# Binomial sum identities

How do you persuade Mathematica to give simple identities involving binomial sums. For example

Sum[Binomial[a,i]*Binomial[b,i],{i,0,n}]


where n is bigger than both a and b.

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Use Assumptions to get the result you expected.
In[1]:= $Assumptions = n > a > 0 && n > b > 0 && a \[Element] Integers && b \[Element] Integers; In[2]:= Sum[Binomial[a, i]*Binomial[b, i], {i, 0, n}] Out[2]= Gamma[1 + a + b]/(Gamma[1 + a] Gamma[1 + b]) In[3]:= FullSimplify[%] Out[3]= (a + b)!/(a! b!)  Note that it does give a generally valid result without these assumptions too. Here the assumptions helped us obtain a simpler result. I used the $Assumptions global variable here to avoid typing the assumptions twice, for Sum and FullSimplify.
@Anush I don't know if it can do that automatically. As you see, FullSimplify doesn't. –  Szabolcs Jan 31 at 20:36
@Anush: If that particular result for this particular case is the desired end result, you can use something like Assuming[n > a > 0 && n > b > 0 && {a, b} \[Element] Integers, FullSimplify[ Sum[Binomial[a, i]*Binomial[b, i], {i, 0, n}]]] /. (x_ + y_)!/(x_! y_!) :> Binomial[x + y, x] to get it. –  rasher Feb 1 at 0:41