cggqrf(3)

NAME

CGGQRF - compute a generalized QR factorization of an N

by-M matrix A and an N-by-P matrix B

SYNOPSIS

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

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'*(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 array, dimension (LDA,M)

On entry, the N-by-M matrix A. On exit, the ele

ments 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) elemen

tary reflectors (see Further Details).

LDA (input) INTEGER

The leading dimension of the array A. LDA >=

max(1,N).

TAUA (output) 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) 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 ma

trix T; if N > P, the elements on and above the (N-P)-th subdiag

onal 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 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 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 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) INTEGER

= 0: successful exit
< 0: if INFO = -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(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 CUNGQR.
To use Q to update another matrix, use LAPACK subroutine

CUNMQR.

The matrix Z is represented as a product of elementary re

flectors

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

CUNMRQ.

LAPACK version 3.0 15 June 2000

CGGQRF(3)