Tournament (graph theory)

Tournament
A tournament on 4 vertices
Vertices	$n$
Edges	${\binom {n}{2}}$
Table of graphs and parameters

A tournament is a directed graph (digraph) obtained by assigning a direction for each edge in an undirected complete graph. That is, it is an orientation of a complete graph, or equivalently a directed graph in which every pair of vertices is connected by a single directed edge.

Many of the important properties of tournaments were first investigated by Landau in order to model dominance relations in flocks of chickens. Current applications of tournaments include the study of voting theory and social choice theory among other things. The name tournament originates from such a graph's interpretation as the outcome of a round-robin tournament in which every player encounters every other player exactly once, and in which no draws occur. In the tournament digraph, the vertices correspond to the players. The edge between each pair of players is oriented from the winner to the loser. If player $a$ beats player $b$, then it is said that $a$ dominates $b$.

Paths and cycles

Any tournament on a finite number $n$ of vertices contains a Hamiltonian path, i.e., directed path on all $n$ vertices (Rédei 1934). This is easily shown by induction on $n$: suppose that the statement holds for $n$, and consider any tournament $T$ on $n+1$ vertices. Choose a vertex $v_{0}$ of $T$ and consider a directed path $v_{1},v_{2},\ldots ,v_{n}$ in $T\setminus \{v_{0}\}$. Now let $i\in \{0,\ldots ,n\}$ be maximal such that for every $j\leq i$ there is a directed edge from $v_{j}$ to $v_{0}$.

$v_{1},\ldots ,v_{i},v_{0},v_{i+1},\ldots ,v_{n}$

is a directed path as desired. This argument also gives an algorithm for finding the Hamiltonian path. More efficient algorithms, that require examining only $\ O(n\log n)$ of the edges, are known.

This implies that a strongly connected tournament has a Hamiltonian cycle (Camion 1959). More strongly, every strongly connected tournament is vertex pancyclic: for each vertex v, and each k in the range from three to the number of vertices in the tournament, there is a cycle of length k containing v. Moreover, if the tournament is 4‑connected, each pair of vertices can be connected with a Hamiltonian path (Thomassen 1980).

Transitivity

A transitive tournament on 8 vertices.

A tournament in which $((a\rightarrow b)$ and $(b\rightarrow c))$ $\Rightarrow$ $(a\rightarrow c)$ is called transitive. The following statements are equivalent for a tournament T on n vertices:

1. T is transitive
2. T is acyclic
3. T does not contain a cycle of length 3
4. The score sequence (set of outdegrees) of T is {0,1,2,...,n − 1}.
5. T has exactly one Hamiltonian path.

Every tournament on n vertices has a transitive subtournament on log₂n vertices. Reid & Parker (1970) showed that this bound is not tight. Erdős & Moser (1964) also proved that there are tournaments on n vertices without a transitive subtournament of size 2log₂n.

A player who wins all games would naturally be the tournament's winner. However, as the above example shows, there might not be such a player. A tournament for which every player loses at least one game is called a 1-paradoxical tournament. More generally, a tournament T=(V,E) is called k-paradoxical if for every k-element subset S of V there is a vertex v₀ in $V\setminus S$ such that $v_{0}\rightarrow v$ for all $v\in S$. By means of the probabilistic method, Paul Erdős showed that for any fixed value of k, if |V| ≥ k²2^kln(2 + o(1)), then almost every tournament on V is k-paradoxical. On the other hand, an easy argument shows that any k-paradoxical tournament must have at least 2^k+1 − 1 players, which was improved to (k + 2)2^k−1 − 1 by Esther and George Szekeres (1965). There is an explicit construction of k-paradoxical tournaments with k²4^k−1(1 + o(1)) players by Graham and Spencer (1971) namely the Paley tournament.

The condensation of any tournament is itself a transitive tournament.

Score sequences and score sets

The score sequence of a tournament is the nondecreasing sequence of outdegrees of the vertices of a tournament. The score set of a tournament is the set of integers that are the outdegrees of vertices in that tournament.

Landau's Theorem (1953) A nondecreasing sequence of integers $(s_{1},s_{2},\cdots ,s_{n})$ is a score sequence if and only if :

1. $0\leq s_{1}\leq s_{2}\leq \cdots \leq s_{n}$
2. $s_{1}+s_{2}+\cdots +s_{i}\geq {i \choose 2},{\mbox{for }}i=1,2,\cdots ,n-1$
3. $s_{1}+s_{2}+\cdots +s_{n}={n \choose 2}.$

Let $s(n)$ be the number of different score sequences of size $n$. The sequence $s(n)$ (sequence A000571 in the OEIS) starts as:

1, 1, 1, 2, 4, 9, 22, 59, 167, 490, 1486, 4639, 14805, 48107, ...

Winston and Kleitman proved that for sufficiently large n:

$s(n)>c_{1}4^{n}n^{-{5 \over 2}},$

where $c_{1}=0.049.$ Takács later showed, using some reasonable but unproven assumptions, that

$s(n)<c_{2}4^{n}n^{-{5 \over 2}},$

where $c_{2}<4.858.$

Together these provide evidence that:

$s(n)\in \Theta (4^{n}n^{-{5 \over 2}}).$

Here $\Theta$ signifies an asymptotically tight bound.

Yao showed that every nonempty set of nonnegative integers is the score set for some tournament.

See also

• Oriented graph
• Paley tournament
• Stockmeyer tournament
• Sumner's conjecture

References

• Bar-Noy, A.; Naor, J. (1990), "Sorting, Minimal Feedback Sets and Hamilton Paths in Tournaments", SIAM Journal on Discrete Mathematics, 3 (1): 7–20, doi:10.1137/0403002.
• Camion, Paul (1959), "Chemins et circuits hamiltoniens des graphes complets", Comptes Rendus de l'Académie des Sciences de Paris, 249: 2151–2152.
• Erdős, P. (1963), "On a problem in graph theory" (PDF), The Mathematical Gazette, 47: 220–223, JSTOR 3613396, MR 0159319.
• Erdős, P.; Moser, L. (1964), "On the representation of directed graphs as unions of orderings" (PDF), Magyar Tud. Akad. Mat. Kutató Int. Közl., 9: 125–132, MR 0168494.
• Graham, R. L.; Spencer, J. H. (1971), "A constructive solution to a tournament problem", Canadian Mathematical Bulletin. Bulletin Canadien de Mathématiques, 14: 45–48, MR 0292715.
• Harary, Frank; Moser, Leo (1966), "The theory of round robin tournaments", American Mathematical Monthly, 73 (3): 231–246, doi:10.2307/2315334, JSTOR 2315334.
• Landau, H.G. (1953), "On dominance relations and the structure of animal societies. III. The condition for a score structure", Bulletin of Mathematical Biophysics, 15 (2): 143–148, doi:10.1007/BF02476378.
• Moon, J. W. (1966), "On subtournaments of a tournament", Canadian Mathematical Bulletin, 9 (3): 297–301, doi:10.4153/CMB-1966-038-7.
• Rédei, Lázló (1934), "Ein kombinatorischer Satz", Acta Litteraria Szeged, 7: 39–43.
• Reid, K.B.; Parker, E.T. (1970), "Disproof of a conjecture of Erdös and Moser", Journal of Combinatorial Theory, 9 (3): 225–238, doi:10.1016/S0021-9800(70)80061-8.
• Szekeres, E.; Szekeres, G. (1965), "On a problem of Schütte and Erdős", The Mathematical Gazette, 49: 290–293, MR 0186566.
• Takács, Lajos; Takacs, Lajos (1991), "A Bernoulli Excursion and Its Various Applications", Advances in Applied Probability, 23 (3), Applied Probability Trust: 557–585, doi:10.2307/1427622, JSTOR 1427622.
• Thomassen, Carsten (1980), "Hamiltonian-Connected Tournaments", Journal of Combinatorial Theory, Series B, 28 (2): 142–163, doi:10.1016/0095-8956(80)90061-1.
• Yao, T.X. (1989), "On Reid Conjecture Of Score Sets For Tournaments", Chinese Sci. Bull., 34: 804–808.

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