Provided by: liblapack-doc_3.3.1-1_all NAME

LAPACK-3  -  improves  the  computed solution to a system of linear equations and provides
error bounds and backward error estimates for the solution

SYNOPSIS

SUBROUTINE DGERFS( TRANS, N, NRHS, A, LDA, AF, LDAF, IPIV, B, LDB,  X,  LDX,  FERR,  BERR,
WORK, IWORK, INFO )

CHARACTER      TRANS

INTEGER        INFO, LDA, LDAF, LDB, LDX, N, NRHS

INTEGER        IPIV( * ), IWORK( * )

DOUBLE         PRECISION  A(  LDA, * ), AF( LDAF, * ), B( LDB, * ), BERR( * ), FERR( *
), WORK( * ), X( LDX, * )

PURPOSE

DGERFS improves the computed solution to a system of linear equations and  provides  error
bounds and backward error estimates for the solution.

ARGUMENTS

TRANS   (input) 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       (input) INTEGER
The order of the matrix A.  N >= 0.

NRHS    (input) INTEGER
The number of right hand sides, i.e., the number of columns
of the matrices B and X.  NRHS >= 0.

A       (input) DOUBLE PRECISION array, dimension (LDA,N)
The original N-by-N matrix A.

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

AF      (input) DOUBLE PRECISION array, dimension (LDAF,N)
The factors L and U from the factorization A = P*L*U
as computed by DGETRF.

LDAF    (input) INTEGER
The leading dimension of the array AF.  LDAF >= max(1,N).

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

B       (input) DOUBLE PRECISION array, dimension (LDB,NRHS)
The right hand side matrix B.

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

X       (input/output) DOUBLE PRECISION array, dimension (LDX,NRHS)
On entry, the solution matrix X, as computed by DGETRS.
On exit, the improved solution matrix X.

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

FERR    (output) DOUBLE PRECISION 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    (output) DOUBLE PRECISION 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    (workspace) DOUBLE PRECISION array, dimension (3*N)

IWORK   (workspace) INTEGER array, dimension (N)

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

PARAMETERS

ITMAX is the maximum number of steps of iterative refinement.

LAPACK routine (version 3.2)               April 2011                            DGERFS(3lapack)