Com10atorics 07.06.2021 20:54 Prove that for all $n\in\mathbb{N}$ we have $\sum_{k=0}^n\dbinom {n}{k}^2=\dbinom {2n}{n}$.
jasperE3 07.06.2021 20:56 Very old. https://aops.com/community/p3719763 https://aops.com/community/p11094024
PickleSauce 07.06.2021 21:01 Hmmm- why is this in HSO?? this identity is well known and proven in the inter c&p book