I am doing the summation over two indices: `m and n`  
Suppose I want to remove those terms from the summation which has `m==n`, what is the command for that? Because at `m==n`, terms are divergent.

I have already tried to do the summation in two cases:   
`m` less than `n` and `m>n`.  
But is there any other method to do it in one step?