Hi,

I have been asked to derive a simple closed form expression for the

following sum:

n

∑ (-1)^(m+i) *(nCi)*(iCm) = S n, m

i = m

[with m = 0,1,2,....,n ; n= 0,1,2,....] andCis the combinations symbol.

I know that we are meant to be using the binomial theorem but I dont quite know where to go from there. Any help would be much appreciated.