CLAMTSQR(3)

🌸 CLAMTSQR(3) — Library Functions Manual 🌸

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🎀 NAME 🎀

/home/abuild/rpmbuild/BUILD/lapack-static-3.12.0-build/lapack-3.12.0/SRC/clamtsqr.f

That's literally the NAME section — just the source path, no cap.

✨ SYNOPSIS ✨

🫶 Functions/Subroutines

• subroutine CLAMTSQR (side, trans, m, n, k, mb, nb, a, lda, t, ldt, c, ldc, work, lwork, info) — CLAMTSQR

💖 Function/Subroutine Documentation 💖

Here's the full signature, verbatim, because we do not mess with code:

subroutine CLAMTSQR (character side, character trans, integer m, integer n, integer k, integer mb, integer nb, complex, dimension( lda, * ) a, integer lda, complex, dimension( ldt, * ) t, integer ldt, complex, dimension(ldc, * ) c, integer ldc, complex, dimension( * ) work, integer lwork, integer info)

CLAMTSQR

🎯 Purpose

CLAMTSQR overwrites the general complex M-by-N matrix C with:

                SIDE = 'L'     SIDE = 'R'
TRANS = 'N':      Q * C          C * Q
TRANS = 'C':      Q**H * C       C * Q**H

where Q is a complex unitary matrix defined as the product of blocked elementary reflectors computed by tall skinny QR factorization (CLATSQR).

🌈 Parameters

SIDE

SIDE is CHARACTER*1

• = 'L': apply Q or Q**H from the Left;
• = 'R': apply Q or Q**H from the Right.

TRANS

TRANS is CHARACTER*1

• = 'N': No transpose, apply Q;
• = 'C': Conjugate Transpose, apply Q**H.

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. N >= 0.

K

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

MB

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

NB

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

A

A is COMPLEX 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 CLATSQR 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 COMPLEX 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 COMPLEX array, dimension (LDC,N) On entry, the M-by-N matrix C. On exit, C is overwritten by Q*C or Q**H*C or C*Q**H or C*Q.

LDC

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

WORK

(workspace) COMPLEX 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 unitary transformations, representing Q as a product of other unitary 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 clamtsqr.f.

📝 Author

Generated automatically by Doxygen for LAPACK from the source code.

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