Hi, I am using FFTW to try another test, f(t)=exp(-x), x>=0, the real part
of the Fourier transforms should be 1/(1+x^2). I use xmax=1000, and
xmin=0,
let N=8192. I found the results seems ****fted upper, every output value is
larger than the analytical solution.
My code is like this:
program test
implicit none
include "fftw3.f"
integer N
parameter(N=8192)
integer pi
parameter(pi=3.14159265357)
double complex in
double complex out
double precision f
dimension out(N)
dimension in(N)
dimension f(N)
integer i
real*8 :: x,y,xmax,xmin,dx
integer*8 plan
OPEN(UNIT=7,FILE='fftin')
OPEN(UNIT=8,FILE='fftout')
! write(*,*) 'Input array:'
xmax=1000.
xmin=0.
dx=(xmax-xmin)/N
write(6,*)
'XMAX',xmax,'xmin',xmin,'dx',dx,'N',N,'omega',1.0/(2.0*dx)*2*pi
do i = 1,N/2+1
x=xmin+dx*(i-1)
in(i)=exp(-x)*(-1)**(i+1)
if(i.gt.N/2+1) in(i)=0.0
enddo
do i=1,N
x=xmin+dx*(i-1)
write(7,*) i,x,real(in(i)), imag(in(i))
enddo
close(7)
!call dfftw_plan_r2r_1d (plan, N, in, out, fftw_r2hc, FFTW_ESTIMATE)
!call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
call dfftw_plan_dft_1d (plan, N, in, out, FFTW_FORWARD, FFTW_ESTIMATE)
call dfftw_execute(plan)
!write(*,*) 'Output array:'
do i = 1,N
f(i)=(-N/2+i-1)/(N*dx)
write(8,*)
i,f(i)*2*pi,real(out(i))*dx,imag(out(i))*dx,1./(1.+(f(i)*2.*pi)**2),-f(i)*2.*pi/(1.+(f(i)*2.*pi)**2)
enddo
close(8)
STOP
can any one help me about it?
Thanks
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