On binomial coefficients modulo squares of primes

Abstract

We give elementary proofs for the Apagodu-Zeilberger-Stanton-Amdeberhan-Tauraso congruences $$\sum\limits_{n=0}^{p-1}\dbinom{2n}{n} \equiv\eta_{p}\mod p^{2},$$ $$\sum\limits_{n=0}^{rp-1}\dbinom{2n}{n} \equiv\eta_{p}\sum\limits_{n=0}^{r-1}\dbinom {2n}{n}\mod p^{2}$$ and $$\sum\limits_{n=0}^{rp-1}\sum\limits_{m=0}^{sp-1}\dbinom{n+m}{m}^{2} \equiv\eta_{p} \sum\limits_{m=0}^{r-1}\sum\limits_{n=0}^{s-1}\dbinom{n+m}{m}^2\mod p^2,$$ where $p$ is an odd prime, $r$ and $s$ are nonnegative integers, and $\eta_{p}= \begin{cases} 0, &\text{if }p\equiv0\mod 3;\\ 1, & \text{if }p\equiv1\mod 3;\\ -1, &\text{if }p\equiv2\mod 3 \end{cases}.$$