Sums of the inverses of binomial coefficients. (English) Zbl 0476.05011
By induction on \(n\) it is shown that
\[
\sum_{k=0}^n \binom{n}{k}^{-1} = \frac{(n+1)}{2^{n+1}} \sum_{k=1}^{n+1} \frac{2^k}{k}
\]
and
\[
\sum_{k=0}^\infty (-1)^k \binom{n+k-1}{k} = \frac{n}{2}\left(2^n \ln 2 - \sum_{k=1}^{n-1}\frac{2^k}{k} \frac{2^k}{n-k}\right)
\]
hold. The special case \(\displaystyle\sum_k \binom{n+k-1}{k} = \frac{n}{n - 1}\) of Lerch’s theorem is also proved.
Reviewer: Johann Cigler (Wien)
MSC:
05A19 | Combinatorial identities, bijective combinatorics |
05A10 | Factorials, binomial coefficients, combinatorial functions |
11B65 | Binomial coefficients; factorials; \(q\)-identities |