# A New Heuristic for Detecting Non-Hamiltonicity in Cubic Graphs

## TL;DR

This paper introduces a new polyhedral heuristic for detecting non-Hamiltonian cubic graphs, showing promising empirical accuracy and potential for polynomial-time algorithms, especially when combined with specific edge cut set analysis.

## Contribution

The paper develops a novel polyhedral approach for identifying non-Hamiltonian cubic graphs and compares its effectiveness to existing relaxations like TSP, demonstrating improved empirical performance.

## Key findings

- The polyhedron is always empty for bridge graphs.
- Some non-bridge non-Hamiltonian graphs also induce empty polyhedra.
- The heuristic correctly identifies non-Hamiltonian graphs in most cases, with very few misdiagnoses.

## Abstract

We analyse a polyhedron which contains the convex hull of all Hamiltonian cycles of a given undirected connected cubic graph. Our constructed polyhedron is defined by polynomially-many linear constraints in polynomially-many continuous (relaxed) variables. Clearly, the emptiness of the constructed polyhedron implies that the graph is non-Hamiltonian. However, whenever a constructed polyhedron is non-empty, the result is inconclusive. Hence, the following natural question arises: if we assume that a non-empty polyhedron implies Hamiltonicity, how frequently is this diagnosis incorrect? We prove that, in the case of bridge graphs, the constructed polyhedron is always empty. We also demonstrate that some non-bridge non-Hamiltonian cubic graphs induce empty polyhedra as well. We compare our approach to the famous Dantzig-Fulkerson-Johnson relaxation of a TSP, and give empirical evidence which suggests that the latter is infeasible if and only if our constructed polyhedron is also empty. By considering special edge cut sets which are present in most cubic graphs, we describe a heuristic approach, built on our constructed polyhedron, for which incorrect diagnoses of non-Hamiltonian graphs as Hamiltonian appear to be very rare. In particular, for cubic graphs containing up to 18 vertices, only four out of 45,982 undirected connected cubic graphs were so misdiagnosed. By constrast, we demonstrate that an equivalent heuristic, when built on the Dantzig-Fulkerson-Johnson relaxation of a TSP, is mostly unsuccessful in identifying additional non-Hamiltonian graphs. These empirical results suggest that polynomial algorithms based on our constructed polyhedron may be able to correctly identify Hamiltonicity of a cubic graph in all but rare cases.