# Occupancy fraction, fractional colouring, and triangle fraction

## TL;DR

This paper establishes improved bounds on independent sets and fractional chromatic numbers in graphs with bounded neighborhood edge density, using a detailed analysis of the hard-core model, extending classical results.

## Contribution

It provides stronger bounds on independent sets and fractional chromatic numbers for graphs with certain neighborhood edge constraints, generalizing classical theorems.

## Key findings

- Expected size of a random independent set is at least a logarithmic factor times n/Δ.
- Fractional chromatic number is bounded by approximately 2 times Δ divided by log f.
- Bounds are asymptotically tight within a factor of 2.

## Abstract

Given $\varepsilon>0$, there exists $f_0$ such that, if $f_0 \le f \le \Delta^2+1$, then for any graph $G$ on $n$ vertices of maximum degree $\Delta$ in which the neighbourhood of every vertex in $G$ spans at most $\Delta^2/f$ edges, (i) an independent set of $G$ drawn uniformly at random has at least $(1/2-\varepsilon)(n/\Delta)\log f$ vertices in expectation, and (ii) the fractional chromatic number of $G$ is at most $(2+\varepsilon)\Delta/\log f$. These bounds cannot in general be improved by more than a factor $2$ asymptotically. One may view these as stronger versions of results of Ajtai, Koml\'os and Szemer\'edi (1981) and Shearer (1983). The proofs use a tight analysis of the hard-core model.