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slamtsqr.f(3) LAPACK slamtsqr.f(3)

NAME

slamtsqr.f

SYNOPSIS

Functions/Subroutines


subroutine slamtsqr (SIDE, TRANS, M, N, K, MB, NB, A, LDA, T, LDT, C, LDC, WORK, LWORK, INFO)

Function/Subroutine Documentation

subroutine slamtsqr (character SIDE, character TRANS, integer M, integer N, integer K, integer MB, integer NB, real, dimension( lda, * ) A, integer LDA, real, dimension( ldt, * ) T, integer LDT, real, dimension(ldc, * ) C, integer LDC, real, dimension( * ) WORK, integer LWORK, integer INFO)

Purpose:

SLAMTSQR overwrites the general real M-by-N matrix C with

SIDE = 'L' SIDE = 'R' TRANS = 'N': Q * C C * Q TRANS = 'T': Q**T * C C * Q**T where Q is a real orthogonal matrix defined as the product of blocked elementary reflectors computed by tall skinny QR factorization (DLATSQR)

Parameters:

SIDE


SIDE is CHARACTER*1
= 'L': apply Q or Q**T from the Left;
= 'R': apply Q or Q**T from the Right.

TRANS


TRANS is CHARACTER*1
= 'N': No transpose, apply Q;
= 'T': Transpose, apply Q**T.

M


M is INTEGER
The number of rows of the matrix A. M >=0.

N


N is INTEGER
The number of columns of the matrix C. M >= N >= 0.

K


K is INTEGER
The number of elementary reflectors whose product defines
the matrix Q.
N >= K >= 0;

MB


MB is INTEGER
The block size to be used in the blocked QR.
MB > N. (must be the same as DLATSQR)

NB


NB is INTEGER
The column block size to be used in the blocked QR.
N >= NB >= 1.

A


A is REAL array, dimension (LDA,K)
The i-th column must contain the vector which defines the
blockedelementary reflector H(i), for i = 1,2,...,k, as
returned by DLATSQR in the first k columns of
its array argument A.

LDA


LDA is INTEGER
The leading dimension of the array A.
If SIDE = 'L', LDA >= max(1,M);
if SIDE = 'R', LDA >= max(1,N).

T


T is REAL array, dimension
( N * Number of blocks(CEIL(M-K/MB-K)),
The blocked upper triangular block reflectors stored in compact form
as a sequence of upper triangular blocks. See below
for further details.

LDT


LDT is INTEGER
The leading dimension of the array T. LDT >= NB.

C


C is REAL array, dimension (LDC,N)
On entry, the M-by-N matrix C.
On exit, C is overwritten by Q*C or Q**T*C or C*Q**T or C*Q.

LDC


LDC is INTEGER
The leading dimension of the array C. LDC >= max(1,M).

WORK


(workspace) REAL array, dimension (MAX(1,LWORK))

LWORK


LWORK is INTEGER
The dimension of the array WORK.
If SIDE = 'L', LWORK >= max(1,N)*NB;
if SIDE = 'R', LWORK >= max(1,MB)*NB.
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


INFO is INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value

Author:

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Further Details:

Tall-Skinny QR (TSQR) performs QR by a sequence of orthogonal transformations, representing Q as a product of other orthogonal matrices Q = Q(1) * Q(2) * . . . * Q(k) where each Q(i) zeros out subdiagonal entries of a block of MB rows of A: Q(1) zeros out the subdiagonal entries of rows 1:MB of A Q(2) zeros out the bottom MB-N rows of rows [1:N,MB+1:2*MB-N] of A Q(3) zeros out the bottom MB-N rows of rows [1:N,2*MB-N+1:3*MB-2*N] of A . . .

Q(1) is computed by GEQRT, which represents Q(1) by Householder vectors stored under the diagonal of rows 1:MB of A, and by upper triangular block reflectors, stored in array T(1:LDT,1:N). For more information see Further Details in GEQRT.

Q(i) for i>1 is computed by TPQRT, which represents Q(i) by Householder vectors stored in rows [(i-1)*(MB-N)+N+1:i*(MB-N)+N] of A, and by upper triangular block reflectors, stored in array T(1:LDT,(i-1)*N+1:i*N). The last Q(k) may use fewer rows. For more information see Further Details in TPQRT.

For more details of the overall algorithm, see the description of Sequential TSQR in Section 2.2 of [1].

[1] “Communication-Optimal Parallel and Sequential QR and LU Factorizations,” J. Demmel, L. Grigori, M. Hoemmen, J. Langou, SIAM J. Sci. Comput, vol. 34, no. 1, 2012

Definition at line 197 of file slamtsqr.f.

Author

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Tue Nov 14 2017 Version 3.8.0