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Chapter Contents
Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_sum_fft_complex_3d (c06px)

## Purpose

nag_sum_fft_complex_3d (c06px) computes the three-dimensional discrete Fourier transform of a trivariate sequence of complex data values (using complex data type).

## Syntax

[x, ifail] = c06px(direct, n1, n2, n3, x)
[x, ifail] = nag_sum_fft_complex_3d(direct, n1, n2, n3, x)

## Description

nag_sum_fft_complex_3d (c06px) computes the three-dimensional discrete Fourier transform of a trivariate sequence of complex data values ${z}_{{j}_{1}{j}_{2}{j}_{3}}$, for $\mathit{j1}=0,1,\dots ,{n}_{1}-1$, $\mathit{j2}=0,1,\dots ,{n}_{2}-1$ and $\mathit{j3}=0,1,\dots ,{n}_{3}-1$.
The discrete Fourier transform is here defined by
 $z^ k1 k2 k3 = 1 n1 n2 n3 ∑ j1=0 n1-1 ∑ j2=0 n2-1 ∑ j3=0 n3-1 z j1 j2 j3 × exp ±2πi j1 k1 n1 + j2 k2 n2 + j3 k3 n3 ,$
where ${k}_{1}=0,1,\dots ,{n}_{1}-1$, ${k}_{2}=0,1,\dots ,{n}_{2}-1$ and ${k}_{3}=0,1,\dots ,{n}_{3}-1$.
(Note the scale factor of $\frac{1}{\sqrt{{n}_{1}{n}_{2}{n}_{3}}}$ in this definition.) The minus sign is taken in the argument of the exponential within the summation when the forward transform is required, and the plus sign is taken when the backward transform is required.
A call of nag_sum_fft_complex_3d (c06px) with ${\mathbf{direct}}=\text{'F'}$ followed by a call with ${\mathbf{direct}}=\text{'B'}$ will restore the original data.
This function performs multiple one-dimensional discrete Fourier transforms by the fast Fourier transform (FFT) algorithm (see Brigham (1974)).

## References

Brigham E O (1974) The Fast Fourier Transform Prentice–Hall
Temperton C (1983) Self-sorting mixed-radix fast Fourier transforms J. Comput. Phys. 52 1–23

## Parameters

### Compulsory Input Parameters

1:     $\mathrm{direct}$ – string (length ≥ 1)
If the forward transform as defined in Description is to be computed, then direct must be set equal to 'F'.
If the backward transform is to be computed then direct must be set equal to 'B'.
Constraint: ${\mathbf{direct}}=\text{'F'}$ or $\text{'B'}$.
2:     $\mathrm{n1}$int64int32nag_int scalar
${n}_{1}$, the first dimension of the transform.
Constraint: ${\mathbf{n1}}\ge 1$.
3:     $\mathrm{n2}$int64int32nag_int scalar
${n}_{2}$, the second dimension of the transform.
Constraint: ${\mathbf{n2}}\ge 1$.
4:     $\mathrm{n3}$int64int32nag_int scalar
${n}_{3}$, the third dimension of the transform.
Constraint: ${\mathbf{n3}}\ge 1$.
5:     $\mathrm{x}\left({\mathbf{n1}}×{\mathbf{n2}}×{\mathbf{n3}}\right)$ – complex array
The complex data values. Data values are stored in x using column-major ordering for storing multidimensional arrays; that is, ${z}_{{j}_{1}{j}_{2}{j}_{3}}$ is stored in ${\mathbf{x}}\left(1+{j}_{1}+{n}_{1}{j}_{2}+{n}_{1}{n}_{2}{j}_{3}\right)$.

None.

### Output Parameters

1:     $\mathrm{x}\left({\mathbf{n1}}×{\mathbf{n2}}×{\mathbf{n3}}\right)$ – complex array
The corresponding elements of the computed transform.
2:     $\mathrm{ifail}$int64int32nag_int scalar
${\mathbf{ifail}}={\mathbf{0}}$ unless the function detects an error (see Error Indicators and Warnings).

## Error Indicators and Warnings

Errors or warnings detected by the function:
${\mathbf{ifail}}=1$
 On entry, ${\mathbf{n1}}<1$.
${\mathbf{ifail}}=2$
 On entry, ${\mathbf{n2}}<1$.
${\mathbf{ifail}}=3$
 On entry, ${\mathbf{n3}}<1$.
${\mathbf{ifail}}=4$
 On entry, ${\mathbf{direct}}\ne \text{'F'}$ or $\text{'B'}$.
${\mathbf{ifail}}=8$
An unexpected error has occurred in an internal call. Check all function calls and array dimensions. Seek expert help.
${\mathbf{ifail}}=-99$
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.

## Accuracy

Some indication of accuracy can be obtained by performing a subsequent inverse transform and comparing the results with the original sequence (in exact arithmetic they would be identical).

The time taken is approximately proportional to ${n}_{1}{n}_{2}{n}_{3}×\mathrm{log}\left({n}_{1}{n}_{2}{n}_{3}\right)$, but also depends on the factorization of the individual dimensions ${n}_{1}$, ${n}_{2}$ and ${n}_{3}$. nag_sum_fft_complex_3d (c06px) is faster if the only prime factors are $2$, $3$ or $5$; and fastest of all if they are powers of $2$.

## Example

This example reads in a trivariate sequence of complex data values and prints the three-dimensional Fourier transform. It then performs an inverse transform and prints the sequence so obtained, which may be compared to the original data values.
```function c06px_example

fprintf('c06px example results\n\n');

n1 = int64(2);
n2 = int64(3);
n3 = int64(4);
x = cat(4,[1             0.994-0.111i  0.903-0.430i;
0.500+0.500i  0.494+0.111i  0.403+0.430i], ...
[0.999-0.040i  0.989-0.151i  0.885-0.466i;
0.499+0.040i  0.489+0.151i  0.385+0.466i], ...
[0.987-0.159i  0.963-0.268i  0.823-0.568i;
0.487+0.159i  0.463+0.268i  0.323+0.568i], ...
[0.936-0.352i  0.891-0.454i  0.694-0.720i;
0.436+0.352i  0.391+0.454i  0.194+0.720i]);

direct = 'F';
[xt, ifail] = c06px(direct, n1, n2, n3, x);
direct = 'B';
[xr, ifail] = c06px(direct, n1, n2, n3, xt);

disp('Original data:');
disp(x);
disp(' ');
disp('Components of discrete Fourier transform:');
disp(xt);
disp(' ');
disp('Original sequence as restored by inverse transform:');
disp(xr);

```
```c06px example results

Original data:

(:,:,1,1) =

1.0000 + 0.0000i   0.9940 - 0.1110i   0.9030 - 0.4300i
0.5000 + 0.5000i   0.4940 + 0.1110i   0.4030 + 0.4300i

(:,:,1,2) =

0.9990 - 0.0400i   0.9890 - 0.1510i   0.8850 - 0.4660i
0.4990 + 0.0400i   0.4890 + 0.1510i   0.3850 + 0.4660i

(:,:,1,3) =

0.9870 - 0.1590i   0.9630 - 0.2680i   0.8230 - 0.5680i
0.4870 + 0.1590i   0.4630 + 0.2680i   0.3230 + 0.5680i

(:,:,1,4) =

0.9360 - 0.3520i   0.8910 - 0.4540i   0.6940 - 0.7200i
0.4360 + 0.3520i   0.3910 + 0.4540i   0.1940 + 0.7200i

Components of discrete Fourier transform:

(:,:,1,1) =

3.2921 + 0.1021i   0.1433 - 0.0860i   0.1433 + 0.2902i
1.2247 - 1.6203i   0.4243 + 0.3197i  -0.4243 + 0.3197i

(:,:,1,2) =

0.0506 - 0.0416i   0.0155 + 0.1527i  -0.0502 + 0.1180i
0.3548 + 0.0833i   0.0204 - 0.1147i   0.0070 - 0.0800i

(:,:,1,3) =

0.1127 + 0.1021i  -0.0245 + 0.1268i  -0.0245 + 0.0773i
0.0000 + 0.1621i   0.0134 - 0.0914i  -0.0134 - 0.0914i

(:,:,1,4) =

0.0506 + 0.2458i  -0.0502 + 0.0861i   0.0155 + 0.0515i
-0.3548 + 0.0833i  -0.0070 - 0.0800i  -0.0204 - 0.1147i

Original sequence as restored by inverse transform:

(:,:,1,1) =

1.0000 - 0.0000i   0.9940 - 0.1110i   0.9030 - 0.4300i
0.5000 + 0.5000i   0.4940 + 0.1110i   0.4030 + 0.4300i

(:,:,1,2) =

0.9990 - 0.0400i   0.9890 - 0.1510i   0.8850 - 0.4660i
0.4990 + 0.0400i   0.4890 + 0.1510i   0.3850 + 0.4660i

(:,:,1,3) =

0.9870 - 0.1590i   0.9630 - 0.2680i   0.8230 - 0.5680i
0.4870 + 0.1590i   0.4630 + 0.2680i   0.3230 + 0.5680i

(:,:,1,4) =

0.9360 - 0.3520i   0.8910 - 0.4540i   0.6940 - 0.7200i
0.4360 + 0.3520i   0.3910 + 0.4540i   0.1940 + 0.7200i

```