Erdős-Rogers functions for arbitrary pairs of graphs
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| Format: | Preprint |
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2024
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| _version_ | 1866929408493748224 |
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| author | Mubayi, Dhruv Verstraete, Jacques |
| author_facet | Mubayi, Dhruv Verstraete, Jacques |
| contents | Let $f_{F,G}(n)$ be the largest size of an induced $F$-free subgraph that every $n$-vertex $G$-free graph is guaranteed to contain. We prove that for any triangle-free graph $F$, \[ f_{F,K_3}(n) = f_{K_2,K_3}(n)^{1 + o(1)} = n^{\frac{1}{2} + o(1)}.\] Along the way we give a slight improvement of a construction of Erd\H os-Frankl-Rödl for the Brown-Erd\H os-Sós $(3r-3,3)$-problem when $r$ is large.
In contrast to our result for $K_3$, for any $K_4$-free graph $F$ containing a cycle, we prove there exists $c_F > 0$ such that $$f_{F,K_4}(n) > f_{K_2,K_4}(n)^{1 + c_F} = n^{\frac{1}{3}+c_F+o(1)}.$$ \iffalse We also observe that our earlier proof for $F=K_3$ generalizes to $f_{F,K_4}(n) = O(\sqrt{n}\log n)$ for all $F$ containing a cycle. \fi
For every graph $G$, we prove that there exists $\varepsilon_G >0$ such that whenever $F$ is a non-empty graph such that $G$ is not contained in any blowup of $F$, then $f_{F,G}(n) = O(n^{1-\varepsilon_G})$. On the other hand, for graph $G$ that is not a clique, and every $\varepsilon>0$, we exhibit a $G$-free graph $F$ such that $f_{F,G}(n) = Ω(n^{1-\varepsilon})$. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2407_03121 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Erdős-Rogers functions for arbitrary pairs of graphs Mubayi, Dhruv Verstraete, Jacques Combinatorics 05C55, 05D10 Let $f_{F,G}(n)$ be the largest size of an induced $F$-free subgraph that every $n$-vertex $G$-free graph is guaranteed to contain. We prove that for any triangle-free graph $F$, \[ f_{F,K_3}(n) = f_{K_2,K_3}(n)^{1 + o(1)} = n^{\frac{1}{2} + o(1)}.\] Along the way we give a slight improvement of a construction of Erd\H os-Frankl-Rödl for the Brown-Erd\H os-Sós $(3r-3,3)$-problem when $r$ is large. In contrast to our result for $K_3$, for any $K_4$-free graph $F$ containing a cycle, we prove there exists $c_F > 0$ such that $$f_{F,K_4}(n) > f_{K_2,K_4}(n)^{1 + c_F} = n^{\frac{1}{3}+c_F+o(1)}.$$ \iffalse We also observe that our earlier proof for $F=K_3$ generalizes to $f_{F,K_4}(n) = O(\sqrt{n}\log n)$ for all $F$ containing a cycle. \fi For every graph $G$, we prove that there exists $\varepsilon_G >0$ such that whenever $F$ is a non-empty graph such that $G$ is not contained in any blowup of $F$, then $f_{F,G}(n) = O(n^{1-\varepsilon_G})$. On the other hand, for graph $G$ that is not a clique, and every $\varepsilon>0$, we exhibit a $G$-free graph $F$ such that $f_{F,G}(n) = Ω(n^{1-\varepsilon})$. |
| title | Erdős-Rogers functions for arbitrary pairs of graphs |
| topic | Combinatorics 05C55, 05D10 |
| url | https://arxiv.org/abs/2407.03121 |