Minimum degree and size conditions for the proper connection number of graphs

Abstract

An edge-coloured graph $G$ is called $properly$ $connected$ if every two vertices are connected by a proper path. The $proper$ $connection$ $number$ of a connected graph $G$, denoted by $pc(G)$, is the smallest number of colours that are needed in order to make $G$ properly connected. Susan A. van Aardt et al. gave a sufficient condition for the proper connection number to be at most $k$ in terms of the size of graphs. In this note, %optimizes the boundary of the number of edges %we study the $proper$ $connection$ $number$ is under the conditions of adding the minimum degree and optimizing the number of edges. our main result is the following, by adding a minimum degree condition: Let $G$ be a connected graph of order $n$, $k\geq3$. If $|E(G)|\geq \binom{n-m-(k+1-m)(\delta+1)}{2} +(k+1-m)\binom{\delta+1}{2}+k+2$, then $pc(G)\leq k$, where $m$ takes the value $t$ if $\delta=1$ and $\lfloor \frac{k}{\delta-1} \rfloor$ if $\delta\geq2$. Furthermore, if $k=2$ and $\delta=2$, %(i.e., $|E(G)|\geq \binom{n-5}{2} +7$) $pc(G)\leq 2$, except $G\in \{G_{1}, G_{n}\}$ ($n\geq8$), where $G_{1}=K_{1}\vee 3K_{2}$ and $G_{n}$ is obtained by taking a complete graph $K_{n-5}$ and $K_{1}\vee (2K_{2}$) with an arbitrary vertex of $K_{n-5}$ and a vertex with $d(v)=4$ in $K_{1}\vee (2K_{2}$) being joined. If $k=2$, $\delta \geq 3$, we conjecture $pc(G)\leq 2$, where $m$ takes the value $1$ if $\delta=3$ and $0$ if $\delta\geq4$ in the assumption.