Stabilities for non-uniform $t$-intersecting families

Abstract

The study of intersection problems on families of sets is one of the most important topics in extremal combinatorics. As we all know, the extremal problems involving certain intersection constraints are equivalent to that with the union properties by taking complement of sets. A family of sets is called $s$-union if the union of any two sets in this family has size at most $s$. Katona [Acta Math. Hungar. 15 (1964)] provided the maximum size of an $s$-union family of sets of $[n]$, and he also determined the extremal families up to isomorphism. Recently, Frankl [J. Combin. Theory Ser. B 122 (2017) 869--876] sharpened this result by establishing the maximum size of an $s$-union family that is not a subfamily of the so-called Katona family. In this paper, we determine the maximum size of an $s$-union family that is neither contained in the Katona family nor in the Frankl family. Moreover, we characterize all extremal families achieving the upper bounds.