sgbtf2(3P)

NAME

sgbtf2 - compute an LU factorization of a real m-by-n band matrix A using partial pivoting with row interchanges

SYNOPSIS

SUBROUTINE SGBTF2( M, N, KL, KU, AB, LDAB, IPIV, INFO )

INTEGER INFO, KL, KU, LDAB, M, N

INTEGER IPIV( ∗ )

REAL AB( LDAB, ∗ )

#include <sunperf.h>

void sgbtf2(int m, int n, int kl, int ku, float ∗sab, int ldab, int ∗ipivot, int ∗info) ;

PURPOSE

SGBTF2 computes an LU factorization of a real m-by-n band matrix A using partial pivoting with row interchanges.

This is the unblocked version of the algorithm, calling Level 2 BLAS.

ARGUMENTS

M (input) INTEGER
The number of rows of the matrix A. M >= 0.

N (input) INTEGER
The number of columns of the matrix A. N >= 0.

KL (input) INTEGER
The number of subdiagonals within the band of A. KL >= 0.

KU (input) INTEGER
The number of superdiagonals within the band of A. KU >= 0.

AB (input/output) REAL array, dimension (LDAB,N)
On entry, the matrix A in band storage, in rows KL+1 to 2∗KL+KU+1; rows 1 to KL of the array need not be set. The j-th column of A is stored in the j-th column of the array AB as follows: AB(kl+ku+1+i-j,j) = A(i,j) for max(1,j-ku)<=i<=min(m,j+kl)

On exit, details of the factorization: U is stored as an upper triangular band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and the multipliers used during the factorization are stored in rows KL+KU+2 to 2∗KL+KU+1. See below for further details.

LDAB (input) INTEGER
The leading dimension of the array AB. LDAB >= 2∗KL+KU+1.

IPIV (output) INTEGER array, dimension (min(M,N))
The pivot indices; for 1 <= i <= min(M,N), row i of the matrix was interchanged with row IPIV(i).

INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = +i, U(i,i) is exactly zero. The factorization has been completed, but the factor U is exactly singular, and division by zero will occur if it is used to solve a system of equations.

FURTHER DETAILS

The band storage scheme is illustrated by the following example, when M = N = 6, KL = 2, KU = 1:

On entry:                       On exit:

   ∗   ∗   ∗   +   +   +       ∗   ∗   ∗ u14 u25 u36
   ∗   ∗   +   +   +   +       ∗   ∗ u13 u24 u35 u46
   ∗ a12 a23 a34 a45 a56      ∗ u12 u23 u34 u45 u56
a11 a22 a33 a44 a55 a66     u11 u22 u33 u44 u55 u66
a21 a32 a43 a54 a65 ∗      m21 m32 m43 m54 m65 ∗
a31 a42 a53 a64 ∗   ∗      m31 m42 m53 m64 ∗   ∗

Array elements marked ∗ are not used by the routine; elements marked + need not be set on entry, but are required by the routine to store elements of U, because of fill-in resulting from the row
interchanges.