If and is intersecting, then .
Katona's circle method places a finite ground set in a uniformly counted cyclic order, proves a bound for the members of a set family that appear as cyclic intervals, and double-counts pairs of a member and a compatible cyclic order.
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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 \( k \leq \frac{n}{2} \).