Visible to Intel only — GUID: GUID-B5130A30-AC70-4D64-8607-7131D5A29750
Visible to Intel only — GUID: GUID-B5130A30-AC70-4D64-8607-7131D5A29750
p?geqr2
Computes a QR factorization of a general rectangular matrix (unblocked algorithm).
Syntax
call psgeqr2(m, n, a, ia, ja, desca, tau, work, lwork, info)
call pdgeqr2(m, n, a, ia, ja, desca, tau, work, lwork, info)
call pcgeqr2(m, n, a, ia, ja, desca, tau, work, lwork, info)
call pzgeqr2(m, n, a, ia, ja, desca, tau, work, lwork, info)
Description
The p?geqr2routine computes a QR factorization of a real/complex distributed m-by-n matrix sub(A) = A(ia:ia+m-1, ja:ja+n-1)= Q*R.
Input Parameters
- m
-
(global) INTEGER.
The number of rows in the distributed matrix sub(A). (m≥0).
- n
-
(global) INTEGER. The number of columns in the distributed matrix sub(A). (n≥0).
- a
-
(local).
REAL for psgeqr2
DOUBLE PRECISION for pdgeqr2
COMPLEX for pcgeqr2
COMPLEX*16 for pzgeqr2.
Pointer into the local memory to an array of size (lld_a, LOCc(ja+n-1)).
On entry, this array contains the local pieces of the m-by-n distributed matrix sub(A) which is to be factored.
- ia, ja
-
(global) INTEGER. The row and column indices in the global matrix A indicating the first row and the first column of sub(A), respectively.
- desca
-
(global and local) INTEGER array of size dlen_. The array descriptor for the distributed matrix A.
- work
-
(local).
REAL for psgeqr2
DOUBLE PRECISION for pdgeqr2
COMPLEX for pcgeqr2
COMPLEX*16 for pzgeqr2.
This is a workspace array of size lwork.
- lwork
-
(local or global) INTEGER.
The size of the array work.
lwork is local input and must be at least lwork≥mp0+max(1, nq0),
where iroff = mod(ia-1, mb_a), icoff = mod(ja-1, nb_a),
iarow = indxg2p(ia, mb_a, myrow, rsrc_a, nprow),
iacol = indxg2p(ja, nb_a, mycol, csrc_a, npcol),
mp0 = numroc(m+iroff, mb_a, myrow, iarow, nprow),
nq0 = numroc(n+icoff, nb_a, mycol, iacol, npcol).
indxg2p and numroc are ScaLAPACK tool functions; myrow, mycol, nprow, and npcol can be determined by calling the subroutine blacs_gridinfo.
If lwork = -1, then lwork is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. Each of these values is returned in the first entry of the corresponding work array, and no error message is issued by pxerbla.
Output Parameters
- a
-
(local).
On exit, the elements on and above the diagonal of sub(A) contain the min(m,n) by n upper trapezoidal matrix R (R is upper triangular if m≥n); the elements below the diagonal, with the array tau, represent the orthogonal/unitary matrix Q as a product of elementary reflectors (see Application Notes below).
- tau
-
(local).
REAL for psgeqr2
DOUBLE PRECISION for pdgeqr2
COMPLEX for pcgeqr2
COMPLEX*16 for pzgeqr2.
Array of size LOCc(ja+min(m,n)-1). This array contains the scalar factors of the elementary reflectors. tau is tied to the distributed matrix A.
- work
-
On exit, work(1) returns the minimal and optimal lwork.
- info
-
(local) INTEGER.
If info = 0, the execution is successful. if info < 0:
If the i-th argument is an array and the j-th entry had an illegal value, then info = - (i*100+j),
if the i-th argument is a scalar and had an illegal value, then info = -i.
Application Notes
The matrix Q is represented as a product of elementary reflectors
Q = H(ja)*H(ja+1)*. . .* H(ja+k-1), where k = min(m,n).
Each H(i) has the form
H(j)= I - tau*v*v',
where tau is a real/complex scalar, and v is a real/complex vector with v(1: i-1) = 0 and v(i) = 1; v(i+1: m) is stored on exit in A(ia+i:ia+m-1, ja+i-1), and tau in tau(ja+i-1).