NAG Library Function Document
nag_superlu_refine_lu (f11mhc)
1
Purpose
nag_superlu_refine_lu (f11mhc) returns error bounds for the solution of a real sparse system of linear equations with multiple right-hand sides, or . It improves the solution by iterative refinement in standard precision, in order to reduce the backward error as much as possible.
2
Specification
#include <nag.h> |
#include <nagf11.h> |
void |
nag_superlu_refine_lu (Nag_OrderType order,
Nag_TransType trans,
Integer n,
const Integer icolzp[],
const Integer irowix[],
const double a[],
const Integer iprm[],
const Integer il[],
const double lval[],
const Integer iu[],
const double uval[],
Integer nrhs,
const double b[],
Integer pdb,
double x[],
Integer pdx,
double ferr[],
double berr[],
NagError *fail) |
|
3
Description
nag_superlu_refine_lu (f11mhc) returns the backward errors and estimated bounds on the forward errors for the solution of a real system of linear equations with multiple right-hand sides or . The function handles each right-hand side vector (stored as a column of the matrix ) independently, so we describe the function of nag_superlu_refine_lu (f11mhc) in terms of a single right-hand side and solution .
Given a computed solution
, the function computes the
component-wise backward error
. This is the size of the smallest relative perturbation in each element of
and
such that if
is the exact solution of a perturbed system:
Then the function estimates a bound for the
component-wise forward error in the computed solution, defined by:
where
is the true solution.
The function uses the
factorization
computed by
nag_superlu_lu_factorize (f11mec) and the solution computed by
nag_superlu_solve_lu (f11mfc).
4
References
Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore
5
Arguments
- 1:
– Nag_OrderTypeInput
-
On entry: the
order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by
. See
Section 3.3.1.3 in How to Use the NAG Library and its Documentation for a more detailed explanation of the use of this argument.
Constraint:
or .
- 2:
– Nag_TransTypeInput
-
On entry: specifies whether
or
is solved.
- is solved.
- is solved.
Constraint:
or .
- 3:
– IntegerInput
-
On entry: , the order of the matrix .
Constraint:
.
- 4:
– const IntegerInput
-
Note: the dimension,
dim, of the array
icolzp
must be at least
.
On entry:
contains the index in
of the start of a new column. See
Section 2.1.3 in the f11 Chapter Introduction.
- 5:
– const IntegerInput
-
Note: the dimension,
dim, of the array
irowix
must be at least
, the number of nonzeros of the sparse matrix
.
On entry: the row index array of sparse matrix .
- 6:
– const doubleInput
-
Note: the dimension,
dim, of the array
a
must be at least
, the number of nonzeros of the sparse matrix
.
On entry: the array of nonzero values in the sparse matrix .
- 7:
– const IntegerInput
-
On entry: the column permutation which defines
, the row permutation which defines
, plus associated data structures as computed by
nag_superlu_lu_factorize (f11mec).
- 8:
– const IntegerInput
-
Note: the dimension,
dim, of the array
il
must be at least
as large as the dimension of the array of the same name in
nag_superlu_lu_factorize (f11mec).
On entry: records the sparsity pattern of matrix
as computed by
nag_superlu_lu_factorize (f11mec).
- 9:
– const doubleInput
-
Note: the dimension,
dim, of the array
lval
must be at least
as large as the dimension of the array of the same name in
nag_superlu_lu_factorize (f11mec).
On entry: records the nonzero values of matrix
and some nonzero values of matrix
as computed by
nag_superlu_lu_factorize (f11mec).
- 10:
– const IntegerInput
-
Note: the dimension,
dim, of the array
iu
must be at least
as large as the dimension of the array of the same name in
nag_superlu_lu_factorize (f11mec).
On entry: records the sparsity pattern of matrix
as computed by
nag_superlu_lu_factorize (f11mec).
- 11:
– const doubleInput
-
Note: the dimension,
dim, of the array
uval
must be at least
as large as the dimension of the array of the same name in
nag_superlu_lu_factorize (f11mec).
On entry: records some nonzero values of matrix
as computed by
nag_superlu_lu_factorize (f11mec).
- 12:
– IntegerInput
-
On entry: , the number of right-hand sides in .
Constraint:
.
- 13:
– const doubleInput
-
Note: the dimension,
dim, of the array
b
must be at least
- when
;
- when
.
The
th element of the matrix
is stored in
- when ;
- when .
On entry: the by right-hand side matrix .
- 14:
– IntegerInput
-
On entry: the stride separating row or column elements (depending on the value of
order) in the array
b.
Constraints:
- if ,
;
- if , .
- 15:
– doubleInput/Output
-
Note: the dimension,
dim, of the array
x
must be at least
- when
;
- when
.
The
th element of the matrix
is stored in
- when ;
- when .
On entry: the
by
solution matrix
, as returned by
nag_superlu_solve_lu (f11mfc).
On exit: the by improved solution matrix .
- 16:
– IntegerInput
-
On entry: the stride separating row or column elements (depending on the value of
order) in the array
x.
Constraints:
- if ,
;
- if , .
- 17:
– doubleOutput
-
On exit: contains an estimated error bound for the th solution vector, that is, the th column of , for .
- 18:
– doubleOutput
-
On exit: contains the component-wise backward error bound for the th solution vector, that is, the th column of , for .
- 19:
– NagError *Input/Output
-
The NAG error argument (see
Section 3.7 in How to Use the NAG Library and its Documentation).
6
Error Indicators and Warnings
- NE_ALLOC_FAIL
-
Dynamic memory allocation failed.
See
Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
- NE_BAD_PARAM
-
On entry, argument had an illegal value.
- NE_INT
-
On entry, .
Constraint: .
On entry, .
Constraint: .
On entry, .
Constraint: .
On entry, .
Constraint: .
- NE_INT_2
-
On entry, and .
Constraint: .
On entry, and .
Constraint: .
On entry, and .
Constraint: .
On entry, and .
Constraint: .
- NE_INTERNAL_ERROR
-
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact
NAG for assistance.
See
Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
- NE_INVALID_PERM_COL
-
Incorrect column permutations in array
iprm.
- NE_INVALID_PERM_ROW
-
Incorrect row permutations in array
iprm.
- NE_NO_LICENCE
-
Your licence key may have expired or may not have been installed correctly.
See
Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
7
Accuracy
The bounds returned in
ferr are not rigorous, because they are estimated, not computed exactly; but in practice they almost always overestimate the actual error.
8
Parallelism and Performance
nag_superlu_refine_lu (f11mhc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_superlu_refine_lu (f11mhc) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the
x06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the
Users' Note for your implementation for any additional implementation-specific information.
At most five steps of iterative refinement are performed, but usually only one or two steps are required.
Estimating the forward error involves solving a number of systems of linear equations of the form or ;
10
Example
This example solves the system of equations
using iterative refinement and to compute the forward and backward error bounds, where
Here
is nonsymmetric and must first be factorized by
nag_superlu_lu_factorize (f11mec).
10.1
Program Text
Program Text (f11mhce.c)
10.2
Program Data
Program Data (f11mhce.d)
10.3
Program Results
Program Results (f11mhce.r)