# NAG FL Interfacec06pzf (fft_​hermitian_​3d)

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## 1Purpose

c06pzf computes the three-dimensional inverse discrete Fourier transform of a trivariate Hermitian sequence of complex data values.

## 2Specification

Fortran Interface
 Subroutine c06pzf ( n1, n2, n3, y, x,
 Integer, Intent (In) :: n1, n2, n3 Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (Out) :: x(n1*n2*n3) Complex (Kind=nag_wp), Intent (In) :: y((n1/2+1)*n2*n3)
#include <nag.h>
 void c06pzf_ (const Integer *n1, const Integer *n2, const Integer *n3, const Complex y[], double x[], Integer *ifail)
The routine may be called by the names c06pzf or nagf_sum_fft_hermitian_3d.

## 3Description

c06pzf computes the three-dimensional inverse discrete Fourier transform of a trivariate Hermitian sequence of complex data values ${z}_{{j}_{1}{j}_{2}{j}_{3}}$, for ${j}_{1}=0,1,\dots ,{n}_{1}-1$, ${j}_{2}=0,1,\dots ,{n}_{2}-1$ and ${j}_{3}=0,1,\dots ,{n}_{3}-1$.
The discrete Fourier transform is here defined by
 $x^ k1 k2 k3 = 1 n1 n2 n3 ∑ j1=0 n1-1 ∑ j2=0 n2-1 ∑ j3=0 n3-1 z j1 j2 j3 × exp(2πi( j1 k1 n1 + j2 k2 n2 + j3 k3 n3 )) ,$
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.)
Because the input data satisfies conjugate symmetry (i.e., ${z}_{{j}_{1}{j}_{2}{j}_{3}}$ is the complex conjugate of ${z}_{\left({n}_{1}-{j}_{1}\right)\left({n}_{2}-{j}_{2}\right)\left({n}_{3}-{j}_{3}\right)}$), the transformed values ${\stackrel{^}{x}}_{{k}_{1}{k}_{2}{k}_{3}}$ are real.
A call of c06pyf followed by a call of c06pzf will restore the original data.
This routine calls c06pqf and c06prf to perform multiple one-dimensional discrete Fourier transforms by the fast Fourier transform (FFT) algorithm in Brigham (1974) and Temperton (1983).
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

## 5Arguments

1: $\mathbf{n1}$Integer Input
On entry: ${n}_{1}$, the first dimension of the transform.
Constraint: ${\mathbf{n1}}\ge 1$.
2: $\mathbf{n2}$Integer Input
On entry: ${n}_{2}$, the second dimension of the transform.
Constraint: ${\mathbf{n2}}\ge 1$.
3: $\mathbf{n3}$Integer Input
On entry: ${n}_{3}$, the third dimension of the transform.
Constraint: ${\mathbf{n3}}\ge 1$.
4: $\mathbf{y}\left(\left({\mathbf{n1}}/2+1\right)×{\mathbf{n2}}×{\mathbf{n3}}\right)$Complex (Kind=nag_wp) array Input
On entry: the Hermitian sequence of complex input dataset $z$, where ${z}_{{j}_{1}{j}_{2}{j}_{3}}$ is stored in ${\mathbf{y}}\left({j}_{3}×\left({n}_{1}/2+1\right){n}_{2}+{j}_{2}×\left({n}_{1}/2+1\right)+{j}_{1}+1\right)$, for ${j}_{1}=0,1,\dots ,{n}_{1}/2$, ${j}_{2}=0,1,\dots ,{n}_{2}-1$ and ${j}_{3}=0,1,\dots ,{n}_{3}-1$. That is, if y is regarded as a three-dimensional array of dimension $\left(0:{\mathbf{n1}}/2,0:{\mathbf{n2}}-1,0:{\mathbf{n3}}-1\right)$, ${\mathbf{y}}\left({j}_{1},{j}_{2},{j}_{3}\right)$ must contain ${z}_{{j}_{1}{j}_{2}{j}_{3}}$.
5: $\mathbf{x}\left({\mathbf{n1}}×{\mathbf{n2}}×{\mathbf{n3}}\right)$Real (Kind=nag_wp) array Output
On exit: the real output dataset $\stackrel{^}{x}$, where ${\stackrel{^}{x}}_{{k}_{1}{k}_{2}{k}_{3}}$ is stored in ${\mathbf{x}}\left({k}_{3}×{n}_{1}{n}_{2}+{k}_{2}×{n}_{1}+{k}_{1}+1\right)$, for ${k}_{1}=0,1,\dots ,{n}_{1}-1$, ${k}_{2}=0,1,\dots ,{n}_{2}-1$ and ${k}_{3}=0,1,\dots ,{n}_{3}-1$. That is, if x is regarded as a three-dimensional array of dimension $\left(0:{\mathbf{n1}}-1,0:{\mathbf{n2}}-1,0:{\mathbf{n3}}-1\right)$, ${\mathbf{x}}\left({k}_{1},{k}_{2},{k}_{3}\right)$ contains ${\stackrel{^}{x}}_{{k}_{1}{k}_{2}{k}_{3}}$.
6: $\mathbf{ifail}$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 $-\mathbf{1}$ or $\mathbf{1}$ is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
On entry, ${\mathbf{n1}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{n1}}\ge 1$.
${\mathbf{ifail}}=2$
On entry, ${\mathbf{n2}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{n2}}\ge 1$.
${\mathbf{ifail}}=3$
On entry, ${\mathbf{n3}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{n3}}\ge 1$.
${\mathbf{ifail}}=4$
An internal error has occurred in this routine. Check the routine call and any array sizes. If the call is correct then please contact NAG for assistance.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
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.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

## 7Accuracy

Some indication of accuracy can be obtained by performing a forward transform using c06pyf and a backward transform using c06pzf, and comparing the results with the original sequence (in exact arithmetic they would be identical).

## 8Parallelism and Performance

c06pzf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c06pzf 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.

The time taken by c06pzf 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 factors of ${n}_{1}$, ${n}_{2}$ and ${n}_{3}$. c06pzf is fastest if the only prime factors of ${n}_{1}$, ${n}_{2}$ and ${n}_{3}$ are $2$, $3$ and $5$, and is particularly slow if one of the dimensions is a large prime, or has large prime factors.
Workspace is internally allocated by c06pzf. The total size of these arrays is approximately proportional to ${n}_{1}{n}_{2}{n}_{3}$.