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Chapter Contents

Chapter Introduction

NAG Toolbox

NAG Toolbox: nag_lapack_dsytrs (f07me)

▸▿ Contents

1 Purpose

2 Syntax

3 Description

4 References

▸▿ 5 Parameters

5.1 Compulsory Input Parameters

5.2 Optional Input Parameters

5.3 Output Parameters

6 Error Indicators and Warnings

7 Accuracy

8 Further Comments

9 Example

Purpose

nag_lapack_dsytrs (f07me) solves a real symmetric indefinite system of linear equations with multiple right-hand sides,

A X = B,

where

A

has been factorized by nag_lapack_dsytrf (f07md).

Syntax

[b, info] = f07me(uplo, a, ipiv, b, 'n', n, 'nrhs_p', nrhs_p)

[b, info] = nag_lapack_dsytrs(uplo, a, ipiv, b, 'n', n, 'nrhs_p', nrhs_p)

Description

nag_lapack_dsytrs (f07me) is used to solve a real symmetric indefinite system of linear equations

A X = B

, this function must be preceded by a call to nag_lapack_dsytrf (f07md) which computes the Bunch–Kaufman factorization of

A

uplo ='U'

A = P U D U^{T} P^{T}

, where

P

is a permutation matrix,

U

is an upper triangular matrix and

D

is a symmetric block diagonal matrix with

1

1

and

2

2

blocks; the solution

X

is computed by solving

P U D Y = B

and then

U^{T} P^{T} X = Y

uplo ='L'

A = P L D L^{T} P^{T}

, where

L

is a lower triangular matrix; the solution

X

is computed by solving

P L D Y = B

and then

L^{T} P^{T} X = Y

References

Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

Parameters

Compulsory Input Parameters

1: $uplo$ – string (length ≥ 1)

Specifies how

A

has been factorized.

$uplo ='U'$: $A = P U D U^{T} P^{T}$ , where $U$ is upper triangular.
$uplo ='L'$: $A = P L D L^{T} P^{T}$ , where $L$ is lower triangular.

Constraint:

uplo ='U'

'L'

2: $a (lda :)$ – double array

The first dimension of the array a must be at least

\max (1, n)

The second dimension of the array a must be at least

\max (1, n)

Details of the factorization of

A

, as returned by nag_lapack_dsytrf (f07md).

3: $ipiv (:)$ – int64int32nag_int array

The dimension of the array ipiv must be at least

\max (1, n)

Details of the interchanges and the block structure of

D

, as returned by nag_lapack_dsytrf (f07md).

4: $b (ldb :)$ – double array

The first dimension of the array b must be at least

\max (1, n)

The second dimension of the array b must be at least

\max (1, nrhs_p)

The

n

r

right-hand side matrix

B

Optional Input Parameters

1: $n$ – int64int32nag_int scalar: Default: the first dimension of the arrays a, b and the second dimension of the arrays a, ipiv.
$n$ , the order of the matrix $A$ .

Constraint: $n \geq 0$ .
2: $nrhs_p$ – int64int32nag_int scalar: Default: the second dimension of the array b.
$r$ , the number of right-hand sides.

Constraint: $nrhs_p \geq 0$ .

Output Parameters

1: $b (ldb :)$ – double array: The first dimension of the array b will be $\max (1, n)$ .
The second dimension of the array b will be $\max (1, nrhs_p)$ .

The $n$ by $r$ solution matrix $X$ .
2: $info$ – int64int32nag_int scalar: $info = 0$ unless the function detects an error (see Error Indicators and Warnings).

Error Indicators and Warnings

$info < 0$: If $info = - i$ , argument $i$ had an illegal value. An explanatory message is output, and execution of the program is terminated.

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

if $uplo ='U'$ , $|E| \leq c (n) ε P |U| |D| |U^{T}| P^{T}$ ;
if $uplo ='L'$ , $|E| \leq c (n) ε P |L| |D| |L^{T}| P^{T}$ ,

c (n)

is a modest linear function of

n

, and

ε

is the machine precision

\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)

Forward and backward error bounds can be computed by calling nag_lapack_dsyrfs (f07mh), and an estimate for

κ_{\infty} (A)

(

= κ_{1} (A)

) can be obtained by calling nag_lapack_dsycon (f07mg).

Further Comments

The total number of floating-point operations is approximately

2 n^{2} r

This function may be followed by a call to nag_lapack_dsyrfs (f07mh) to refine the solution and return an error estimate.

The complex analogues of this function are nag_lapack_zhetrs (f07ms) for Hermitian matrices and nag_lapack_zsytrs (f07ns) for symmetric matrices.

Example

This example solves the system of equations

A X = B

, where

A = (\begin{matrix} 2.07 & 3.87 & 4.20 & - 1.15 \\ 3.87 & - 0.21 & 1.87 & 0.63 \\ 4.20 & 1.87 & 1.15 & 2.06 \\ - 1.15 & 0.63 & 2.06 & - 1.81 \end{matrix}) and B = (\begin{matrix} - 9.50 & 27.85 \\ - 8.38 & 9.90 \\ - 6.07 & 19.25 \\ - 0.96 & 3.93 \end{matrix}) .

Here

A

is symmetric indefinite and must first be factorized by nag_lapack_dsytrf (f07md).

Open in the MATLAB editor: f07me_example

function f07me_example


fprintf('f07me example results\n\n');

% Indefinite matrix A (lower triangular part stored)
uplo = 'L';
a = [ 2.07,  0,    0,     0; 
      3.87, -0.21, 0,     0;
      4.20,  1.87, 1.15,  0;
     -1.15,  0.63, 2.06, -1.81];

% RHS
b = [-9.50, 27.85;
     -8.38,  9.90;
     -6.07, 19.25;
     -0.96,  3.93];

% Factorize
[af, ipiv, info] = f07md( ...
                          uplo, a);

% Solve
[x, info] = f07me( ...
                   uplo, af, ipiv, b);

disp('Solution(s)');
disp(x);

f07me example results

Solution(s)
   -4.0000    1.0000
   -1.0000    4.0000
    2.0000    3.0000
    5.0000    2.0000

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Chapter Contents

Chapter Introduction

NAG Toolbox