Implement a mathematical expression

Hello to all,

I have to implement the followin expression in Matlab:

X(m) * conj ( X(m -p)), where X(m) is the FFT of x(t) and p is a delay factor in frequency.

The problem is I am not a mathematician to figure out the correct (and otimum) way to do it.

Can you please help me?

Best regards, Jean

[Walter Roberson]

Is p an integer, or could it have a fractional part?

What happens for the places where m-p is negative?