NAG CL Interface
c06pwc (fft_​hermitian_​2d)

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1 Purpose

c06pwc computes the two-dimensional inverse discrete Fourier transform of a bivariate Hermitian sequence of complex data values.

2 Specification

#include <nag.h>
void  c06pwc (Integer m, Integer n, const Complex y[], double x[], NagError *fail)
The function may be called by the names: c06pwc or nag_sum_fft_hermitian_2d.

3 Description

c06pwc computes the two-dimensional inverse discrete Fourier transform of a bivariate Hermitian sequence of complex data values zj1j2, for j1=0,1,,m-1 and j2=0,1,,n-1.
The discrete Fourier transform is here defined by
x^ k1 k2 = 1mn j1=0 m-1 j2=0 n-1 z j1 j2 × exp(2πi( j1 k1 m + j2 k2 n )) ,  
where k1=0,1,,m-1 and k2=0,1,,n-1. (Note the scale factor of 1mn in this definition.)
Because the input data satisfies conjugate symmetry (i.e., z j1 j2 is the complex conjugate of z (m-j1) (n-j2) , the transformed values x^ k1 k2 are real.
A call of c06pvc followed by a call of c06pwc will restore the original data.
This function performs multiple one-dimensional discrete Fourier transforms by the fast Fourier transform (FFT) algorithm in Brigham (1974) and Temperton (1983).

4 References

Brigham E O (1974) The Fast Fourier Transform Prentice–Hall
Temperton C (1983) Fast mixed-radix real Fourier transforms J. Comput. Phys. 52 340–350

5 Arguments

1: m Integer Input
On entry: m, the first dimension of the transform.
Constraint: m1.
2: n Integer Input
On entry: n, the second dimension of the transform.
Constraint: n1.
3: y[ (m/2+1)×n ] const Complex Input
On entry: the Hermitian sequence of complex input dataset z, where z j1 j2 is stored in y[ j2 × (m/2+1) + j1], for j1=0,1,,m/2 and j2=0,1,,n-1.
4: x[ m×n ] double Output
On exit: the real output dataset x^, where x^ k1 k2 is stored in x[ k2 × m+ k1], for k1=0,1,,m-1 and k2=0,1,,n-1.
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, m=value.
Constraint: m1.
On entry, n=value.
Constraint: n1.
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.

7 Accuracy

Some indication of accuracy can be obtained by performing a forward transform using c06pvc and a backward transform using c06pwc, and comparing the results with the original sequence (in exact arithmetic they would be identical).

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
c06pwc is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c06pwc 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 function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The time taken by c06pwc is approximately proportional to mn log(mn) , but also depends on the factors of m and n. c06pwc is fastest if the only prime factors of m and n are 2, 3 and 5, and is particularly slow if m or n is a large prime, or has large prime factors.
Workspace is internally allocated by c06pwc. The total size of these arrays is approximately proportional to mn.

10 Example

See c06pvc.