The number of induced paths in outerplanar graphs

Abstract

Let $P_k$ denote the path with $k$ vertices, and $\mathrm{ex}_{\mathcal{OP}}(n,H^{\mathrm{ind}},\emptyset)$ be the maximum number of induced copies of $H$ in an $n$-vertex outerplanar graph. In this paper, we determine the exact value of $\mathrm{ex}_{\mathcal{OP}}(n,P_3^{\mathrm{ind}},\emptyset)$ for all $n$, and give an asymptotic value of $\mathrm{ex}_{\mathcal{OP}}(n,P_4^{\mathrm{ind}},\emptyset)$. For general $k$, Matolcsi and Nagy proved that $\lim_{k\to \infty} {\left( \mathrm{ex}_{\mathcal{OP}}(n, P_{k+1},\emptyset)\right)^{1/k}} =4$. In the induced case, we prove that \[ fib(k-1)\frac{{(n-2k+3)}^2}{4} \le \mathrm{ex}_{\mathcal{OP}}(n, P_{k+1}^{\mathrm{ind}},\emptyset) \le fib(k+1) \binom{n}{2}, \] where $fib(k)$ is the Fibonacci number. This implies that $\lim_{k\to \infty} {\left( \mathrm{ex}_{\mathcal{OP}}(n, P_{k+1}^{\mathrm{ind}},\emptyset)\right)^{1/k}} = \frac{\sqrt{5}+1}{2}\approx 1.618$.