The rainbow Tur\'an number of $P_5$

TL;DR

This paper determines the maximum number of edges in a properly edge-colored graph with no rainbow $P_5$, establishing an exact asymptotic value and supporting a broader conjecture for rainbow Turán numbers of paths.

Contribution

It proves that the rainbow Turán number for $P_5$ is exactly $rac{5n}{2}$ when $n$ is divisible by 16, settling an open case in rainbow Turán theory.

Findings

$ex^*(n,P_5) \,=\, \frac{5n}{2}$ for divisible $n$

Confirms the conjecture $ex^*(n,P_{\ell}) = \frac{\ell}{2}n + O(1)$ for $\ell=5$

Provides asymptotic exactness for the rainbow Turán number of $P_5$

Abstract

An edge-colored graph $F$ is rainbow if each edge of $F$ has a unique color. The rainbow Tur\'an number $ex^*(n,F)$ of a graph $F$ is the maximum possible number of edges in a properly edge-colored $n$-vertex graph with no rainbow copy of $F$. The study of rainbow Tur\'an numbers was introduced by Keevash, Mubayi, Sudakov, and Verstra\"ete in 2007. In this paper we focus on $ex^*(n,P_5)$. While several recent papers have investigated rainbow Tur\'an numbers for $\ell$-edge paths $P_{\ell}$, exact results have only been obtained for $\ell < 5$, and $P_5$ represents one of the smallest cases left open in rainbow Tur\'{a}n theory. In this paper, we prove that $ex^*(n,P_5) \leq \frac{5n}{2}$. Combined with a lower-bound construction due to Johnston and Rombach, this result shows that $ex^*(n,P_5) = \frac{5n}{2} $ when $n$ is divisible by $16$, thereby settling the question asymptotically for all $n$. In addition, this result strengthens the conjecture that $ex^*(n,P_{\ell}) = \frac{\ell}{2}n + O(1)$ for all $\ell \geq 3$.