Transversal factors and spanning trees

- University of Warwick

- University College London

- University of Warwick

### Editorial introduction

Since the proof of a “colorful” version of Caratheodory’s theorem by Bárány in 1982, it has been an important problem to obtain colorful extensions of other classical results in discrete geometry (for instance Tverberg’s theorem). The present paper continues this line of research, but in the context of extremal graph theory rather than discrete geometry.

Mantel’s classical theorem from 1907 states that every n-vertex graph on more than n2/4 edges contains a triangle. In Ron Aharoni, Matt DeVos, Sebastián González Hermosillo de la Maza, Amanda Montejano, and Robert Šámal, A rainbow version of Mantel’s Theorem, Advances in Combinatorics 2020:2, 12 pp, a “rainbow”, “colored”, or “colorful” variant of this problem was considered : given three graphs G1,G2,G3 on the same vertex set of size n, what average degree conditions on G1,G2,G3 force the existence of a “rainbow triangle” (a triangle {e1,e2,e3} such that each edge ei belongs to Gi)? By taking three copies of the same graph G we see that the colored version is at least as hard as the original problem, and the paper cited above provided a construction showing that in this case the colorful variant is strictly harder than Mantel’s problem.

It was suggested to study average degree or minimum degree thresholds for colorful variants of classical problems in extremal combinatorics, such as Dirac’s theorem (every n-vertex graph of minimum degree at least n/2 has a Hamiltonian cycle). In particular, Joos and Kim proved in 2020 that the same minimum degree condition as in Dirac’s theorem guarantees a rainbow n-cycle: namely if we are given n graphs of minimum degree at least n/2 on the same set of n vertices, then there is an n-cycle comprising one edge of each graph.

The results in the present paper follow the same line of research. The two major results that are extended to the colorful setting here are a theorem of Kühn and Osthus (a sharp minimum degree condition to obtain a perfect packing of copies of any given graph F, generalizing the Hajnal-Szemerédi theorem), and a theorem of Komlós, Sárközy and Szemerédi (a sharp degree condition to contain any given spanning tree without large degree vertices). Amazingly, the minimum degree conditions in the (stronger) colorful versions are the same as the original minimum degree conditions.