dgbsv (l)  Linux Man Pages
dgbsv: computes the solution to a real system of linear equations A * X = B, where A is a band matrix of order N with KL subdiagonals and KU superdiagonals, and X and B are NbyNRHS matrices
NAME
DGBSV  computes the solution to a real system of linear equations A * X = B, where A is a band matrix of order N with KL subdiagonals and KU superdiagonals, and X and B are NbyNRHS matricesSYNOPSIS
 SUBROUTINE DGBSV(
 N, KL, KU, NRHS, AB, LDAB, IPIV, B, LDB, INFO )
 INTEGER INFO, KL, KU, LDAB, LDB, N, NRHS
 INTEGER IPIV( * )
 DOUBLE PRECISION AB( LDAB, * ), B( LDB, * )
PURPOSE
DGBSV computes the solution to a real system of linear equations A * X = B, where A is a band matrix of order N with KL subdiagonals and KU superdiagonals, and X and B are NbyNRHS matrices. The LU decomposition with partial pivoting and row interchanges is used to factor A as A = L * U, where L is a product of permutation and unit lower triangular matrices with KL subdiagonals, and U is upper triangular with KL+KU superdiagonals. The factored form of A is then used to solve the system of equations A * X = B.ARGUMENTS
 N (input) INTEGER
 The number of linear equations, i.e., the order 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.
 NRHS (input) INTEGER
 The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 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 jth column of A is stored in the jth column of the array AB as follows: AB(KL+KU+1+ij,j) = A(i,j) for max(1,jKU)<=i<=min(N,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 (N)
 The pivot indices that define the permutation matrix P; row i of the matrix was interchanged with row IPIV(i).
 B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
 On entry, the NbyNRHS right hand side matrix B. On exit, if INFO = 0, the NbyNRHS solution matrix X.
 LDB (input) INTEGER
 The leading dimension of the array B. LDB >= max(1,N).
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith 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 the solution has not been computed.
FURTHER DETAILS
The band storage scheme is illustrated by the following example, when M = N = 6, KL = 2, KU = 1:On entry: On exit:
a11
a21
a31