nag_fft_hermitian (c06ebc) (PDF version)
c06 Chapter Contents
c06 Chapter Introduction
NAG C Library Manual

NAG Library Function Document

nag_fft_hermitian (c06ebc)

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

nag_fft_hermitian (c06ebc) calculates the discrete Fourier transform of a Hermitian sequence of n complex data values.

2  Specification

#include <nag.h>
#include <nagc06.h>
void  nag_fft_hermitian (Integer n, double x[], NagError *fail)

3  Description

Given a Hermitian sequence of n complex data values zj  (i.e., a sequence such that z0  is real and zn-j  is the complex conjugate of zj , for j=1,2,,n-1), nag_fft_hermitian (c06ebc) calculates their discrete Fourier transform defined by
x^k = 1n j=0 n-1 zj exp -i 2πjk n ,   k= 0, 1, , n-1 .
(Note the scale factor of 1n  in this definition.) The transformed values x^k  are purely real.
To compute the inverse discrete Fourier transform defined by
y^k = 1n j=0 n-1 zj exp +i 2πjk n ,   ​ k= 0, 1, ,n -1 ,
this function should be preceded by a call of nag_conjugate_hermitian (c06gbc) to form the complex conjugates of the zj .
nag_fft_hermitian (c06ebc) uses the fast Fourier transform (FFT) algorithm (see Brigham (1974)). There are some restrictions on the value of n (see Section 5).

4  References

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

5  Arguments

1:     nIntegerInput
On entry: n, the number of data values.
Constraint: n>1 . The largest prime factor of n must not exceed 19, and the total number of prime factors of n, counting repetitions, must not exceed 20.
2:     x[n]doubleInput/Output
On entry: the sequence to be transformed stored in Hermitian form. If the data values z j  are written as x j + iy j , then for 0 j n/2 , x j  is contained in x[j] , and for 1 j n-1/2 , y j  is contained in x[n-j] . It is not necessary for other elements of the sequence to be explicitly stored. (See also Section 9.)
On exit: the components of the discrete Fourier transform x^ k . x^ k  is stored in x[k] , for k=0,1,,n-1.
3:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

6  Error Indicators and Warnings

At least one of the prime factors of n is greater than 19.
n has more than 20 prime factors.
On entry, n=value.
Constraint: n>1.

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  Further Comments

The time taken is approximately proportional to nlogn , but also depends on the factorization of n. nag_fft_hermitian (c06ebc) is somewhat faster than average if the only prime factors of n are 2, 3 or 5; and fastest of all if n is a power of 2.
On the other hand, nag_fft_hermitian (c06ebc) is particularly slow if n has several unpaired prime factors, i.e., if the ‘square-free’ part of n has several factors.

9  Example

This example reads in a sequence of real data values which is assumed to be a Hermitian sequence of complex data values stored in Hermitian form. The input sequence is expanded into a full complex sequence and printed alongside the original sequence. The discrete Fourier transform (as computed by nag_fft_hermitian (c06ebc)) is printed out. It then performs an inverse transform using nag_fft_real (c06eac) and nag_conjugate_hermitian (c06gbc), and prints the sequence so obtained alongside the original data values.

9.1  Program Text

Program Text (c06ebce.c)

9.2  Program Data

Program Data (c06ebce.d)

9.3  Program Results

Program Results (c06ebce.r)

nag_fft_hermitian (c06ebc) (PDF version)
c06 Chapter Contents
c06 Chapter Introduction
NAG C Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2012