NAG Library Routine Document

F07PUF (ZHPCON)

+− 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

1 Purpose

F07PUF (ZHPCON) estimates the condition number of a complex Hermitian indefinite matrix

A

, where

A

has been factorized by F07PRF (ZHPTRF), using packed storage.

2 Specification

SUBROUTINE F07PUF (

UPLO, N, AP, IPIV, ANORM, RCOND, WORK, INFO)

INTEGER	N, IPIV(*), INFO
REAL (KIND=nag_wp)	ANORM, RCOND
COMPLEX (KIND=nag_wp)	AP(), WORK(2N)
CHARACTER(1)	UPLO

The routine may be called by its LAPACK name zhpcon.

3 Description

F07PUF (ZHPCON) estimates the condition number (in the

1

-norm) of a complex Hermitian indefinite matrix

A

κ_{1} (A) = {‖A‖}_{1} {‖A^{- 1}‖}_{1} .

Since

A

is Hermitian,

κ_{1} (A) = κ_{\infty} (A) = {‖A‖}_{\infty} {‖A^{- 1}‖}_{\infty}

Because

κ_{1} (A)

is infinite if

A

is singular, the routine actually returns an estimate of the reciprocal of

κ_{1} (A)

The routine should be preceded by a call to F06UDF to compute

{‖A‖}_{1}

and a call to F07PRF (ZHPTRF) to compute the Bunch–Kaufman factorization of

A

. The routine then uses Higham's implementation of Hager's method (see Higham (1988)) to estimate

{‖A^{- 1}‖}_{1}

4 References

Higham N J (1988) FORTRAN codes for estimating the one-norm of a real or complex matrix, with applications to condition estimation ACM Trans. Math. Software 14 381–396

5 Parameters

1: UPLO – CHARACTER(1)Input

On entry: specifies how

A

has been factorized.

$UPLO ='U'$: $A = P U D U^{H} P^{T}$ , where $U$ is upper triangular.
$UPLO ='L'$: $A = P L D L^{H} P^{T}$ , 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: 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 factorization of

A

stored in packed form, as returned by F07PRF (ZHPTRF).

4: 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 F07PRF (ZHPTRF).

5: ANORM – REAL (KIND=nag_wp)Input

On entry: the

1

-norm of the original matrix

A

, which may be computed by calling F06UDF with its parameter

NORM ='1'

. ANORM must be computed either before calling F07PRF (ZHPTRF) or else from a copy of the original matrix

A

Constraint:

ANORM \geq 0.0

6: RCOND – REAL (KIND=nag_wp)Output

On exit: an estimate of the reciprocal of the condition number of

A

. RCOND is set to zero if exact singularity is detected or the estimate underflows. If RCOND is less than machine precision,

A

is singular to working precision.

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

8: 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 computed estimate RCOND is never less than the true value

ρ

, and in practice is nearly always less than

10 ρ

, although examples can be constructed where RCOND is much larger.

8 Further Comments

A call to F07PUF (ZHPCON) 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 floating point operations but takes considerably longer than a call to F07PSF (ZHPTRS) with one right-hand side, because extra care is taken to avoid overflow when

A

is approximately singular.

The real analogue of this routine is F07PGF (DSPCON).

9 Example

This example estimates the condition number in the

1

-norm (or

\infty

-norm) of the matrix

A

, where

A = (\begin{array}{r} - 1.36 + 0.00 i & 1.58 + 0.90 i & 2.21 - 0.21 i & 3.91 + 1.50 i \\ 1.58 - 0.90 i & - 8.87 + 0.00 i & - 1.84 - 0.03 i & - 1.78 + 1.18 i \\ 2.21 + 0.21 i & - 1.84 + 0.03 i & - 4.63 + 0.00 i & 0.11 + 0.11 i \\ 3.91 - 1.50 i & - 1.78 - 1.18 i & 0.11 - 0.11 i & - 1.84 + 0.00 i \end{array}) .

Here

A

is Hermitian indefinite, stored in packed form, and must first be factorized by F07PRF (ZHPTRF). The true condition number in the

1

-norm is

9.10

NAG Library Routine DocumentF07PUF (ZHPCON)

+− 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

F07PUF (ZHPCON)