﻿ c06eb Method
c06eb calculates the discrete Fourier transform of a Hermitian sequence of $n$ complex data values. (No extra workspace required.)

# Syntax

C#
```public static void c06eb(
double[] x,
int n,
out int ifail
)```
Visual Basic
```Public Shared Sub c06eb ( _
x As Double(), _
n As Integer, _
<OutAttribute> ByRef ifail As Integer _
)```
Visual C++
```public:
static void c06eb(
array<double>^ x,
int n,
[OutAttribute] int% ifail
)```
F#
```static member c06eb :
x : float[] *
n : int *
ifail : int byref -> unit
```

#### Parameters

x
Type: array<System..::..Double>[]()[][]
An array of size [n]
On entry: the sequence to be transformed stored in Hermitian form. If the data values ${z}_{j}$ are written as ${x}_{j}+i{y}_{j}$, and if x is declared with bounds $\left(0:{\mathbf{n}}-1\right)$ in the method from which c06eb is called, then for $0\le j\le n/2$, ${x}_{j}$ is contained in ${\mathbf{x}}\left[j-1\right]$, and for $1\le j\le \left(n-1\right)/2$, ${y}_{j}$ is contained in ${\mathbf{x}}\left[n-j\right]$. (See also [] in the C06 class Chapter Introduction and [Example].)
On exit: the components of the discrete Fourier transform ${\stackrel{^}{x}}_{k}$. If x is declared with bounds $\left(0:{\mathbf{n}}-1\right)$ in the method from which c06eb is called, then ${\stackrel{^}{x}}_{\mathit{k}}$ is stored in ${\mathbf{x}}\left[\mathit{k}\right]$, for $\mathit{k}=0,1,\dots ,n-1$.
n
Type: System..::..Int32
On entry: $n$, the number of data values. The largest prime factor of n must not exceed $19$, and the total number of prime factors of n, counting repetitions, must not exceed $20$.
Constraint: ${\mathbf{n}}>1$.
ifail
Type: System..::..Int32%
On exit: ${\mathbf{ifail}}={0}$ unless the method detects an error or a warning has been flagged (see [Error Indicators and Warnings]).

# Description

Given a Hermitian sequence of $n$ complex data values ${z}_{\mathit{j}}$ (i.e., a sequence such that ${z}_{0}$ is real and ${z}_{n-\mathit{j}}$ is the complex conjugate of ${z}_{\mathit{j}}$, for $\mathit{j}=1,2,\dots ,n-1$), c06eb calculates their discrete Fourier transform defined by
 $x^k=1n∑j=0n-1zj×exp-i2πjkn, k=0,1,…,n-1.$
(Note the scale factor of $\frac{1}{\sqrt{n}}$ in this definition.) The transformed values ${\stackrel{^}{x}}_{k}$ are purely real (see also the C06 class).
To compute the inverse discrete Fourier transform defined by
 $y^k=1n∑j=0n-1zj×exp+i2πjkn,$
this method should be preceded by a call of c06gb to form the complex conjugates of the ${z}_{j}$.
c06eb uses the fast Fourier transform (FFT) algorithm (see Brigham (1974)). There are some restrictions on the value of $n$ (see [Parameters]).

# References

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

# Error Indicators and Warnings

Errors or warnings detected by the method:
${\mathbf{ifail}}=1$
At least one of the prime factors of n is greater than $19$.
${\mathbf{ifail}}=2$
n has more than $20$ prime factors.
${\mathbf{ifail}}=3$
 On entry, ${\mathbf{n}}\le 1$.
${\mathbf{ifail}}=4$
An unexpected error has occurred in an internal call. Check all method calls and array dimensions. Seek expert help.
${\mathbf{ifail}}=-9000$
An error occured, see message report.
${\mathbf{ifail}}=-8000$
Negative dimension for array $〈\mathit{\text{value}}〉$
${\mathbf{ifail}}=-6000$
Invalid Parameters $〈\mathit{\text{value}}〉$

# 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).

# Parallelism and Performance

None.

The time taken is approximately proportional to $n×\mathrm{log}\left(n\right)$, but also depends on the factorization of $n$. c06eb is faster 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, c06eb is particularly slow if $n$ has several unpaired prime factors, i.e., if the ‘square-free’ part of $n$ has several factors. For such values of $n$, (C06FBF not in this release) (which requires an additional $n$ elements of workspace) is considerably faster.