NAG Library Routine Document

F07QVF (ZSPRFS)

INTEGER	N, NRHS, IPIV(*), LDB, LDX, INFO
REAL (KIND=nag_wp)	FERR(NRHS), BERR(NRHS), RWORK(N)
COMPLEX (KIND=nag_wp)	AP(), AFP(), B(LDB,), X(LDX,), WORK(2*N)
CHARACTER(1)	UPLO

The routine may be called by its LAPACK name zsprfs.

3 Description

F07QVF (ZSPRFS) returns the backward errors and estimated bounds on the forward errors for the solution of a complex symmetric system of linear equations with multiple right-hand sides

A X = B

, using packed storage. The routine handles each right-hand side vector (stored as a column of the matrix

B

) independently, so we describe the function of F07QVF (ZSPRFS) in terms of a single right-hand side

b

and solution

x

Given a computed solution

x

, the routine computes the component-wise backward error

β

. This is the size of the smallest relative perturbation in each element of

A

and

b

such that

x

is the exact solution of a perturbed system

\begin{matrix} (A + δ A) x = b + δ b \\ |δ a_{i j}| \leq β |a_{i j}| and |δ b_{i}| \leq β |b_{i}| . \end{matrix}

Then the routine estimates a bound for the component-wise forward error in the computed solution, defined by:

\max_{i} |x_{i} - {\hat{x}}_{i}| / \max_{i} |x_{i}|

where

\hat{x}

is the true solution.

For details of the method, see the F07 Chapter Introduction.

4 References

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

5 Parameters

1: UPLO – CHARACTER(1)Input

On entry: specifies whether the upper or lower triangular part of

A

is stored and how

A

is to be factorized.

$UPLO ='U'$: The upper triangular part of $A$ is stored and $A$ is factorized as $P U D U^{T} P^{T}$ , where $U$ is upper triangular.
$UPLO ='L'$: The lower triangular part of $A$ is stored and $A$ is factorized as $P L D L^{T} P^{T}$ , where $L$ is lower triangular.

Constraint:

UPLO ='U'

'L'

2: N – INTEGERInput

On entry:

n

, the order of the matrix

A

Constraint:

N \geq 0

3: NRHS – INTEGERInput

On entry:

r

, the number of right-hand sides.

Constraint:

NRHS \geq 0

4: AP( $*$ ) – COMPLEX (KIND=nag_wp) arrayInput

Note: the dimension of the array AP must be at least

\max (1, N \times (N + 1) / 2)

On entry: the

n

n

original symmetric matrix

A

as supplied to F07QRF (ZSPTRF).

5: AFP( $*$ ) – COMPLEX (KIND=nag_wp) arrayInput

Note: the dimension of the array AFP must be at least

\max (1, N \times (N + 1) / 2)

On entry: the factorization of

A

stored in packed form, as returned by F07QRF (ZSPTRF).

6: IPIV( $*$ ) – INTEGER arrayInput

Note: the dimension of the array IPIV must be at least

\max (1, N)

On entry: details of the interchanges and the block structure of

D

, as returned by F07QRF (ZSPTRF).

7: B(LDB, $*$ ) – COMPLEX (KIND=nag_wp) arrayInput

Note: the second dimension of the array B must be at least

\max (1, NRHS)

On entry: the

n

r

right-hand side matrix

B

8: LDB – INTEGERInput

On entry: the first dimension of the array B as declared in the (sub)program from which F07QVF (ZSPRFS) is called.

Constraint:

LDB \geq \max (1, N)

9: X(LDX, $*$ ) – COMPLEX (KIND=nag_wp) arrayInput/Output

Note: the second dimension of the array X must be at least

\max (1, NRHS)

On entry: the

n

r

solution matrix

X

, as returned by F07QSF (ZSPTRS).

On exit: the improved solution matrix

X

10: LDX – INTEGERInput

On entry: the first dimension of the array X as declared in the (sub)program from which F07QVF (ZSPRFS) is called.

Constraint:

LDX \geq \max (1, N)

11: FERR(NRHS) – REAL (KIND=nag_wp) arrayOutput

On exit:

FERR (j)

contains an estimated error bound for the

j

th solution vector, that is, the

j

th column of

X

, for

j = 1, 2, \dots, r

12: BERR(NRHS) – REAL (KIND=nag_wp) arrayOutput

On exit:

BERR (j)

contains the component-wise backward error bound

β

for the

j

th solution vector, that is, the

j

th column of

X

, for

j = 1, 2, \dots, r

13: WORK( $2 \times N$ ) – COMPLEX (KIND=nag_wp) arrayWorkspace

14: RWORK(N) – REAL (KIND=nag_wp) arrayWorkspace

15: INFO – INTEGEROutput

On exit:

INFO = 0

unless the routine detects an error (see Section 6).

6 Error Indicators and Warnings

Errors or warnings detected by the routine:

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

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 Further Comments

For each right-hand side, computation of the backward error involves a minimum of

16 n^{2}

real floating point operations. Each step of iterative refinement involves an additional

24 n^{2}

real operations. 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

A x = b

; the number is usually

5

and never more than

11

. Each solution involves approximately

8 n^{2}

real operations.

The real analogue of this routine is F07PHF (DSPRFS).

9 Example

This example solves the system of equations

A X = B

using iterative refinement and to compute the forward and backward error bounds, where

A = (\begin{array}{r} - 0.39 - 0.71 i & 5.14 - 0.64 i & - 7.86 - 2.96 i & 3.80 + 0.92 i \\ 5.14 - 0.64 i & 8.86 + 1.81 i & - 3.52 + 0.58 i & 5.32 - 1.59 i \\ - 7.86 - 2.96 i & - 3.52 + 0.58 i & - 2.83 - 0.03 i & - 1.54 - 2.86 i \\ 3.80 + 0.92 i & 5.32 - 1.59 i & - 1.54 - 2.86 i & - 0.56 + 0.12 i \end{array})

and

B = (\begin{array}{r} - 55.64 + 41.22 i & - 19.09 - 35.97 i \\ - 48.18 + 66.00 i & - 12.08 - 27.02 i \\ - 0.49 - 1.47 i & 6.95 + 20.49 i \\ - 6.43 + 19.24 i & - 4.59 - 35.53 i \end{array}) .

Here

A

is symmetric, stored in packed form, and must first be factorized by F07QRF (ZSPTRF).

NAG Library Routine DocumentF07QVF (ZSPRFS)

+− Contents

1 Purpose

2 Specification

3 Description

4 References

5 Parameters

6 Error Indicators and Warnings

7 Accuracy

8 Further Comments

9 Example

9.1 Program Text

9.2 Program Data

9.3 Program Results

NAG Library Routine Document

F07QVF (ZSPRFS)