p?geqlf

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 12/16/2022

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p?geqlf

Computes the QL factorization of a general matrix.

Syntax

call psgeqlf(m, n, a, ia, ja, desca, tau, work, lwork, info)

call pdgeqlf(m, n, a, ia, ja, desca, tau, work, lwork, info)

call pcgeqlf(m, n, a, ia, ja, desca, tau, work, lwork, info)

call pzgeqlf(m, n, a, ia, ja, desca, tau, work, lwork, info)

Include Files

Description

The p?geqlf routine forms the QL factorization of a real/complex distributed m-by-n matrix sub(A)= A(ia:ia+m-1, ja:ja+n-1) = Q*L.

Input Parameters

m

(global) INTEGER. The number of rows in the matrix sub(Q); (m≥ 0).

n

(global) INTEGER. The number of columns in the matrix sub(Q) (n≥ 0).

a

(local)

REAL for psgeqlf

DOUBLE PRECISION for pdgeqlf

COMPLEX for pcgeqlf

DOUBLE COMPLEX for pzgeqlf

Pointer into the local memory to an array of local size (lld_a,LOCc(ja+n-1)). Contains the local pieces of the distributed matrix sub(A) 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 the submatrix A(ia:ia+m-1, ja:ja+n-1), respectively.

desca

(global and local) INTEGER array of size dlen_. The array descriptor for the distributed matrix A.

work

(local)

REAL for psgeqlf

DOUBLE PRECISION for pdgeqlf

COMPLEX for pcgeqlf

DOUBLE COMPLEX for pzgeqlf

Workspace array of size of lwork.

lwork

(local or global) INTEGER, size of work, must be at least lwork≥nb_a*(mp0 + nq0 + nb_a), 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)

NOTE:

mod(x,y) is the integer remainder of x/y.

numroc and indxg2p 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

On exit, if m≥n, the lower triangle of the distributed submatrix A(ia+m-n:ia+m-1, ja:ja+n-1) contains the n-by-n lower triangular matrix L; if m≤n, the elements on and below the (n - m)-th superdiagonal contain the m-by-n lower trapezoidal matrix L; the remaining elements, with the array tau, represent the orthogonal/unitary matrix Q as a product of elementary reflectors (see Application Notes below).

tau

(local)

REAL for psgeqlf

DOUBLE PRECISION for pdgeqlf

COMPLEX for pcgeqlf

DOUBLE COMPLEX for pzgeqlf

Array of size LOCc(ja+n-1).

Contains the scalar factors of elementary reflectors. tau is tied to the distributed matrix A.

work(1)

On exit, work(1) contains the minimum value of lwork required for optimum performance.

info

(global) INTEGER.

= 0: the execution is successful.

< 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+k-1)*...*H(ja+1)*H(ja)

where k = min(m,n)

Each H(i) has the form

H(i) = I - tau*v*v'

where tau is a real/complex scalar, and v is a real/complex vector with v(m-k+i+1:m) = 0 and v(m-k+i) = 1; v(1:m-k+i-1) is stored on exit in A(ia:ia+m-k+i-2, ja+n-k+i-1), and tau in tau(ja+n-k+i-1).

Parent topic: Orthogonal Factorizations: ScaLAPACK Computational Routines

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