dgbtf2(3)

NAME

DGBTF2 - compute an LU factorization of a real m-by-n band

matrix A using partial pivoting with row interchanges

SYNOPSIS

SUBROUTINE DGBTF2( M, N, KL, KU, AB, LDAB, IPIV, INFO )
    INTEGER        INFO, KL, KU, LDAB, M, N
    INTEGER        IPIV( * )
    DOUBLE         PRECISION AB( LDAB, * )

PURPOSE

DGBTF2 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) DOUBLE PRECISION 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 factoriza

tion are stored in rows KL+KU+2 to 2*KL+KU+1. See below for fur

ther 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 ille

gal 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; ele

ments 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.

LAPACK version 3.0 15 June 2000

DGBTF2(3)