Evaluation

For all positive integers n n what is the value of:

k = 1 n k × k ! × ( n k ) n k = ? \large \sum_{k=1}^n \frac{ k\times k! \times \binom nk}{n^k} = ?

n ! n! e e n n e 2 e^2 n 2 n^2

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1 solution

Mark Hennings
May 13, 2019

Note that k × k ! × ( n k ) n k = k ! ( n k ) n k 1 ( k + 1 ) ! ( n k + 1 ) n k 1 k n 1 \frac{k \times k! \times \binom{n}{k}}{n^k}\; = \; \frac{k! \binom{n}{k}}{n^{k-1}} - \frac{(k+1)!\binom{n}{k+1}}{n^k} \hspace{2cm} 1 \le k \le n-1 and hence k = 1 n k × k ! × ( n k ) n k = k = 1 n 1 [ k ! ( n k ) n k 1 ( k + 1 ) ! ( n k + 1 ) n k ] + n ! n n 1 = [ ( n 1 ) n ! n n 1 ] + n ! n n 1 = n \begin{aligned} \sum_{k=1}^n \frac{k \times k! \times \binom{n}{k}}{n^k} & = \; \sum_{k=1}^{n-1}\left[ \frac{k! \binom{n}{k}}{n^{k-1}} - \frac{(k+1)!\binom{n}{k+1}}{n^k} \right] + \frac{n!}{n^{n-1}} \\ & = \; \left[\binom{n}{1} - \frac{n!}{n^{n-1}}\right] + \frac{n!}{n^{n-1}} \; = \; \boxed{n} \end{aligned}

Thank you for sharing a nice solution.

Hana Wehbi - 2 years ago

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