Equating two maximum degrees

Abstract

Given a graph $G$, we would like to find (if it exists) the largest induced subgraph $H$ in which there are at least $k$ vertices realizing the maximum degree of $H$. This problem was first posed by Caro and Yuster. They proved, for example, that for every graph $G$ on $n$ vertices we can guarantee, for $k = 2$, such an induced subgraph $H$ by deleting at most $2\sqrt{n}$ vertices, but the question if $2\sqrt{n}$ is best possible remains open. Among the results obtained in this paper we prove that: 1. For every graph $G$ on $n \geq 4$ vertices we can delete at most $\lceil \frac{- 3 + \sqrt{ 8n- 15}}{2 } \rceil$ vertices to get an induced subgraph $H$ with at least two vertices realizing $\Delta(H)$, and this bound is sharp, solving the problems left open by Caro and Yuster. 2.For every graph $G$ with maximum degree $\Delta \geq 1$ we can delete at most $\lceil \frac{ -3 + \sqrt{8\Delta +1}}{2 } \rceil$ vertices to get an induced subgraph $H$ with at least two vertices realizing $\Delta(H)$, and this bound is sharp. 3. Every graph $G$ with $\Delta(G) \leq 2$ and least $2k - 1$ vertices (respectively $2k - 2$ vertices if k is even) contains an induced subgraph $H$ in which at least $k$ vertices realise $\Delta(H)$, and these bound are sharp.