NAG Library Function Document
nag_dgtrfs (f07chc)
1
Purpose
nag_dgtrfs (f07chc) computes error bounds and refines the solution to a real system of linear equations
or
, where
is an
by
tridiagonal matrix and
and
are
by
matrices, using the
factorization returned by
nag_dgttrf (f07cdc) and an initial solution returned by
nag_dgttrs (f07cec). Iterative refinement is used to reduce the backward error as much as possible.
2
Specification
#include <nag.h> |
#include <nagf07.h> |
void |
nag_dgtrfs (Nag_OrderType order,
Nag_TransType trans,
Integer n,
Integer nrhs,
const double dl[],
const double d[],
const double du[],
const double dlf[],
const double df[],
const double duf[],
const double du2[],
const Integer ipiv[],
const double b[],
Integer pdb,
double x[],
Integer pdx,
double ferr[],
double berr[],
NagError *fail) |
|
3
Description
nag_dgtrfs (f07chc) should normally be preceded by calls to
nag_dgttrf (f07cdc) and
nag_dgttrs (f07cec).
nag_dgttrf (f07cdc) uses Gaussian elimination with partial pivoting and row interchanges to factorize the matrix
as
where
is a permutation matrix,
is unit lower triangular with at most one nonzero subdiagonal element in each column, and
is an upper triangular band matrix, with two superdiagonals.
nag_dgttrs (f07cec) then utilizes the factorization to compute a solution,
, to the required equations. Letting
denote a column of
,
nag_dgtrfs (f07chc) computes a
component-wise backward error,
, the smallest relative perturbation in each element of
and
such that
is the exact solution of a perturbed system
The function also estimates a bound for the component-wise forward error in the computed solution defined by , where is the corresponding column of the exact solution, .
4
References
Anderson E, Bai Z, Bischof C, Blackford S, Demmel J, Dongarra J J, Du Croz J J, Greenbaum A, Hammarling S, McKenney A and Sorensen D (1999)
LAPACK Users' Guide (3rd Edition) SIAM, Philadelphia
http://www.netlib.org/lapack/lug
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 the equations to be solved as follows:
- Solve for .
- or
- Solve for .
Constraint:
, or .
- 3:
– IntegerInput
-
On entry: , the order of the matrix .
Constraint:
.
- 4:
– IntegerInput
-
On entry: , the number of right-hand sides, i.e., the number of columns of the matrix .
Constraint:
.
- 5:
– const doubleInput
-
Note: the dimension,
dim, of the array
dl
must be at least
.
On entry: must contain the subdiagonal elements of the matrix .
- 6:
– const doubleInput
-
Note: the dimension,
dim, of the array
d
must be at least
.
On entry: must contain the diagonal elements of the matrix .
- 7:
– const doubleInput
-
Note: the dimension,
dim, of the array
du
must be at least
.
On entry: must contain the superdiagonal elements of the matrix .
- 8:
– const doubleInput
-
Note: the dimension,
dim, of the array
dlf
must be at least
.
On entry: must contain the multipliers that define the matrix of the factorization of .
- 9:
– const doubleInput
-
Note: the dimension,
dim, of the array
df
must be at least
.
On entry: must contain the diagonal elements of the upper triangular matrix from the factorization of .
- 10:
– const doubleInput
-
Note: the dimension,
dim, of the array
duf
must be at least
.
On entry: must contain the elements of the first superdiagonal of .
- 11:
– const doubleInput
-
Note: the dimension,
dim, of the array
du2
must be at least
.
On entry: must contain the elements of the second superdiagonal of .
- 12:
– const IntegerInput
-
Note: the dimension,
dim, of the array
ipiv
must be at least
.
On entry: must contain the pivot indices that define the permutation matrix . At the th step, row of the matrix was interchanged with row , and must always be either or , indicating that a row interchange was not performed.
- 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 matrix of right-hand sides .
- 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 initial solution matrix .
On exit: the by refined 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: estimate of the forward error bound for each computed solution vector, such that
, where
is the
th column of the computed solution returned in the array
x and
is the corresponding column of the exact solution
. The estimate is almost always a slight overestimate of the true error.
- 18:
– doubleOutput
-
On exit: estimate of the component-wise relative backward error of each computed solution vector (i.e., the smallest relative change in any element of or that makes an exact solution).
- 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_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 computed solution for a single right-hand side,
, satisfies an equation of the form
where
and
is the
machine precision. An approximate error bound for the computed solution is given by
where
, the condition number of
with respect to the solution of the linear equations. See Section 4.4 of
Anderson et al. (1999) for further details.
Function
nag_dgtcon (f07cgc) can be used to estimate the condition number of
.
8
Parallelism and Performance
nag_dgtrfs (f07chc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_dgtrfs (f07chc) 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.
The total number of floating-point operations required to solve the equations or is proportional to . At most five steps of iterative refinement are performed, but usually only one or two steps are required.
The complex analogue of this function is
nag_zgtrfs (f07cvc).
10
Example
This example solves the equations
where
is the tridiagonal matrix
Estimates for the backward errors and forward errors are also output.
10.1
Program Text
Program Text (f07chce.c)
10.2
Program Data
Program Data (f07chce.d)
10.3
Program Results
Program Results (f07chce.r)