Bounds for identifying codes in terms of degree parameters

An identifying code is a subset of vertices of a graph such that each vertex is uniquely determined by its neighbourhood within the identifying code. If $\M(G)$ denotes the minimum size of an identifying code of a graph $G$, it was conjectured by F. Foucaud, R. Klasing, A. Kosowski and A. Raspaud that there exists a constant $c$ such that if a connected graph $G$ with $n$ vertices and maximum degree $d$ admits an identifying code, then $\M(G)\leq n-\tfrac{n}{d}+c$. We use probabilistic tools to show that for any $d\geq 3$, $\M(G)\leq n-\tfrac{n}{\Theta(d)}$ holds for a large class of graphs containing, among others, all regular graphs and all graphs of bounded clique number. This settles the conjecture (up to constants) for these classes of graphs. In the general case, we prove $\M(G)\leq n-\tfrac{n}{\Theta(d^{3})}$. In a second part, we prove that in any graph $G$ of minimum degree $\delta$ and girth at least 5, $\M(G)\leq(1+o_\delta(1))\tfrac{3\log\delta}{2\delta}n$. Using the former result, we give sharp estimates for the size of the minimum identifying code of random $d$-regular graphs, which is about $\tfrac{\log d}{d}n$.

💡 Research Summary

The paper investigates upper bounds on the minimum size γ_ID(G) of an identifying code in a graph G, focusing on how these bounds depend on degree parameters such as maximum degree d, minimum degree δ, and structural constraints like girth. An identifying code is a set C ⊆ V(G) that simultaneously dominates the graph (every vertex outside C has a neighbor in C) and separates every pair of vertices (the closed neighborhoods intersect C in distinct ways). Only twin‑free graphs admit such codes, so the study is restricted to this class.

Main contributions

1. Maximum‑degree bound (general graphs).
   For any twin‑free graph with maximum degree d ≥ 3, the authors prove
   \