Given the wave function
we are to find the values of such that has a meaningful physical value. And by we mean the following integral
First, normalization will result in
We can find taking this integral to the complex plane, and observing that if we allow the function to take complex values and we define
Then the residue of at will be
And according to a suitable theorem, we are able to write
And then we are allowed to say that only when takes the values with the integral has a physical meaning.