Hostname: page-component-78c5997874-ndw9j Total loading time: 0 Render date: 2024-11-08T05:01:14.840Z Has data issue: false hasContentIssue false

On the Maximum Number of Spanning Copies of an Orientation in a Tournament

Published online by Cambridge University Press:  06 June 2017

RAPHAEL YUSTER*
Affiliation:
Department of Mathematics, University of Haifa, Haifa 31905, Israel (e-mail: [email protected])

Abstract

For an orientation H with n vertices, let T(H) denote the maximum possible number of labelled copies of H in an n-vertex tournament. It is easily seen that T(H) ≥ n!/2e(H), as the latter is the expected number of such copies in a random tournament. For n odd, let R(H) denote the maximum possible number of labelled copies of H in an n-vertex regular tournament. In fact, Adler, Alon and Ross proved that for H=Cn, the directed Hamilton cycle, T(Cn) ≥ (e−o(1))n!/2n, and it was observed by Alon that already R(Cn) ≥ (e−o(1))n!/2n. Similar results hold for the directed Hamilton path Pn. In other words, for the Hamilton path and cycle, the lower bound derived from the expectation argument can be improved by a constant factor. In this paper we significantly extend these results, and prove that they hold for a larger family of orientations H which includes all bounded-degree Eulerian orientations and all bounded-degree balanced orientations, as well as many others. One corollary of our method is that for any fixed k, every k-regular orientation H with n vertices satisfies T(H) ≥ (eko(1))n!/2e(H), and in fact, for n odd, R(H) ≥ (eko(1))n!/2e(H).

Type
Paper
Copyright
Copyright © Cambridge University Press 2017 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

[1] Adler, I., Alon, N. and Ross, S. (2001) On the maximum number of Hamiltonian paths in tournaments. Random Struct. Alg. 18 291296.CrossRefGoogle Scholar
[2] Alon, N. (1990) The maximum number of Hamiltonian paths in tournaments. Combinatorica 10 319324.CrossRefGoogle Scholar
[3] Alon, N. (2016) Problems and results in Extremal Combinatorics, III. J. Combin. 7 233256.Google Scholar
[4] Bollobás, B. (1978) Extremal Graph Theory, Academic Press.Google Scholar
[5] Bondy, A. (1995) Basic graph theory: Paths and circuits. In Handbook of Combinatorics (Graham, R., Grötschel, M. and Lovász, L., eds), Vol. 1, North-Holland, pp. 3110.Google Scholar
[6] Corrádi, K. and Hajnal, A. (1963) On the maximal number of independent circuits in a graph. Acta Math. Hungar. 14 423439.CrossRefGoogle Scholar
[7] Cuckler, B. (2007) Hamiltonian cycles in regular tournaments. Combin. Probab. Comput. 16 239249.CrossRefGoogle Scholar
[8] Ferber, A., Krivelevich, M. and Sudakov, B. (2012) Counting and packing Hamilton cycles in dense graphs and oriented graphs. J. Combin. Theory Ser. B 122 (2017), 196220.CrossRefGoogle Scholar
[9] Friedgut, E. and Kahn, J. (2005) On the number of Hamiltonian cycles in a tournament. Combin. Probab. Comput. 14 769781.CrossRefGoogle Scholar
[10] Gustavsson, T. (1991) Decompositions of large graphs and digraphs with high minimum degree. PhD thesis, University of Stockholm.Google Scholar
[11] Keevash, P. (2014) The existence of designs. arXiv:1401.3665 Google Scholar
[12] Kühn, D. and Osthus, D. (2012) A survey on Hamilton cycles in directed graphs. European J. Combin. 33 750766.CrossRefGoogle Scholar
[13] Rödl, V. (1985) On a packing and covering problem. European J. Combin. 6 6978.CrossRefGoogle Scholar
[14] Szele, T. (1943) Kombinatorikai vizsgálatok az irányitott teljes gráffal kapcsolatban. Mat. Fiz. Lapok 50 223256.Google Scholar
[15] Thomassen, C. (1985) Hamilton circuits in regular tournaments. Ann. Discrete Math. 27 159162.Google Scholar
[16] Turán, P. (1941) On an extremal problem in graph theory (in Hungarian). Mat. Fiz. Lapok 48 436452.Google Scholar
[17] Wilson, R. (1975) Decomposition of complete graphs into subgraphs isomorphic to a given graph. Congress. Numer. XV 647659.Google Scholar
[18] Wormald, N. Tournaments with many Hamilton cycles. Manuscript.Google Scholar