The $(k,\ell)$-proper index of graphs

TL;DR

This paper introduces the $(k, ext{l})$-proper index of graphs, studying the minimum colors needed for edge-colorings that ensure multiple disjoint proper trees connecting any $k$-subset of vertices, with results on complete, bipartite, and random graphs.

Contribution

It establishes bounds and thresholds for the $(k, ext{l})$-proper index in complete, bipartite, and Erdős-Rényi random graphs, advancing understanding of graph connectivity under proper edge-colorings.

Findings

For large enough $n$, $px_{k, ext{l}}(K_n)=2$.

For bipartite graphs $K_{m,n}$ with $m=O(n^r)$, $px_{k, ext{l}}(K_{m,n})=2$ for large $n$.

In Erdős-Rényi graphs $G_{n,p}$, the index is almost surely at most 2 when $p$ exceeds a certain threshold.

Abstract

A tree $T$ in an edge-colored graph is called a {\it proper tree} if no two adjacent edges of $T$ receive the same color. Let $G$ be a connected graph of order $n$ and $k$ be an integer with $2\leq k \leq n$. For $S\subseteq V(G)$ and $|S| \ge 2$, an $S$-tree is a tree containing the vertices of $S$ in $G$. Suppose $\{T_1,T_2,\ldots,T_\ell\}$ is a set of $S$-trees, they are called \emph{internally disjoint} if $E(T_i)\cap E(T_j)=\emptyset$ and $V(T_i)\cap V(T_j)=S$ for $1\leq i\neq j\leq \ell$. For a set $S$ of $k$ vertices of $G$, the maximum number of internally disjoint $S$-trees in $G$ is denoted by $\kappa(S)$. The $\kappa$-connectivity $\kappa_k(G)$ of $G$ is defined by $\kappa_k(G)=\min\{\kappa(S)\mid S$ is a $k$-subset of $V(G)\}$. For a connected graph $G$ of order $n$ and for two integers $k$ and $\ell$ with $2\le k\le n$ and $1\leq \ell \leq \kappa_k(G)$, the \emph{$(k,\ell)$-proper index $px_{k,\ell}(G)$} of $G$ is the minimum number of colors that are needed in an edge-coloring of $G$ such that for every $k$-subset $S$ of $V(G)$, there exist $\ell$ internally disjoint proper $S$-trees connecting them. In this paper, we show that for every pair of positive integers $k$ and $\ell$ with $k \ge 3$, there exists a positive integer $N_1=N_1(k,\ell)$ such that $px_{k,\ell}(K_n) = 2$ for every integer $n \ge N_1$, and also there exists a positive integer $N_2=N_2(k,\ell)$ such that $px_{k,\ell}(K_{m,n}) = 2$ for every integer $n \ge N_2$ and $m=O(n^r) (r \ge 1)$. In addition, we show that for every $p \ge c\sqrt[k]{\frac{\log_a n}{n}}$ ($c \ge 5$), $px_{k,\ell}(G_{n,p})\le 2$ holds almost surely, where $G_{n,p}$ is the Erd\"{o}s-R\'{e}nyi random graph model.