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Improved bounds for hypo-Hamiltonian graphs. (English) Zbl 1380.05034
Summary: A graph $$G$$ is hypo-Hamiltonian if $$G$$ is non-Hamiltonian and $$G-v$$ is hamiltonian for every $$v\in V(G)$$. In the following, every graph is assumed to be hypo-Hamiltonian. R. E. L. Aldred et al. [J. Comb. Math. Comb. Comput. 23, 143–152 (1997; Zbl 0880.05061)] gave a list of all graphs of order at most 17. In this article, we present an algorithm to generate all graphs of a given order and apply it to prove that there exist exactly 14 graphs of order 18 and 34 graphs of order 19. We also extend their results in the cubic case. Furthermore, we show that (i) the smallest graph of girth 6 has order 25, (ii) the smallest planar graph has order at least 23, (iii) the smallest cubic planar graph has order at least 54, and (iv) the smallest cubic planar graph of girth 5 with non-trivial automorphism group has order 78.

##### MSC:
 05C10 Planar graphs; geometric and topological aspects of graph theory 05C38 Paths and cycles 05C45 Eulerian and Hamiltonian graphs 05C85 Graph algorithms (graph-theoretic aspects)
##### Software:
GenHypohamiltonian; GENREG; nauty; plantri; snarkhunter; Traces
Full Text:
##### References:
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