NAG Library Function Document

nag_superlu_solve_lu (f11mfc)

void	nag_superlu_solve_lu (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)

3 Description

nag_superlu_solve_lu (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 nag_superlu_lu_factorize (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_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

order = Nag_RowMajor

. See Section 3.2.1.3 in the Essential Introduction for a more detailed explanation of the use of this argument.

Constraint:

order = Nag_RowMajor

Nag_ColMajor

2: trans – Nag_TransTypeInput

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 – IntegerInput

On entry:

n

, the order of the matrix

A

Constraint:

n \geq 0

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

On entry: the column permutation which defines

P_{c}

, the row permutation which defines

P_{r}

, plus associated data structures as computed by nag_superlu_lu_factorize (f11mec).

5: il[ $\dim$ ] – 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

L

as computed by nag_superlu_lu_factorize (f11mec).

6: lval[ $\dim$ ] – 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

L

and some nonzero values of matrix

U

as computed by nag_superlu_lu_factorize (f11mec).

7: iu[ $\dim$ ] – 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

U

as computed by nag_superlu_lu_factorize (f11mec).

8: uval[ $\dim$ ] – 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

U

as computed by nag_superlu_lu_factorize (f11mec).

9: nrhs – IntegerInput

On entry:

nrhs

, the number of right-hand sides in

B

Constraint:

nrhs \geq 0

10: b[ $\dim$ ] – doubleInput/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 – IntegerInput

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 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_ALLOC_FAIL: Dynamic memory allocation failed.
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.
NE_INVALID_PERM_COL: Incorrect column permutations in array iprm.
NE_INVALID_PERM_ROW: Incorrect Row Permutations in array iprm.

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 nag_superlu_refine_lu (f11mhc), and an estimate for

κ_{\infty} (A)

can be obtained by calling nag_superlu_condition_number_lu (f11mgc).

8 Parallelism and Performance

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

nag_superlu_solve_lu (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 Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

nag_superlu_solve_lu (f11mfc) may be followed by a call to nag_superlu_refine_lu (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 nag_superlu_lu_factorize (f11mec).

NAG Library Function Documentnag_superlu_solve_lu (f11mfc)

+− 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 Library Function Document

nag_superlu_solve_lu (f11mfc)