Integer, Intent (In)	::	n, kd, ldab
Integer, Intent (Out)	::	iwork(n), info
Real (Kind=nag_wp), Intent (In)	::	ab(ldab,*)
Real (Kind=nag_wp), Intent (Out)	::	rcond, work(3*n)
Character (1), Intent (In)	::	norm, uplo, diag

C Header Interface

#include <nag.h>

void	f07vgf_ (const char norm, const char uplo, const char diag, const Integer n, const Integer kd, const double ab[], const Integer ldab, double rcond, double work[], Integer iwork[], Integer info, const Charlen length_norm, const Charlen length_uplo, const Charlen length_diag)

The routine may be called by the names f07vgf, nagf_lapacklin_dtbcon or its LAPACK name dtbcon.

3 Description

f07vgf estimates the condition number of a real triangular band matrix

A

, in either the

1

-norm or the

\infty

-norm:

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

Note that

κ_{\infty} (A) = κ_{1} (A^{T})

Because the condition number is infinite if

A

is singular, the routine actually returns an estimate of the reciprocal of the condition number.

The routine computes

{‖ A ‖}_{1}

{‖ A ‖}_{\infty}

exactly, and uses Higham's implementation of Hager's method (see Higham (1988)) to estimate

{‖ A^{- 1} ‖}_{1}

{‖ A^{- 1} ‖}_{\infty}

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 Arguments

1: $norm$ – Character(1) Input

On entry: indicates whether

κ_{1} (A)

κ_{\infty} (A)

is estimated.

$norm ='1'$ or $'O'$: $κ_{1} (A)$ is estimated.
$norm ='I'$: $κ_{\infty} (A)$ is estimated.

Constraint:

norm ='1'

'O'

'I'

2: $uplo$ – Character(1) Input

On entry: specifies whether

A

is upper or lower triangular.

$uplo ='U'$: $A$ is upper triangular.
$uplo ='L'$: $A$ is lower triangular.

Constraint:

uplo ='U'

'L'

3: $diag$ – Character(1) Input

On entry: indicates whether

A

is a nonunit or unit triangular matrix.

$diag ='N'$: $A$ is a nonunit triangular matrix.
$diag ='U'$: $A$ is a unit triangular matrix; the diagonal elements are not referenced and are assumed to be $1$ .

Constraint:

diag ='N'

'U'

4: $n$ – Integer Input

On entry:

n

, the order of the matrix

A

Constraint:

n \geq 0

5: $kd$ – Integer Input

On entry:

k_{d}

, the number of superdiagonals of the matrix

A

uplo ='U'

, or the number of subdiagonals if

uplo ='L'

Constraint:

kd \geq 0

6: $ab (ldab, *)$ – Real (Kind=nag_wp) array Input

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

\max (1, n)

On entry: the

n \times n

triangular band matrix

A

The matrix is stored in rows

1

k_{d} + 1

, more precisely,

if $uplo ='U'$ , the elements of the upper triangle of $A$ within the band must be stored with element $A_{i j}$ in $ab (k_{d} + 1 + i - j, j) for \max (1, j - k_{d}) \leq i \leq j$ ;
if $uplo ='L'$ , the elements of the lower triangle of $A$ within the band must be stored with element $A_{i j}$ in $ab (1 + i - j, j) for j \leq i \leq \min (n, j + k_{d}) .$

diag ='U'

, the diagonal elements of

A

are assumed to be

1

, and are not referenced.

7: $ldab$ – Integer Input

On entry: the first dimension of the array ab as declared in the (sub)program from which f07vgf is called.

Constraint:

ldab \geq kd + 1

8: $rcond$ – 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.

9: $work (3 \times n)$ – Real (Kind=nag_wp) array Workspace

10: $iwork (n)$ – Integer array Workspace

11: $info$ – Integer Output

On exit:

info = 0

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

6 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.

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 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.

f07vgf 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 routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

A call to f07vgf involves solving a number of systems of linear equations of the form

A x = b

A^{T} x = b

; the number is usually

4

5

and never more than

11

. Each solution involves approximately

2 n k

floating-point operations (assuming

n ≫ k

) but takes considerably longer than a call to f07vef with one right-hand side, because extra care is taken to avoid overflow when

A

is approximately singular.

The complex analogue of this routine is f07vuf.

10 Example

This example estimates the condition number in the

1

-norm of the matrix

A

, where

A = (\begin{matrix} - 4.16 & 0.00 & 0.00 & 0.00 \\ - 2.25 & 4.78 & 0.00 & 0.00 \\ 0.00 & 5.86 & 6.32 & 0.00 \\ 0.00 & 0.00 & - 4.82 & 0.16 \end{matrix}) .

Here

A

is treated as a lower triangular band matrix with one subdiagonal. The true condition number in the

1

-norm is

69.62

f07vg: FL CL CPP AD PY MB

NAG FL Interfacef07vgf (dtbcon)

▸▿ 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 FL Interface
f07vgf (dtbcon)