# Triangle-free planar graphs with small independence number

## TL;DR

This paper investigates the independence number of triangle-free planar graphs, showing it can be significantly larger than n/3 unless certain obstructions are present, and provides tools to analyze and transform such graphs.

## Contribution

It introduces a new lower bound on the independence number for triangle-free planar graphs and offers a reduction rule to handle obstructions, advancing structural understanding.

## Key findings

- Independence number exceeds n/3 unless obstructions occur
- Reduction rule transforms graphs while preserving independence number properties
- Structural characterization of graphs with independence number close to n/3

## Abstract

Since planar triangle-free graphs are 3-colourable, such a graph with n vertices has an independent set of size at least n/3. We prove that unless the graph contains a certain obstruction, its independence number is at least n/(3-epsilon) for some fixed epsilon>0. We also provide a reduction rule for this obstruction, which enables us to transform any plane triangle-free graph G into a plane triangle-free graph G' such that alpha(G')-|G'|/3=alpha(G)-|G|/3 and |G'|<=(alpha(G)-|G|/3)/epsilon. We derive a number of algorithmic consequences as well as a structural description of n-vertex plane triangle-free graphs whose independence number is close to n/3.