 Erdős–Ko–Rado theorem

If $n\ge 2k$ and $\mathcal{A}\subseteq [n{]}^{(k)}$ is intersecting, then $|\mathcal{A}|\le \left(\genfrac{}{}{0ex}{}{n-1}{k-1}\right)$.

The Erdős–Ko–Rado theorem is a fundamental result in combinatorial set theory, particularly in the area concerning intersecting families of sets. It was first proved by Paul Erdős, Chao Ko, and Ronald Rado in 1961. ### Statement of the Theorem: For a finite set \( X \) with \( n \) elements, let \( k \) be a positive integer such that \( k \leq \frac{n}{2} \).