[通用算法] 傅里叶逆变换

!      傅里叶变换
      PROGRAM FOURIER 
      IMPLICIT NONE
      REAL(8)::DT,t,FCT,df,pi,w
      CHARACTER::U
      INTEGER::I,j
      REAL(8),DIMENSION(10001)::CT
      OPEN(12,FILE='VACFHI.txt')
      OPEN(13,FILE='testFT.txt')
      dt=1.2     ! fs
      df=1.d-4   ! 1 fs^-1 = 33000 cm^-1
      pi=4.d0*atan(1.d0)
      DO i=1,10001
        READ(12,*)U,CT(I)
      ENDDO
      DO j=1,12500 
         FCT=0
         w=2.d0*pi*dble(j)*df
        DO i=0,10000
          t=dble(i)*DT
          FCT=FCT+2.d0*(CT(I+1)*dCOS(w*t)*DT)
        ENDDO
        WRITE(13,*)w/pi/2.d0*3.3d4,FCT        
      ENDDO
      CLOSE(12)
      CLOSE(13)
      END      
!傅里叶逆变换
      program inverse FT
      implicit none
      integer,parameter::N=12500
      real(8),dimension(N)::FT,u
      integer::i,j
      real(8)::t,dw,CT,w
      open(11,file='testFT.txt')
      open(12,file='inverseFT.txt')
  
      do i=1,N
      read(11,*)u(i),FT(i)
      enddo
      
      do i=1,12500
        CT=0
        t=i
        dw=3.3     
        do j= 1,N
          w=3.3*j
          CT=CT+2.d0*(FT(j)*cos(w*t)*dw)
        enddo
        write(12,*)t,CT
      enddo
      close(11)
      close(12)
      end

PROGRAM FOURIER 
      IMPLICIT NONE
      Real(8) , parameter :: PI = acos(-1.d0)
      REAL(8)::FCT,w
      CHARACTER::U
      INTEGER::I,j
      Integer , parameter :: N = 10001
      Integer , parameter :: M = 12501
      REAL(8),DIMENSION(N)::CT
      OPEN(12,FILE='VACFHI.txt')
      OPEN(13,FILE='testFT.txt')
      DO i=1,N
        READ(12,*)U,CT(I)
      ENDDO
      DO j=0,M-1
         FCT=0
         w=2.d0*pi*j/M
        DO i=0,N-1
          FCT=FCT+(CT(I+1)*COS(w*i))
        ENDDO
        WRITE(13,*)w/pi/2.d0*3.3d4,FCT        
      ENDDO
      CLOSE(12)
      CLOSE(13)
      END