dggrqf(3)

NAME

DGGRQF - compute a generalized RQ factorization of an M

by-N matrix A and a P-by-N matrix B

SYNOPSIS

SUBROUTINE  DGGRQF(  M,  P, N, A, LDA, TAUA, B, LDB, TAUB,
WORK, LWORK, INFO )
    INTEGER        INFO, LDA, LDB, LWORK, M, N, P
    DOUBLE         PRECISION A( LDA, * ),  B(  LDB,  *  ),
TAUA( * ), TAUB( * ), WORK( * )

PURPOSE

DGGRQF 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 orthogonal matrix, Z is a P-by-P or

thogonal 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 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) DOUBLE PRECISION 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 trape

zoidal matrix R; the remaining elements, with the array TAUA,

represent the orthogonal matrix Q as a product of elementary re

flectors (see Further Details).

LDA (input) INTEGER

The leading dimension of the array A. LDA >=

max(1,M).

TAUA (output) DOUBLE PRECISION array, dimension

(min(M,N))

The scalar factors of the elementary reflectors

which represent the orthogonal matrix Q (see Further Details). B

(input/output) DOUBLE PRECISION array, dimension (LDB,N) On en

try, 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 orthogonal ma

trix Z as a product of elementary reflectors (see Further De

tails). LDB (input) INTEGER The leading dimension of the ar

ray B. LDB >= max(1,P).

TAUB (output) DOUBLE PRECISION array, dimension

(min(P,N))

The scalar factors of the elementary reflectors

which represent the orthogonal matrix Z (see Further Details).

WORK (workspace/output) DOUBLE PRECISION array, dimension

(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 DORMRQ.

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 INF0= -i, the i-th argument had an ille

gal value.

FURTHER DETAILS

The matrix Q is represented as a product of elementary re

flectors

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 real scalar, and v is a real 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 DORGRQ.
To use Q to update another matrix, use LAPACK subroutine

DORMRQ.

The matrix Z is represented as a product of elementary re

flectors

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 real scalar, and v is a real 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 DORGQR.
To use Z to update another matrix, use LAPACK subroutine

DORMQR.

LAPACK version 3.0 15 June 2000

DGGRQF(3)