dimension of the array A. LDA >= max(1,M).

TAUA    (output) COMPLEX*16 array, dimension (min(M,N))
    The scalar factors of the elementary reflectors which represent the unitary matrix Q (see Further Details). B (input/output) COMPLEX*16 array, dimension (LDB,N) On entry, the P-by-N matrix B. On exit, the elements on and above the diagonal of the array contain the min(P,N)-by-N upper trapezoidal matrix T (T is upper triangular if P >= N); the elements below the diagonal, 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,P).

TAUB    (output) COMPLEX*16 array, dimension (min(P,N))
    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 RQ factorization of an M-by-N matrix, NB2 is the optimal blocksize for the QR factorization of a P-by-N matrix, and NB3 is the optimal blocksize for a call of ZUNMRQ. 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(m,n).
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(n-k+i+1:n) = 0 and v(n-k+i) = 1; v(1:n-k+i-1) is stored on exit in A(m-k+i,1:n-k+i-1), and taua in TAUA(i).
To form Q explicitly, use LAPACK subroutine ZUNGRQ.
To use Q to update another matrix, use LAPACK subroutine ZUNMRQ. The matrix Z is represented as a product of elementary reflectors
   Z = H(1) H(2) . . . H(k), where k = min(p,n).
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(1:i-1) = 0 and v(i) = 1; v(i+1:p) is stored on exit in B(i+1:p,i), and taub in TAUB(i).
To form Z explicitly, use LAPACK subroutine ZUNGQR.
To use Z to update another matrix, use LAPACK subroutine ZUNMQR.