NAG FL Interface
c06pqf (fft_​realherm_​1d_​multi_​col)

1 Purpose

c06pqf computes the discrete Fourier transforms of m sequences, each containing n real data values or a Hermitian complex sequence stored column-wise in a complex storage format.

2 Specification

Fortran Interface
Subroutine c06pqf ( direct, n, m, x, work, ifail)
Integer, Intent (In) :: n, m
Integer, Intent (Inout) :: ifail
Real (Kind=nag_wp), Intent (Inout) :: x((n+2)*m), work(*)
Character (1), Intent (In) :: direct
C Header Interface
#include <nag.h>
void  c06pqf_ (const char *direct, const Integer *n, const Integer *m, double x[], double work[], Integer *ifail, const Charlen length_direct)
The routine may be called by the names c06pqf or nagf_sum_fft_realherm_1d_multi_col.

3 Description

Given m sequences of n real data values xjp , for j=0,1,,n-1 and p= 1,2,,m, c06pqf simultaneously calculates the Fourier transforms of all the sequences defined by
z^kp = 1n j=0 n-1 xjp × exp -i 2πjk n ,   k=0,1,,n-1 ​ and ​ p=1,2,,m .  
The transformed values z^ k p are complex, but for each value of p the z^ k p form a Hermitian sequence (i.e., z^ n-k p is the complex conjugate of z^ k p ), so they are completely determined by mn real numbers (since z^0p is real, as is z^ n/2 p for n even).
Alternatively, given m Hermitian sequences of n complex data values zjp , this routine simultaneously calculates their inverse (backward) discrete Fourier transforms defined by
x^kp = 1n j=0 n-1 zjp × exp i 2πjk n ,   k=0,1,,n-1 ​ and ​ p=1,2,,m .  
The transformed values x^kp are real.
(Note the scale factor 1n in the above definition.)
A call of c06pqf with direct='F' followed by a call with direct='B' will restore the original data.
The routine uses a variant of the fast Fourier transform (FFT) algorithm (see Brigham (1974)) known as the Stockham self-sorting algorithm, which is described in Temperton (1983). Special coding is provided for the factors 2, 3, 4 and 5.

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: direct Character(1) Input
On entry: if the forward transform as defined in Section 3 is to be computed, direct must be set equal to 'F'.
If the backward transform is to be computed, direct must be set equal to 'B'.
Constraint: direct='F' or 'B'.
2: n Integer Input
On entry: n, the number of real or complex values in each sequence.
Constraint: n1.
3: m Integer Input
On entry: m, the number of sequences to be transformed.
Constraint: m1.
4: x n+2×m Real (Kind=nag_wp) array Input/Output
On entry: the m real or Hermitian data sequences to be transformed.
  • if direct='F', the m real data sequences, xp = x1p , x2p , , xnp , for p=1,2,,m, should be stored sequentially in x, with a stride of n+2 between sequences.
  • if direct='B', the m Hermitian data sequences, z^p = z^1p , z^2p , , z^n/2+1p = Re(z^1p) , Im(z^1p) , Re(z^2p) , Im(z^2p) , , Re(z^n/2+1p) , Im(z^n/2+1p) , for p=1,2,,m, should be stored sequentially in x, with a stride of n+2 between sequences.
In other words:
  • if direct='F', xp-1×n+2+j must contain xjp, for j=1,2,,n and p=1,2,,m;
  • if direct='B', xp-1 × n+2 + 2×k-1 and xp-1 × n+2 + 2×k must contain the real and imaginary parts respectively of z^kp, for k=1,2,,n/2+1 and p=1,2,,m. (Note that for the sequence z^kp to be Hermitian, the imaginary part of z^1p, and of z^ n/2+1 p for n even, must be zero.)
On exit:
  • if direct='F' then the m sequences, z^p, for p=1,2,,m stored as described on entry for direct='B'
  • if direct='B' then the m sequences, xp, for p=1,2,,m stored as described on entry for direct='F'
5: work* Real (Kind=nag_wp) array Workspace
Note: the dimension of the array work must be at least m+2 × n + 15.
The workspace requirements as documented for c06pqf may be an overestimate in some implementations.
On exit: work1 contains the minimum workspace required for the current values of m and n with this implementation.
6: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1. If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1 or 1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this argument, the recommended value is 0. 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 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
On entry, m=value.
Constraint: m1.
On entry, n=value.
Constraint: n1.
On entry, direct=value.
Constraint: direct='F' or 'B'.
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.
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.
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.
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

c06pqf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c06pqf 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 by c06pqf is approximately proportional to nm logn, but also depends on the factors of n. c06pqf is fastest if the only prime factors of n are 2, 3 and 5, and is particularly slow if n is a large prime, or has large prime factors.

10 Example

This example reads in sequences of real data values and prints their discrete Fourier transforms (as computed by c06pqf with direct='F'), after expanding them from complex Hermitian form into full complex sequences.
Inverse transforms are then calculated by calling c06pqf with direct='B' showing that the original sequences are restored.

10.1 Program Text

Program Text (c06pqfe.f90)

10.2 Program Data

Program Data (c06pqfe.d)

10.3 Program Results

Program Results (c06pqfe.r)