The Pentagon Graph Operator

TL;DR

This paper studies the iterative behavior of the pentagon graph operator, classifying graphs into vanishing, periodic, or expanding types, and explores broader operators based on induced cycles.

Contribution

It introduces a classification of graphs under the pentagon operator and constructs explicit examples, extending the theory of induced-cycle graph operators.

Findings

Every graph is classified as vanishing, periodic, or expanding under the pentagon operator.

Explicit examples include the dodecahedron and icosahedron as periodic cases.

An icosahedral tadpole-hat construction yields expanding families.

Abstract

For a graph $G$, let $\mathscr{C}_5(G)$ denote the graph whose vertices are the induced $5$-cycles of $G$, where two vertices are adjacent whenever the corresponding cycles share an edge. We investigate the iterative behavior of the pentagon graph operator $\mathscr{C}_5(G)$ , positioning it as the natural continuation of the quadrangle graph operator and the broader induced-cycle graph operator program. We construct explicit pentagon-vanishing, pentagon-periodic, and pentagon-expanding graphs. In particular, the dodecahedron and the icosahedron provide natural periodic examples, while an icosahedral tadpole-hat construction yields expanding families. Our main result proves that every graph is exactly one of three types with respect to $\mathscr{C}_5(G)$: vanishing, periodic, or expanding. The paper suggests a broader theory for the operators $C_k$ generated by induced cycles of fixed length $k$.