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Title:
Walecki tournaments with an arc that lies in a unique directed triangle
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Discussiones Mathematicae Graph Theory
Received: 2024-08-19 , Revised: 2024-12-07 , Accepted: 2024-12-09 , Available online: 2025-01-10 , https://doi.org/10.7151/dmgt.2573
Abstract:
A Walecki tournament is any tournament that can be formed by choosing an
orientation for each of the Hamilton cycles in the Walecki decomposition of
a complete graph on an odd number of vertices. In this paper, we show that if
some arc in a Walecki tournament on at least $7$ vertices lies in exactly one
directed triangle, then there is a vertex of the tournament (the vertex
typically labelled $*$ in the decomposition) that is fixed under every
automorphism of the tournament. Furthermore, any isomorphism between such
Walecki tournaments maps the vertex labelled $*$ in one to the vertex labelled
$*$ in the other.
We also show that among Walecki tournaments with a signature of even length
$2k$, of the $2^{2k}$ possible signatures, at least $2^k$ produce tournaments
that have an arc that lies in a unique directed triangle (and therefore to
which our result applies).
Keywords:
tournaments, Walecki decomposition, Walecki tournaments, automorphisms, isomorphisms
References:
- J. Aleš, Automorphism Groups of Walecki Tournaments, Ph.D Thesis (Simon Fraser University, Burnaby, 1999).
- J. Aleš, Automorphism groups of Walecki tournaments with zero and odd signatures, Art Discrete Appl. Math. 2 (2018) #P1.06.
https://doi.org/10.26493/2590-9770.1266.729 - B. Alspach, A Class of Tournaments, Ph.D Thesis (University of California, Santa Barbara, 1966).
- B. Alspach, K.B. Reid and D.P. Roselle, Bypasses in asymmetric digraphs, J. Combin. Theory Ser. B 17 (1974) 11–18.
https://doi.org/10.1016/0095-8956(74)90041-0
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