2D spatial triple (auto)correlation of 2D images

I would like to write some code to calculate a 2D spatial triple correlation using fft2.

For a 2D spatial (auto)correlation , I do:

fftshift(real(ifft2(fft2(image).*conj(fft2(image)))))

The resulting 2D spatial triple (auto)correlation is a 4D matrix and I tried to calculate it in this way:

imfft = fft2(image);

    for i=1:1:size(image,1)
       for j=1:1:size(image,1)

    tcorr(:,:,i,j)=fftshift(real(ifft2(imfft(i,j).*imftt.*conj(fft2(image.*exp(-2*pi*1i/size(image,1)*(x*(i-1)+y*(j-1))))))));

       end
  end

But I am not sure if it's right or not.

[Charlie]

Hi Stefano,

I'm trying to calculate a triple correlation of a 1D signal. The expression I'm calculating is

P=x(t)x(t+2\tau)x(t+3\tau)

Do you have any suggestions for how I could calculate this easily in matlab?

Thanks, Charlie