TITLE: Expansion of $ (a_1 + a_2 + \cdots + a_k)^n $
QUESTION [7 upvotes]: Is there an expansion for the following summation?
$$ (a_1 + a_2 + \cdots + a_k)^n $$

REPLY [10 votes]: http://en.wikipedia.org/wiki/Multinomial_theorem
This is what you seek.