A Construction of a 3/2-Tough Plane Triangulation With No 2-Factor

Abstract

In 1956, Tutte proved the celebrated theorem that every 4-connected planar graph is Hamiltonian. This result implies that every more than $\frac{3}{2}$-tough planar graph on at least three vertices is Hamiltonian and so has a 2-factor. Owens in 1999 constructed non-Hamiltonian maximal planar graphs of toughness arbitrarily close to $\frac{3}{2}$ and asked whether there exists a maximal non-Hamiltonian planar graph of toughness exactly $\frac{3}{2}$. In fact, the graphs Owens constructed do not even contain a 2-factor. Thus the toughness of exactly $\frac{3}{2}$ is the only case left in asking the existence of 2-factors in tough planar graphs. This question was also asked by Bauer, Broersma, and Schmeichel in a survey. In this paper, we close this gap by constructing a maximal $\frac{3}{2}$-tough plane graph with no 2-factor, answering the question asked by Owens as well as by Bauer, Broersma, and Schmeichel.