Decomposing random regular graphs into stars

TL;DR

This paper investigates the existence of $k$-star decompositions in random $d$-regular graphs, establishing thresholds and conditions for their existence using probabilistic and combinatorial methods.

Contribution

It provides new existence thresholds for $k$-star decompositions in random regular graphs, combining probabilistic techniques with combinatorial conditions.

Findings

Existence of $k$-star decompositions for $d/2 < k \\leq d/2 + \\max\{1, \\frac{1}{6}\\log d\}$.

Thresholds for $k$-star decompositions in graphs with $d \\leq 100$ and $k > d/2$.

Conditions under which $k$-star decompositions exist for smaller $k$, based on $eta$-orientations.

Abstract

We study $k$-star decompositions, that is, partitions of the edge set into disjoint stars with $k$ edges, in the uniformly random $d$-regular graph model $\mathcal{G}_{n,d}$. Using the small subgraph conditioning method, we prove an existence result for such decompositions for all $d,k$ such that $d/2 < k \leq d/2 + \max\{1,\frac{1}{6}\log d\}$. More generally, we give a sufficient existence condition that can be checked numerically for any given values of $d$ and $k$. Complementary negative results are obtained using the independence ratio of random regular graphs. Our results establish an existence threshold for $k$-star decompositions in $\mathcal{G}_{n,d}$ for all $d\leq 100$ and $k > d/2$. For smaller values of $k$, the connection between $k$-star decompositions and $\beta$-orientations allows us to apply results of Thomassen (2012) and Lov\'asz, Thomassen, Wu and Zhang (2013). We prove that random $d$-regular graphs satisfy their assumptions with high probability, thus establishing a.a.s. existence of $k$-star decompositions (i) when $2k^2+k\leq d$, and (ii) when $k$ is odd and $k < d/2$.