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Discrete Applied Mathematics
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Improving the Caro–Wei bound and applications to Turán stability

Improving the Caro-Wei bound and applications to Turán stability
Authors: Tom Kelly; Luke Postle;

Improving the Caro–Wei bound and applications to Turán stability

Abstract

We prove that if $G$ is a graph and $f(v) \leq 1/(d(v) + 1/2)$ for each $v\in V(G)$, then either $G$ has an independent set of size at least $\sum_{v\in V(G)}f(v)$ or $G$ contains a clique $K$ such that $\sum_{v\in K}f(v) > 1$. This result implies that for any $σ\leq 1/2$, if $G$ is a graph and every clique $K\subseteq V(G)$ has at most $(1 - σ)(|K| - σ)$ simplicial vertices, then $α(G) \geq \sum_{v\in V(G)} 1 / (d(v) + 1 - σ)$. Letting $σ= 0$ implies the famous Caro-Wei Theorem, and letting $σ= 1/2$ implies that if fewer than half of the vertices in each clique of $G$ are simplicial, then $α(G) \geq \sum_{v\in V(G)}1/(d(v) + 1/2)$, which is tight for the 5-cycle. When applied to the complement of a graph, this result implies the following new Tur\' an stability result. If $G$ is a $K_{r + 1}$-free graph with more than $(1 - 1/r)n^2/2 - n/4$ edges, then $G$ contains an independent set $I$ such that at least half of the vertices in $I$ are complete to $G - I$. Applying this stability result iteratively provides a new proof of the stability version of Tur\' an's Theorem in which $K_{r + 1}$-free graphs with close to the extremal number of edges are $r$-partite.

16 pages. Derived from the preprint arxiv:1811.11806v1

Keywords

Extremal problems in graph theory, Vertex subsets with special properties (dominating sets, independent sets, cliques, etc.), Coloring of graphs and hypergraphs, Brooks' theorem, Caro-Wei theorem, FOS: Mathematics, Mathematics - Combinatorics, Combinatorics (math.CO), independence number, simplicial vertices, Turán stability

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selected citations
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This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
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