Integer, Intent (In)	::	ndim, l, nd(ndim), n, lwork
Integer, Intent (Inout)	::	ifail
Real (Kind=nag_wp), Intent (Inout)	::	x(n), y(n)
Real (Kind=nag_wp), Intent (Out)	::	work(lwork)

C Header Interface

#include <nag.h>

void	c06fff_ (const Integer ndim, const Integer l, const Integer nd[], const Integer n, double x[], double y[], double work[], const Integer lwork, Integer *ifail)

The routine may be called by the names c06fff or nagf_sum_fft_complex_multid_1d_sep.

3 Description

c06fff computes the discrete Fourier transform of one variable (the

l

th say) in a multivariate sequence of complex data values

z_{j_{1} j_{2} \dots j_{m}}

, for

j_{1} = 0, 1, \dots, n_{1} - 1

and

j_{2} = 0, 1, \dots, n_{2} - 1

, and so on. Thus the individual dimensions are

n_{1}, n_{2}, \dots, n_{m}

, and the total number of data values is

n = n_{1} \times n_{2} \times \dots \times n_{m}

The routine computes

n / n_{l}

one-dimensional transforms defined by

{\hat{z}}_{j_{1} \dots k_{l} \dots j_{m}} = \frac{1}{\sqrt{n_{l}}} \sum_{j_{l} = 0}^{n_{l} - 1} z_{j_{1} \dots j_{l} \dots j_{m}} \times \exp (- \frac{2 π i j_{l} k_{l}}{n_{l}}),

where

k_{l} = 0, 1, \dots, n_{l} - 1

(Note the scale factor of

\frac{1}{\sqrt{n_{l}}}

in this definition.)

To compute the inverse discrete Fourier transforms, defined with

\exp (+ \frac{2 π i j_{l} k_{l}}{n_{l}})

in the above formula instead of

\exp (- \frac{2 π i j_{l} k_{l}}{n_{l}})

, this routine should be preceded and followed by the complex conjugation of the data values and the transform (by negating the imaginary parts stored in

y

The data values must be supplied in a pair of one-dimensional arrays (real and imaginary parts separately), in accordance with the Fortran convention for storing multidimensional data (i.e., with the first subscript

j_{1}

varying most rapidly).

This routine calls c06fcf to perform one-dimensional discrete Fourier transforms by the fast Fourier transform (FFT) algorithm in Brigham (1974).

4 References

Brigham E O (1974) The Fast Fourier Transform Prentice–Hall

5 Arguments

1: $ndim$ – Integer Input: On entry: $m$ , the number of dimensions (or variables) in the multivariate data.

Constraint: $ndim \geq 1$ .
2: $l$ – Integer Input: On entry: $l$ , the index of the variable (or dimension) on which the discrete Fourier transform is to be performed.

Constraint: $1 \leq l \leq ndim$ .
3: $nd (ndim)$ – Integer array Input: On entry: $nd (i)$ must contain $n_{i}$ (the dimension of the $i$ th variable), for $i = 1, 2, \dots, m$ .

Constraint: $nd (i) \geq 1$ , for $i = 1, 2, \dots, ndim$ .
4: $n$ – Integer Input: On entry: $n$ , the total number of data values.

Constraint: $n = nd (1) \times nd (2) \times \dots \times nd (ndim)$ .
5: $x (n)$ – Real (Kind=nag_wp) array Input/Output: On entry: $x (1 + j_{1} + n_{1} j_{2} + n_{1} n_{2} j_{3} + \dots)$ must contain the real part of the complex data value $z_{j_{1} j_{2} \dots j_{m}}$ , for $0 \leq j_{1} \leq n_{1} - 1, 0 \leq j_{2} \leq n_{2} - 1, \dots$ ; i.e., the values are stored in consecutive elements of the array according to the Fortran convention for storing multidimensional arrays.

On exit: the real parts of the corresponding elements of the computed transform.
6: $y (n)$ – Real (Kind=nag_wp) array Input/Output: On entry: the imaginary parts of the complex data values, stored in the same way as the real parts in the array x.

On exit: the imaginary parts of the corresponding elements of the computed transform.
7: $work (lwork)$ – Real (Kind=nag_wp) array Workspace
8: $lwork$ – Integer Input: On entry: the dimension of the array work as declared in the (sub)program from which c06fff is called.

Constraint: $lwork \geq 3 \times nd (l)$ .
9: $ifail$ – Integer Input/Output: On entry: ifail must be set to $0$ , $- 1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $- 1$ means that an error message is printed while a value of $1$ means that it is not.

If halting is not appropriate, the value $- 1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

- 1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $ndim = ⟨ value ⟩$ .
Constraint: $ndim \geq 1$ .

$ifail = 2$: On entry, $n = ⟨ value ⟩$ .
Constraint: $n = nd (1) \times nd (2) \times \dots \times nd (ndim)$ .

$ifail = 3$: On entry, $l = ⟨ value ⟩$ and $ndim = ⟨ value ⟩$ .
Constraint: $1 \leq l \leq ndim$ .

$ifail = 10 \times l + 3$: On entry, $l = ⟨ value ⟩$ .
Constraint: $nd (l) \geq 1$ .

$ifail = 10 \times l + 4$: On entry, $lwork = ⟨ value ⟩$ and $nd (⟨ value ⟩) = ⟨ value ⟩$ .
Constraint: $lwork \geq 3 \times nd (l)$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 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).

8 Parallelism and Performance

c06fff is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

c06fff 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

The time taken is approximately proportional to

n \times \log n_{l}

, but also depends on the factorization of

n_{l}

. c06fff is faster if the only prime factors of

n_{l}

are

2

3

5

; and fastest of all if

n_{l}

is a power of

2

10 Example

This example reads in a bivariate sequence of complex data values and prints the discrete Fourier transform of the second variable. It then performs an inverse transform and prints the sequence so obtained, which may be compared with the original data values.

c06ff: FL CL CPP AD

NAG FL Interfacec06fff (fft_​complex_​multid_​1d_​sep)

▸▿ 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
c06fff (fft_complex_multid_1d_sep)