NAG FL Interface
c06rbf (fft_​real_​cosine_​simple)

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1 Purpose

c06rbf computes the discrete Fourier cosine transforms of m sequences of real data values.

2 Specification

Fortran Interface
Subroutine c06rbf ( m, n, x, work, ifail)
Integer, Intent (In) :: m, n
Integer, Intent (Inout) :: ifail
Real (Kind=nag_wp), Intent (Inout) :: x(m*(n+3)), work(*)
C Header Interface
#include <nag.h>
void  c06rbf_ (const Integer *m, const Integer *n, double x[], double work[], Integer *ifail)
The routine may be called by the names c06rbf or nagf_sum_fft_real_cosine_simple.

3 Description

Given m sequences of n+1 real data values xjp , for j=0,1,,n and p=1,2,,m, c06rbf simultaneously calculates the Fourier cosine transforms of all the sequences defined by
x^ k p = 2n (12x0p+ j=1 n-1 xjp×cos(jkπn)+12(-1)kxnp) ,   k= 0, 1, , n ​ and ​ p= 1, 2, , m .  
(Note the scale factor 2n in this definition.)
Since the Fourier cosine transform is its own inverse, two consecutive calls of c06rbf will restore the original data.
The transform calculated by this routine can be used to solve Poisson's equation when the derivative of the solution is specified at both left and right boundaries (see Swarztrauber (1977)).
The routine uses a variant of the fast Fourier transform (FFT) algorithm (see Brigham (1974)) known as the Stockham self-sorting algorithm, described in Temperton (1983), together with pre- and post-processing stages described in Swarztrauber (1982). Special coding is provided for the factors 2, 3, 4 and 5.

4 References

Brigham E O (1974) The Fast Fourier Transform Prentice–Hall
Swarztrauber P N (1977) The methods of cyclic reduction, Fourier analysis and the FACR algorithm for the discrete solution of Poisson's equation on a rectangle SIAM Rev. 19(3) 490–501
Swarztrauber P N (1982) Vectorizing the FFT's Parallel Computation (ed G Rodrique) 51–83 Academic Press
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 number of sequences to be transformed.
Constraint: m1.
2: n Integer Input
On entry: one less than the number of real values in each sequence, i.e., the number of values in each sequence is n+1.
Constraint: n1.
3: x( m×(n+3) ) Real (Kind=nag_wp) array Input/Output
On entry: the data must be stored in x as if in a two-dimensional array of dimension (1:m,0:n+2); each of the m sequences is stored in a row of the array. In other words, if the (n+1) data values of the pth sequence to be transformed are denoted by xjp, for j=0,1,,n and p=1,2,,m, the first m(n+1) elements of the array x must contain the values
x01 , x02 ,, x0m , x11 , x12 ,, x1m ,, xn1 , xn2 ,, xnm .  
The (n+2)th and (n+3)th elements of each row x n+2 p , x n+3 p , for p=1,2,,m, are required as workspace. These 2m elements may contain arbitrary values as they are set to zero by the routine.
On exit: the m Fourier cosine transforms stored as if in a two-dimensional array of dimension (1:m,0:n+2). Each of the m transforms is stored in a row of the array, overwriting the corresponding original data. If the (n+1) components of the pth Fourier cosine transform are denoted by x^kp, for k=0,1,,n and p=1,2,,m, the m(n+3) elements of the array x contain the values
x^01 , x^02 ,, x^0m , x^11 , x^12 ,, x^1m ,, x^n1 , x^n2 ,, x^nm , 0 , 0 ,,0  (2m times) .  
4: work(*) Real (Kind=nag_wp) array Workspace
Note: the dimension of the array work must be at least m×n + 2×n + 2×m + 15.
The workspace requirements as documented for c06rbf may be an overestimate in some implementations.
On exit: work(1) contains the minimum workspace required for the current values of m and n with this implementation.
5: 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 -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.
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

Background information to multithreading can be found in the Multithreading documentation.
c06rbf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c06rbf 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 c06rbf is approximately proportional to nm log(n), but also depends on the factors of n. c06rbf 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 Fourier cosine transforms (as computed by c06rbf). It then calls the routine again and prints the results which may be compared with the original sequence.

10.1 Program Text

Program Text (c06rbfe.f90)

10.2 Program Data

Program Data (c06rbfe.d)

10.3 Program Results

Program Results (c06rbfe.r)