The transform calculated by this routine can be used to solve Poisson's equation when the solution is specified at the left boundary, and the derivative of the solution is specified at the right boundary (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: $direct$ – 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'

'B'

2: $m$ – Integer Input

On entry:

m

, the number of sequences to be transformed.

Constraint:

m \geq 1

3: $n$ – Integer Input

On entry:

n

, the number of real values in each sequence.

Constraint:

n \geq 1

4: $x (m \times (n + 2))$ – 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, 1 : n + 2)

; each of the

m

sequences is stored in a row of the array. In other words, if the data values of the

p

th sequence to be transformed are denoted by

x_{j}^{p}

, for

j = 1, 2, \dots, n

and

p = 1, 2, \dots, m

, the first

m n

elements of the array x must contain the values

x_{1}^{1}, x_{1}^{2}, \dots, x_{1}^{m}, x_{2}^{1}, x_{2}^{2}, \dots, x_{2}^{m}, \dots, x_{n}^{1}, x_{n}^{2}, \dots, x_{n}^{m} .

The

(n + 1)

th and

(n + 2)

th elements of each row

x_{n + 1}^{p}, x_{n + 2}^{p}

, for

p = 1, 2, \dots, m

, are required as workspace. These

2 m

elements may contain arbitrary values as they are set to zero by the routine.

On exit: the

m

quarter-wave sine transforms stored as if in a two-dimensional array of dimension

(1 : m, 1 : n + 2)

. Each of the

m

transforms is stored in a row of the array, overwriting the corresponding original sequence. If the

n

components of the

p

th quarter-wave sine transform are denoted by

{\hat{x}}_{k}^{p}

, for

k = 1, 2, \dots, n

and

p = 1, 2, \dots, m

, the

m (n + 2)

elements of the array x contain the values

{\hat{x}}_{1}^{1}, {\hat{x}}_{1}^{2}, \dots, {\hat{x}}_{1}^{m}, {\hat{x}}_{2}^{1}, {\hat{x}}_{2}^{2}, \dots, {\hat{x}}_{2}^{m}, \dots, {\hat{x}}_{n}^{1}, {\hat{x}}_{n}^{2}, \dots, {\hat{x}}_{n}^{m}, 0, 0, \dots, 0 (2 m times) .

5: $work (*)$ – Real (Kind=nag_wp) array Workspace

Note: the dimension of the array work must be at least

m \times n + 2 \times n + 2 \times m + 15

The workspace requirements as documented for c06rcf 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.

6: $ifail$ – Integer Input/Output

On entry: ifail must be set to

0

−1

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

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

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $m = ⟨ value ⟩$ .
Constraint: $m \geq 1$ .

$ifail = 2$: On entry, $n = ⟨ value ⟩$ .
Constraint: $n \geq 1$ .

$ifail = 3$: On entry, $direct = ⟨ value ⟩$ .
Constraint: $direct ='F'$ or $'B'$ .

$ifail = 4$: 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.

$ifail = - 99$: 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.

$ifail = - 399$: 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.

$ifail = - 999$: 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.

c06rcf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

c06rcf 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 c06rcf is approximately proportional to

n m \log (n)

, but also depends on the factors of

n

. c06rcf 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 quarter-wave sine transforms as computed by c06rcf with

direct ='F'

. It then calls the routine again with

direct ='B'

and prints the results which may be compared with the original data.

c06rc: FL CL CPP AD PY MB

NAG FL Interfacec06rcf (fft_​real_​qtrsine_​simple)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
c06rcf (fft_real_qtrsine_simple)