Stability and exact extremal colourings for the Alon--Frankl--Lovász bound

Abstract

The Alon--Frankl--Lovász theorem states that every $q$-colouring of the edges of the complete $r$-uniform hypergraph on $n$ vertices contains a monochromatic matching of size $\lfloor(n+q-1)/(r+q-1)\rfloor$. For fixed uniformity $r$ and number of colours $q$, we prove a stability theorem: every colouring whose largest monochromatic matching has size at most $n/(r+q-1)+o(n)$ differs from a threshold extremal colouring on only $o(n^r)$ edges. We also give a necessary and sufficient structural characterization of all extremal colourings for sufficiently large $n$. The proofs combine rigidity of fractional covering weights with an extension argument for prescribed matchings. These results answer, for sufficiently large orders, the stability and extremal-colouring questions posed by Gishboliner, Glock, Michaeli and Sgueglia.