Set Systems with No Singleton Intersection

Let F be a k-uniform set system defined on a ground set of size n with no singleton intersection; i.e., no pair A,B ∈ F has |A ∩ B| = 1. Frankl showed that |F| ≤ (n−2 k−2 ) for k ≥ 4 and n sufficiently large, confirming a conjecture of Erdős and Sós. We determine the maximum size of F for k = 4 and all n, and also establish a stability result for general k, showing that any F with size asymptotic to that of the best construction must be structurally similar to it.