void	f11mfc (Nag_OrderType order, Nag_TransType trans, Integer n, const Integer iprm[], const Integer il[], const double lval[], const Integer iu[], const double uval[], Integer nrhs, double b[], Integer pdb, NagError *fail)

The function may be called by the names: f11mfc, nag_sparse_direct_real_gen_solve or nag_superlu_solve_lu.

3 Description

f11mfc solves a real system of linear equations with multiple right-hand sides

A X = B

A^{T} X = B

, according to the value of the argument trans, where the matrix factorization

P_{r} A P_{c} = L U

corresponds to an

L U

decomposition of a sparse matrix stored in compressed column (Harwell–Boeing) format, as computed by f11mec.

In the above decomposition

L

is a lower triangular sparse matrix with unit diagonal elements and

U

is an upper triangular sparse matrix;

P_{r}

and

P_{c}

are permutation matrices.

4 References

None.

5 Arguments

1: $order$ – Nag_OrderType Input

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

order = Nag_RowMajor

. See Section 3.1.3 in the Introduction to the NAG Library CL Interface for a more detailed explanation of the use of this argument.

Constraint:

order = Nag_RowMajor

Nag_ColMajor

2: $trans$ – Nag_TransType Input

On entry: specifies whether

A X = B

A^{T} X = B

is solved.

$trans = Nag_NoTrans$: $A X = B$ is solved.
$trans = Nag_Trans$: $A^{T} X = B$ is solved.

Constraint:

trans = Nag_NoTrans

Nag_Trans

3: $n$ – Integer Input

On entry:

n

, the order of the matrix

A

Constraint:

n \geq 0

4: $iprm [7 \times n]$ – const Integer Input

On entry: the column permutation which defines

P_{c}

, the row permutation which defines

P_{r}

, plus associated data structures as computed by f11mec.

5: $il [\dim]$ – const Integer Input

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 f11mec.

On entry: records the sparsity pattern of matrix

L

as computed by f11mec.

6: $lval [\dim]$ – const double Input

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 f11mec.

On entry: records the nonzero values of matrix

L

and some nonzero values of matrix

U

as computed by f11mec.

7: $iu [\dim]$ – const Integer Input

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 f11mec.

On entry: records the sparsity pattern of matrix

U

as computed by f11mec.

8: $uval [\dim]$ – const double Input

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 f11mec.

On entry: records some nonzero values of matrix

U

as computed by f11mec.

9: $nrhs$ – Integer Input

On entry:

nrhs

, the number of right-hand sides in

B

Constraint:

nrhs \geq 0

10: $b [\dim]$ – double Input/Output

Note: the dimension, dim, of the array b must be at least

$\max (1, pdb \times nrhs)$ when $order = Nag_ColMajor$ ;
$\max (1, n \times pdb)$ when $order = Nag_RowMajor$ .

The

(i, j)

th element of the matrix

B

is stored in

$b [(j - 1) \times pdb + i - 1]$ when $order = Nag_ColMajor$ ;
$b [(i - 1) \times pdb + j - 1]$ when $order = Nag_RowMajor$ .

On entry: the

n

nrhs

right-hand side matrix

B

On exit: the

n

nrhs

solution matrix

X

11: $pdb$ – Integer Input

On entry: the stride separating row or column elements (depending on the value of order) in the array b.

Constraints:

if $order = Nag_ColMajor$ , $pdb \geq \max (1, n)$ ;
if $order = Nag_RowMajor$ , $pdb \geq \max (1, nrhs)$ .

12: $fail$ – NagError * Input/Output

The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

NE_ALLOC_FAIL: Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
NE_BAD_PARAM: On entry, argument $〈value〉$ had an illegal value.
NE_INT: On entry, $n = 〈value〉$ .
Constraint: $n \geq 0$ .

On entry, $nrhs = 〈value〉$ .
Constraint: $nrhs \geq 0$ .

On entry, $pdb = 〈value〉$ .
Constraint: $pdb > 0$ .
NE_INT_2: On entry, $pdb = 〈value〉$ and $n = 〈value〉$ .
Constraint: $pdb \geq \max (1, n)$ .

On entry, $pdb = 〈value〉$ and $nrhs = 〈value〉$ .
Constraint: $pdb \geq \max (1, nrhs)$ .
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 7.5 in the Introduction to the NAG Library CL Interface 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 8 in the Introduction to the NAG Library CL Interface for further information.

7 Accuracy

For each right-hand side vector

b

, the computed solution

x

is the exact solution of a perturbed system of equations

(A + E) x = b

, where

|E| \leq c (n) ε |L| |U|,

c (n)

is a modest linear function of

n

, and

ε

is the machine precision, when partial pivoting is used.

\hat{x}

is the true solution, then the computed solution

x

satisfies a forward error bound of the form

\frac{{‖x - \hat{x}‖}_{\infty}}{{‖x‖}_{\infty}} \leq c (n) cond (A, x) ε

where

cond (A, x) = {‖|A^{- 1}| |A| |x|‖}_{\infty} / {‖x‖}_{\infty} \leq cond (A) = {‖|A^{- 1}| |A|‖}_{\infty} \leq κ_{\infty} (A)

. Note that

cond (A, x)

can be much smaller than

cond (A)

, and

cond (A^{T})

can be much larger (or smaller) than

cond (A)

Forward and backward error bounds can be computed by calling f11mhc, and an estimate for

κ_{\infty} (A)

can be obtained by calling f11mgc.

8 Parallelism and Performance

f11mfc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

f11mfc 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.

9 Further Comments

f11mfc may be followed by a call to f11mhc to refine the solution and return an error estimate.

10 Example

This example solves the system of equations

A X = B

, where

A = (\begin{matrix} 2.00 & 1.00 & 0 & 0 & 0 \\ 0 & 0 & 1.00 & - 1.00 & 0 \\ 4.00 & 0 & 1.00 & 0 & 1.00 \\ 0 & 0 & 0 & 1.00 & 2.00 \\ 0 & - 2.00 & 0 & 0 & 3.00 \end{matrix}) and B = (\begin{matrix} 1.56 & 3.12 \\ - 0.25 & - 0.50 \\ 3.60 & 7.20 \\ 1.33 & 2.66 \\ 0.52 & 1.04 \end{matrix}) .

Here

A

is nonsymmetric and must first be factorized by f11mec.

Interfaces: FL CL AD

NAG CL Interface Introduction

F11 (Sparse) Chapter Contents

F11 (Sparse) Chapter Introduction

f11mf: FL CL AD

NAG CL Interfacef11mfc (direct_​real_​gen_​solve)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG CL Interface
f11mfc (direct_real_gen_solve)