3-Coloring on Regular, Planar, and Ordered Hamiltonian Graphs

We prove that 3-Coloring remains NP-hard on 4- and 5-regular planar Hamiltonian graphs, strengthening the results of Dailey [Disc. Math.'80] and Fleischner and Sabidussi [J. Graph. Theor.'02]. Moreover, we prove that 3-Coloring remains NP-hard on $p$-regular Hamiltonian graphs for every $p\geq 6$and$p$-orderedregularHamiltoniangraphsforevery$p\geq 3$.