On some Binomial Coefficient Identities with Applications

TL;DR

This paper provides new proofs and generalizations of binomial coefficient identities, introduces novel binomial sum identities, and applies these to derive harmonic number sum identities using advanced combinatorial techniques.

Contribution

It offers a new proof of Munarini's binomial identity, generalizes identities of Alzer and Kouba, and introduces new binomial sum identities with applications to harmonic number sums.

Findings

New proof of Munarini's binomial identity

Generalizations of identities by Alzer and Kouba

New binomial sum identities and harmonic number sum formulas

Abstract

We present a different proof of the following identity due to Munarini, which generalizes a curious binomial identity of Simons. \begin{align*} \sum_{k=0}^{n}\binom{\alpha}{n-k}\binom{\beta+k}{k}x^k &=\sum_{k=0}^{n}(-1)^{n+k}\binom{\beta-\alpha+n}{n-k}\binom{\beta+k}{k}(x+1)^k, \end{align*} where $n$ is a non-negative integer and $\alpha$ and $\beta$ are complex numbers, which are not negative integers. Our approach is based on a particularly interesting combination of the Taylor theorem and the Wilf-Zeilberger algorithm. We also generalize a combinatorial identity due to Alzer and Kouba, and offer a new binomial sum identity. Furthermore, as applications, we give many harmonic number sum identities. As examples, we prove that \begin{equation*} H_n=\frac{1}{2}\sum_{k=1}^{n}(-1)^{n+k}\binom{n}{k}\binom{n+k}{k}H_k \end{equation*} and \begin{align*} \sum_{k=0}^{n}\binom{n}{k}^2H_kH_{n-k}=\binom{2n}{n} \left((H_{2n}-2H_n)^2+H_{n}^{(2)}-H_{2n}^{(2)}\right). \end{align*}