Planar Tur\'an number of the 6-cycle

TL;DR

This paper establishes a new sharp upper bound for the maximum number of 6-cycles in large planar graphs without 6-cycles, improving previous results and proposing a conjecture for longer cycles.

Contribution

It provides an improved upper bound for the planar Turán number of 6-cycles and introduces a conjecture for the Turán number of longer cycles.

Findings

Sharp upper bound for ex_P(n,C_6) is (5/2)n - 7 for n ≥ 18

Improves previous bound of (18(n-2))/7 by Lan

Proposes a conjecture for ex_P(n,C_k) for k ≥ 7

Abstract

Let ${\rm ex}_{\mathcal{P}}(n,T,H)$ denote the maximum number of copies of $T$ in an $n$-vertex planar graph which does not contain $H$ as a subgraph. When $T=K_2$, ${\rm ex}_{\mathcal{P}}(n,T,H)$ is the well studied function, the planar Tur\'an number of $H$, denoted by ${\rm ex}_{\mathcal{P}}(n,H)$. The topic of extremal planar graphs was initiated by Dowden (2016). He obtained sharp upper bound for both ${\rm ex}_{\mathcal{P}}(n,C_4)$ and ${\rm ex}_{\mathcal{P}}(n,C_5)$. Later on, Y. Lan, et al. continued this topic and proved that ${\rm ex}_{\mathcal{P}}(n,C_6)\leq \frac{18(n-2)}{7}$. In this paper, we give a sharp upper bound ${\rm ex}_{\mathcal{P}}(n,C_6) \leq \frac{5}{2}n-7$, for all $n\geq 18$, which improves Lan's result. We also pose a conjecture on ${\rm ex}_{\mathcal{P}}(n,C_k)$, for $k\geq 7$.