e01ab: FL CL CPP AD PY MB

NAG CL Interface
e01abc (dim1_everett)

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NAG Library Manual, Mark 30

Interfaces: FL CL CPP AD PY MB

NAG CL Interface Introduction

E01 (Interp) Chapter Contents

E01 (Interp) Chapter Introduction

e01ab: FL CL CPP AD PY MB

▸▿ 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

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Long (impl_details)
Full (nag_info_impl_details)

1 Purpose

e01abc interpolates a function of one variable at a given point

x

from a table of function values evaluated at equidistant points, using Everett's formula.

2 Specification

#include <nag.h>

void	e01abc (Integer n, double p, double a[], double g[], NagError *fail)

The function may be called by the names: e01abc, nag_interp_dim1_everett or nag_1d_everett_interp.

3 Description

e01abc interpolates a function of one variable at a given point

x = x_{0} + p h,

where

- 1 \leq p \leq 1

and

h

is the interval of differencing, from a table of values

x_{m} = x_{0} + m h

and

y_{m}

where

m = - (n - 1), - (n - 2), \dots, - 1, 0, 1, \dots, n

. The formula used is that of Fröberg (1970), neglecting the remainder term:

y_{p} = \sum_{r = 0}^{n - 1} (\frac{1 - p + r}{2 r + 1}) δ^{2 r} y_{0} + \sum_{r = 0}^{n - 1} (\frac{p + r}{2 r + 1}) δ^{2 r} y_{1} .

The values of

δ^{2 r} y_{0}

and

δ^{2 r} y_{1}

are stored on exit from the function in addition to the interpolated function value

y_{p}

4 References

Fröberg C E (1970) Introduction to Numerical Analysis Addison–Wesley

5 Arguments

1: $n$ – Integer Input

On entry:

n

, half the number of points to be used in the interpolation.

Constraint:

n > 0

2: $p$ – double Input

On entry: the point

p

at which the interpolated function value is required, i.e.,

p = (x - x_{0}) / h

with

- 1.0 < p < 1.0

Constraint:

- 1.0 \leq p \leq 1.0

3: $a [2 \times n]$ – double Input/Output

On entry:

a [i - 1]

must be set to the function value

y_{i - n}

, for

i = 1, 2, \dots, 2 n

On exit: the contents of a are unspecified.

4: $g [2 \times n + 1]$ – double Output

On exit: the array contains

$y_{0}$	in $g [0]$
$y_{1}$	in $g [1]$
$δ^{2 r} y_{0}$	in $g [2 r]$
$δ^{2 r} y_{1}$	in $g [2 r + 1]$ , for $r = 1, 2, \dots, n - 1$ .

The interpolated function value

y_{p}

is stored in

g [2 n]

5: $fail$ – NagError * Input/Output

The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

NE_ALLOC_FAIL: Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
NE_BAD_PARAM: On entry, argument $⟨ value ⟩$ had an illegal value.
NE_INT: On entry, $n = ⟨ value ⟩$ .
Constraint: $n > 0$ .
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.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_LICENCE: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
NE_REAL: On entry, $p = ⟨ value ⟩$ .
Constraint: $p \leq 1.0$ .

On entry, $p = ⟨ value ⟩$ .
Constraint: $p \geq - 1.0$ .

7 Accuracy

In general, increasing

n

improves the accuracy of the result until full attainable accuracy is reached, after which it might deteriorate. If

x

lies in the central interval of the data (i.e.,

0.0 \leq p < 1.0

), as is desirable, an upper bound on the contribution of the highest order differences (which is usually an upper bound on the error of the result) is given approximately in terms of the elements of the array g by

a \times (| g [2 n - 2] | + | g [2 n - 1] |)

, where

a = 0.1

0.02

0.005

0.001

0.0002

for

n = 1, 2, 3, 4, 5

respectively, thereafter decreasing roughly by a factor of

4

each time.

Note that if

p = 1

y_{1}

is returned. If

p = −1

and

n > 1

y_{−1}

is returned. In these cases, no interpolation is necessary and there is no loss of accuracy.

8 Parallelism and Performance

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

e01abc is not threaded in any implementation.

9 Further Comments

The computation time increases as the order of

n

increases.

10 Example

This example interpolates at the point

x = 0.28

from the function values

(\begin{array}{r} x_{i} & - 1.00 & - 0.50 & 0.00 & 0.50 & 1.00 & 1.50 \\ y_{i} & 0.00 & - 0.53 & - 1.00 & - 0.46 & 2.00 & 11.09 \end{array}) .

We take

n = 3

and

p = 0.56

e01ab: FL CL CPP AD PY MB

NAG CL Interfacee01abc (dim1_​everett)

▸▿ 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 CL Interface
e01abc (dim1_everett)