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Omni-MATH / Let k and n be positive integers and let S={(a₁,…,a_(k))∈Z^(k)∣0≤a_(k)≤⋯≤a₁≤n,a₁+⋯+a_(k)=k} Determine, with proof, the value of …

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problem

Let kk and nn be positive integers and let S={(a1,…,ak)∈Zk∣0≤ak≤⋯≤a1≤n,a1+⋯+ak=k} S=\left\{\left(a_{1}, \ldots, a_{k}\right) \in \mathbb{Z}^{k} \mid 0 \leq a_{k} \leq \cdots \leq a_{1} \leq n, a_{1}+\cdots+a_{k}=k\right\} Determine, with proof, the value of $$ \sum_{\left(a_{1}, \ldots, a_{k}\right) \in S}\binom{n}{a_{1}}\binom{a_{1}}{a_{2}} \cdots\binom{a_{k-1}}{a_{k}} $$ in terms of kk and nn, where the sum is over all kk-tuples (a1,…,ak)\left(a_{1}, \ldots, a_{k}\right) in SS.
Plain-text mathematical notation (without MathML)
Let k and n be positive integers and let S={(a₁,…,a_(k))∈Z^(k)∣0≤a_(k)≤⋯≤a₁≤n,a₁+⋯+a_(k)=k} Determine, with proof, the value of $$ \sum_{\left(a_{1}, \ldots, a_{k}\right) \in S}\binom{n}{a_{1}}\binom{a_{1}}{a_{2}} \cdots\binom{a_{k-1}}{a_{k}} $$ in terms of k and n, where the sum is over all k-tuples (a₁,…,a_(k)) in S.
Original LaTeX notation
Let $k$ and $n$ be positive integers and let $$ S=\left\{\left(a_{1}, \ldots, a_{k}\right) \in \mathbb{Z}^{k} \mid 0 \leq a_{k} \leq \cdots \leq a_{1} \leq n, a_{1}+\cdots+a_{k}=k\right\} $$ Determine, with proof, the value of $$ \sum_{\left(a_{1}, \ldots, a_{k}\right) \in S}\binom{n}{a_{1}}\binom{a_{1}}{a_{2}} \cdots\binom{a_{k-1}}{a_{k}} $$ in terms of $k$ and $n$, where the sum is over all $k$-tuples $\left(a_{1}, \ldots, a_{k}\right)$ in $S$.

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