%discrete fourier transform
%page 133
%DFT
function Xk=dft(x)
[N,M]=size(x);  %make sure that x is a column vector
if M~=1
    x=x';
    N=M;
end
Xk=zeros(N,1);
n=0:N-1;             %有些疑問，求和過程體現(xiàn)在哪里？？？？？？？？？？？
for k=0:N-1
    Xk(k+1)=exp(-j*2*pi*k*n/N)*x;
end