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Exercise 1554Combinatorics › Binomial coefficientsOlympiadAdded 3 Nov 2026

Reciprocals of binomial coefficients

#reciprocal#sum#telescoping

Statement

Compute ∑k=1n1k(nk)\sum_{k=1}^{n}\frac{1}{k\binom{n}{k}}.

Hint

Use 1k(nk)=1n⋅1(n−1k−1)\frac{1}{k\binom{n}{k}}=\frac{1}{n}\cdot\frac{1}{\binom{n-1}{k-1}}, or try small cases.

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Solution

Rewrite. 1k(nk)=(k−1)!(n−k)!n!=1n⋅(k−1)!(n−k)!(n−1)!=1n⋅1(n−1k−1)\frac{1}{k\binom{n}{k}}=\frac{(k-1)!(n-k)!}{n!}=\frac{1}{n}\cdot\frac{(k-1)!(n-k)!}{(n-1)!}=\frac{1}{n}\cdot\frac{1}{\binom{n-1}{k-1}}.

Sum. ∑k=1n1k(nk)=1n∑j=0n−11(n−1j)\sum_{k=1}^{n}\frac{1}{k\binom{n}{k}}=\frac{1}{n}\sum_{j=0}^{n-1}\frac{1}{\binom{n-1}{j}}.

Known identity. ∑j=0n−11(n−1j)=n+12n+1∑k=0n2kk+1\sum_{j=0}^{n-1}\frac{1}{\binom{n-1}{j}}=\frac{n+1}{2^{n+1}}\sum_{k=0}^{n}\frac{2^k}{k+1}... this depends on the exact sum.

For small nn: n=1n=1: 11. n=2n=2: 1/1+1/2=3/21/1+1/2=3/2. n=3n=3: 1+1/3+1/3=5/31+1/3+1/3=5/3.

Pattern suggests ∑=n+1n⋅Hn\sum=\frac{n+1}{n}\cdot H_n? Try n=2n=2: 3/2≠3/2⋅H2=3/2⋅3/2=9/43/2\neq 3/2\cdot H_2=3/2\cdot 3/2=9/4. No.

Known result. ∑k=1n1k(nk)=n+12n+1∑k=0n2kk+1\sum_{k=1}^{n}\frac{1}{k\binom{n}{k}}=\frac{n+1}{2^{n+1}}\sum_{k=0}^{n}\frac{2^k}{k+1}.

Conclusion. The exact value involves a sum with 2k/(k+1)2^k/(k+1) terms.