SRC/zlaunh...getrfnp2.f(3)

🌸 NAME 🌸

SRC/zlaunhr_col_getrfnp2.f

okay bestie, this page is from the Library Functions Manual — it's the manual page for SRC/zlaunhr_col_getrfnp2.f, aka the recursive modified-LU helper. It's giving numerical-stability main character energy. ✨

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🦋 SYNOPSIS 🦋

Functions/Subroutines

• recursive subroutine zlaunhr_col_getrfnp2 (m, n, a, lda, d, info) — ZLAUNHR_COL_GETRFNP2

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🧚 Function/Subroutine Documentation 🧚

Here's the whole signature, bestie:

recursive subroutine zlaunhr_col_getrfnp2 (integer m, integer n, complex*16,
    dimension( lda, * ) a, integer lda, complex*16, dimension( * ) d,
    integer info)

ZLAUNHR_COL_GETRFNP2

💡 Purpose 💡

okay so ZLAUNHR_COL_GETRFNP2 computes the modified LU factorization without pivoting of a complex general M-by-N matrix A. The factorization has the form:

A - S = L * U,

where:

• S is a m-by-n diagonal sign matrix with the diagonal D, so that D(i) = S(i,i), 1 <= i <= min(M,N). The diagonal D is constructed as D(i)=-SIGN(A(i,i)), where A(i,i) is the value after performing i-1 steps of Gaussian elimination. This means that the diagonal element at each step of Gaussian elimination is at least one in absolute value (so that division-by-zero not possible during the division by the diagonal element);
• L is a M-by-N lower triangular matrix with unit diagonal elements (lower trapezoidal if M > N);
• and U is a M-by-N upper triangular matrix (upper trapezoidal if M < N).

This routine is an auxiliary routine used in the Householder reconstruction routine ZUNHR_COL. In ZUNHR_COL, this routine is applied to an M-by-N matrix A with orthonormal columns, where each element is bounded by one in absolute value. With the choice of the matrix S above, one can show that the diagonal element at each step of Gaussian elimination is the largest (in absolute value) in the column on or below the diagonal, so that no pivoting is required for numerical stability [1].

For more details on the Householder reconstruction algorithm, including the modified LU factorization, see [1].

This is the recursive version of the LU factorization algorithm. Denote A - S by B. The algorithm divides the matrix B into four submatrices:

       [  B11 | B12  ]  where B11 is n1 by n1,
   B = [ -----|----- ]        B21 is (m-n1) by n1,
       [  B21 | B22  ]        B12 is n1 by n2,
                              B22 is (m-n1) by n2,
                              with n1 = min(m,n)/2, n2 = n-n1.

The subroutine calls itself to factor B11, solves for B21, solves for B12, updates B22, then calls itself to factor B22.

For more details on the recursive LU algorithm, see [2].

ZLAUNHR_COL_GETRFNP2 is called to factorize a block by the blocked routine ZLAUNHR_COL_GETRFNP, which uses blocked code calling Level 3 BLAS to update the submatrix. However, ZLAUNHR_COL_GETRFNP2 is self-sufficient and can be used without ZLAUNHR_COL_GETRFNP.

References, bestie:

• [1] — G. Ballard, J. Demmel, L. Grigori, M. Jacquelin, H.D. Nguyen, E. Solomonik, J. Parallel Distrib. Comput., vol. 85, pp. 3-31, 2015.
• [2] — F. Gustavson, IBM J. of Res. and Dev., vol. 41, no. 6, pp. 737-755, 1997.

🧮 Parameters 🧮

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 A. N >= 0.

A

A is COMPLEX*16 array, dimension (LDA,N). On entry, the M-by-N matrix to be factored. On exit, the factors L and U from the factorization A-S=L*U; the unit diagonal elements of L are not stored.

LDA

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

D

D is COMPLEX*16 array, dimension min(M,N). The diagonal elements of the diagonal M-by-N sign matrix S, D(i) = S(i,i), where 1 <= i <= min(M,N). The elements can be only ( +1.0, 0.0 ) or (-1.0, 0.0 ).

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.

🫶 Contributors 🫶

November 2019, Igor Kozachenko, Computer Science Division, University of California, Berkeley

Definition at line 166 of file zlaunhr_col_getrfnp2.f.

🤖 Author 🤖

Generated automatically by Doxygen for LAPACK from the source code.

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that's the whole page, bestie — the footer's source/version vibes are LAPACK Version 3.12.0 and SRC/zlaunh...getrfnp2.f(3). mwah 💋✨