FFTCF
Computes the Fourier coefficients of a complex periodic sequence.
Required Arguments
N — Length of the sequence to be transformed. (Input)
SEQ — Complex array of length N containing the periodic sequence. (Input)
COEF — Complex array of length N containing the Fourier coefficients. (Output)
FORTRAN 90 Interface
Generic: CALL FFTCF (N, SEQ, COEF)
Specific: The specific interface names are S_FFTCF and D_FFTCF.
FORTRAN 77 Interface
Single: CALL FFTCF (N, SEQ, COEF)
Double: The double precision name is DFFTCF.
Description
The routine FFTCF computes the discrete complex Fourier transform of a complex vector of size N. It uses the Intel® Math Kernel Library, Sun Performance Library or IBM Engineering and Scientific Subroutine Library for the computation, if available. Otherwise, the method used is a variant of the Cooley-Tukey algorithm, which is most efficient when N is a product of small prime factors. If N satisfies this condition, then the computational effort is proportional to N log N. This considerable savings has historically led people to refer to this algorithm as the “fast Fourier transform” or FFT.
Specifically, given an N-vector x, FFTCF returns in c = COEF
Furthermore, a vector of Euclidean norm S is mapped into a vector of norm
Finally, note that we can invert the Fourier transform as follows:
This formula reveals the fact that, after properly normalizing the Fourier coefficients, one has the coefficients for a trigonometric interpolating polynomial to the data. An unnormalized inverse is implemented in FFTCB. FFTCF is based on the complex FFT in FFTPACK. The package FFTPACK was developed by Paul Swarztrauber at the National Center for Atmospheric Research.
1. Workspace may be explicitly provided, if desired, by use of F2TCF/DF2TCF. The reference is:
CALL F2TCF (N, SEQ, COEF, WFFTC, CPY)
The additional arguments are as follows:
WFFTC — Real array of length 4 * N + 15 initialized by FFTCI. The initialization depends on N. (Input)
CPY — Real array of length 2 * N. (Workspace)
If the Intel® Math Kernel Library, Sun Performance Library or IBM Engineering and Scientific Subroutine Library is used, WFFTC and CPY are not referenced.
2. The routine FFTCF is most efficient when N is the product of small primes.
3. The arrays COEF and SEQ may be the same.
4. If FFTCF/FFTCB is used repeatedly with the same value of N, then call FFTCI followed by repeated calls to F2TCF/F2TCB. This is more efficient than repeated calls to FFTCF/FFTCB.
If the Intel® Math Kernel Library, Sun Performance Library or IBM Engineering and Scientific Subroutine Library is used, parameters computed by FFTCI are not used. In this case, there is no need to call FFTCI.
Example
In this example, we input a pure exponential data vector and recover its Fourier series, which is a vector with all components zero except at the appropriate frequency where it has an N. Notice that the norm of the input vector is
but the norm of the output vector is N.

USE FFTCF_INT
USE CONST_INT
USE UMACH_INT

IMPLICIT NONE
INTEGER N
PARAMETER (N=7)
!
INTEGER I, NOUT
REAL TWOPI
COMPLEX C, CEXP, COEF(N), H, SEQ(N)
INTRINSIC CEXP
!
C = (0.,1.)
TWOPI = CONST('PI')
TWOPI = 2.0 * TWOPI
! Here we compute (2*pi*i/N)*3.
H = (TWOPI*C/N)*3.
! This loop fills out the data vector
! with a pure exponential signal of
! frequency 3.
DO 10 I=1, N
SEQ(I) = CEXP((I-1)*H)
10 CONTINUE
! Compute the Fourier transform of SEQ
CALL FFTCF (N, SEQ, COEF)
! Get output unit number and print
! results
CALL UMACH (2, NOUT)
WRITE (NOUT,99998)
99998 FORMAT (9X, 'INDEX', 8X, 'SEQ', 15X, 'COEF')
WRITE (NOUT,99999) (I, SEQ(I), COEF(I), I=1,N)
99999 FORMAT (1X, I11, 5X,'(',F5.2,',',F5.2,')', &
5X,'(',F5.2,',',F5.2,')')
END
Output

INDEX SEQ COEF
1 ( 1.00, 0.00) ( 0.00, 0.00)
2 (-0.90, 0.43) ( 0.00, 0.00)
3 ( 0.62,-0.78) ( 0.00, 0.00)
4 (-0.22, 0.97) ( 7.00, 0.00)
5 (-0.22,-0.97) ( 0.00, 0.00)
6 ( 0.62, 0.78) ( 0.00, 0.00)
7 (-0.90,-0.43) ( 0.00, 0.00)