English

Independence, induced subgraphs, and domination in $K_{1,r}$-free graphs

Combinatorics 2026-05-12 v2

Abstract

Let GG be a graph and F\mathcal{F} a family of graphs. Define αF(G)\alpha_{\mathcal{F}}(G) as the maximum order of any induced subgraph of GG that belongs to the family F\mathcal{F}. For the family F\mathcal{F} of graphs with \emph{chromatic number} at most~kk, we prove that if GG is K1,rK_{1,r}-free, then αF(G)(r1)kγ(G)\alpha_{\mathcal{F}}(G) \le (r-1)k\gamma(G), where γ(G)\gamma(G) is the \emph{domination number}. When F\mathcal{F} is the family of empty graphs, this bound simplifies to α(G)2γ(G)\alpha(G) \le 2\gamma(G) for K1,3K_{1,3}-free (claw-free) graphs, where α(G)\alpha(G) is the \emph{independence number} of GG. For dd-regular graphs, this is further refined to the bound α(G)2(d+1d+2)γ(G)\alpha(G) \le 2\left(\frac{d+1}{d+2}\right)\gamma(G), which is tight for d{2,3,4}d \in \{2, 3, 4\}. Using Ramsey theory, we extend this framework to edge-hereditary graph families, showing that for K1,rK_{1,r}-free graphs, we have αF(G)r(Kr,F)γ(G)\alpha_{\mathcal{F}}(G) \le r(K_r, \mathcal{F^*})\gamma(G), where F\mathcal{F^*} is the set of graphs not in F\mathcal{F}. Specializing to KqK_q-free graphs, we show αF(G)(r(Kq,Kr)1)γ(G)\alpha_{\mathcal{F}}(G) \le (r(K_q, K_r) - 1)\gamma(G). Finally, for the \emph{kk-independence number} αk(G)\alpha_k(G), we prove that if GG is K1,rK_{1,r}-free with order nn and minimum degree δk+1\delta \ge k+1, αk(G)((r1)(k+1)δk+(r1)(k+1))n, \alpha_k(G) \le \left( \frac{(r-1)(k+1)}{\delta - k + (r-1)(k+1)} \right) n, and this bound is sharp for all parameters.

Keywords

Cite

@article{arxiv.2501.05291,
  title  = {Independence, induced subgraphs, and domination in $K_{1,r}$-free graphs},
  author = {Yair Caro and Randy Davila and Michael A. Henning and Ryan Pepper},
  journal= {arXiv preprint arXiv:2501.05291},
  year   = {2026}
}