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sgtrfs.f(3) LAPACK sgtrfs.f(3)

NAME

sgtrfs.f -

SYNOPSIS

Functions/Subroutines


subroutine sgtrfs (TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF, DU2, IPIV, B, LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO)
SGTRFS

Function/Subroutine Documentation

subroutine sgtrfs (characterTRANS, integerN, integerNRHS, real, dimension( * )DL, real, dimension( * )D, real, dimension( * )DU, real, dimension( * )DLF, real, dimension( * )DF, real, dimension( * )DUF, real, dimension( * )DU2, integer, dimension( * )IPIV, real, dimension( ldb, * )B, integerLDB, real, dimension( ldx, * )X, integerLDX, real, dimension( * )FERR, real, dimension( * )BERR, real, dimension( * )WORK, integer, dimension( * )IWORK, integerINFO)

SGTRFS

Purpose:


SGTRFS improves the computed solution to a system of linear
equations when the coefficient matrix is tridiagonal, and provides
error bounds and backward error estimates for the solution.

Parameters:

TRANS


TRANS is CHARACTER*1
Specifies the form of the system of equations:
= 'N': A * X = B (No transpose)
= 'T': A**T * X = B (Transpose)
= 'C': A**H * X = B (Conjugate transpose = Transpose)

N


N is INTEGER
The order of the matrix A. N >= 0.

NRHS


NRHS is INTEGER
The number of right hand sides, i.e., the number of columns
of the matrix B. NRHS >= 0.

DL


DL is REAL array, dimension (N-1)
The (n-1) subdiagonal elements of A.

D


D is REAL array, dimension (N)
The diagonal elements of A.

DU


DU is REAL array, dimension (N-1)
The (n-1) superdiagonal elements of A.

DLF


DLF is REAL array, dimension (N-1)
The (n-1) multipliers that define the matrix L from the
LU factorization of A as computed by SGTTRF.

DF


DF is REAL array, dimension (N)
The n diagonal elements of the upper triangular matrix U from
the LU factorization of A.

DUF


DUF is REAL array, dimension (N-1)
The (n-1) elements of the first superdiagonal of U.

DU2


DU2 is REAL array, dimension (N-2)
The (n-2) elements of the second superdiagonal of U.

IPIV


IPIV is INTEGER array, dimension (N)
The pivot indices; for 1 <= i <= n, row i of the matrix was
interchanged with row IPIV(i). IPIV(i) will always be either
i or i+1; IPIV(i) = i indicates a row interchange was not
required.

B


B is REAL array, dimension (LDB,NRHS)
The right hand side matrix B.

LDB


LDB is INTEGER
The leading dimension of the array B. LDB >= max(1,N).

X


X is REAL array, dimension (LDX,NRHS)
On entry, the solution matrix X, as computed by SGTTRS.
On exit, the improved solution matrix X.

LDX


LDX is INTEGER
The leading dimension of the array X. LDX >= max(1,N).

FERR


FERR is REAL array, dimension (NRHS)
The estimated forward error bound for each solution vector
X(j) (the j-th column of the solution matrix X).
If XTRUE is the true solution corresponding to X(j), FERR(j)
is an estimated upper bound for the magnitude of the largest
element in (X(j) - XTRUE) divided by the magnitude of the
largest element in X(j). The estimate is as reliable as
the estimate for RCOND, and is almost always a slight
overestimate of the true error.

BERR


BERR is REAL array, dimension (NRHS)
The componentwise relative backward error of each solution
vector X(j) (i.e., the smallest relative change in
any element of A or B that makes X(j) an exact solution).

WORK


WORK is REAL array, dimension (3*N)

IWORK


IWORK is INTEGER array, dimension (N)

INFO


INFO is INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value

Internal Parameters:


ITMAX is the maximum number of steps of iterative refinement.

Author:

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Date:

September 2012

Definition at line 208 of file sgtrfs.f.

Author

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Tue Sep 25 2012 Version 3.4.2