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If $k$ and $n$ are positive integers, how do I give a simple closed form expression for the sum $\sum_{a_1+···+a_k=n} {n \choose a_1,...,a_k}$
I'm not sure the process of finding a closed form expression and I think understanding this general case will help for explicit questions like ${9 \choose a, b, c}$
By the multinomial theorem, the sum in question evaluates to $(1 + 1 + \cdots + 1)^n = k^n$. For $k = 2$ this reduces to the famous binomial identity $\sum_a \binom n a = 2^n$.
More explicitly: For $k = 2$ we get $\sum_a \binom n a = \sum_a \binom n a 1^a 1^{n-a} = (1+1)^n = 2^n$. And for $k = 3$ we get $\sum_{a_1+a_2+a_3 = n} \binom {n} {a_1,a_2,a_3}1^{a_1} 1^{a_2} 1^{a_3} = (1+1+1)^n = 3^n$.
