A classification of locally semicomplete digraphs

Recently, Huang (1995) gave a characterization of local tournaments. His characterization involves arc-reversals and therefore may not be easily used to solve other structural problems on locally semicomplete digraphs (where one deals with a fixed locally semicomplete digraph). In this paper we derive a classification of locally semicomplete digraphs which is very useful for studying structural properties of locally semicomplete digraphs and which does not depend on Huang's characterization. An advantage of this new classification of locally semicomplete digraphs is that it allows one to prove results for locally semicomplete digraphs without reproving the same statement for tournaments. We use our result to characterize pancyclic and vertex pancyclic locally semicomplete digraphs and to show the existence of a polynomial algorithm to decide whether a given locally semicomplete digraph has a kernel

A new framework for analysis of coevolutionary systems:Directed graph representation and random walks

Studying coevolutionary systems in the context of simplified models (i.e. games with pairwise interactions between coevolving solutions modelled as self plays) remains an open challenge since the rich underlying structures associated with pairwise comparison-based fitness measures are often not taken fully into account. Although cyclic dynamics have been demonstrated in several contexts (such as intransitivity in coevolutionary problems), there is no complete characterization of cycle structures and their effects on coevolutionary search. We develop a new framework to address this issue. At the core of our approach is the directed graph (digraph) representation of coevolutionary problem that fully captures structures in the relations between candidate solutions. Coevolutionary processes are modelled as a specific type of Markov chains ? random walks on digraphs. Using this framework, we show that coevolutionary problems admit a qualitative characterization: a coevolutionary problem is either solvable (there is a subset of solutions that dominates the remaining candidate solutions) or not. This has an implication on coevolutionary search. We further develop our framework that provide the means to construct quantitative tools for analysis of coevolutionary processes and demonstrate their applications through case studies. We show that coevolution of solvable problems corresponds to an absorbing Markov chain for which we can compute the expected hitting time of the absorbing class. Otherwise, coevolution will cycle indefinitely and the quantity of interest will be the limiting invariant distribution of the Markov chain. We also provide an index for characterizing complexity in coevolutionary problems and show how they can be generated in a controlled mannerauthorsversionPeer reviewe

Extremal and degree donditions for path extendability in digraphs

In the study of cycles and paths, the meta-conjecture of Bondy that sufficient conditions for Hamiltonicity often imply pancyclicity has motivated research on the existence of cycles and paths of many lengths. Hendry further introduced the stronger concepts of cycle extendability and path extendability, which require that every cycle or path can be extended to another one with one additional vertex. These concepts have been studied extensively, but there exist few results on path extendability in digraphs, as far as we know. In this paper, we make the first attempt in this direction. We establish a number of extremal and degree conditions for path extendability in general digraphs. Moreover, we prove that every path of length at least two in a regular tournament is extendable, with some exceptions. One of our proof approaches is a new contraction operation to transform nonextendable paths into nonextendable cycles

Moon-type theorems on circuits in strongly connected tournaments of order NN and diameter DD

Let $T$ be a strongly connected tournament of order $n\ge 4$ whose diameter does not exceed $d\ge 3.$ Denote by $c_{\ell}(T)$ the number of circuits of length $\ell$ in $T.$ In our recent paper, we construct a strongly connected tournament $T_{d,n}$ of order $n$ with diameter $d$ and conjecture that $c_{\ell}(T)\ge c_{\ell}(T_{d,n})$ for any $\ell=3,...,n.$ In particular, for $d=n-1,$ this inequality is true and yields the known Moon (lower) bound $c_{\ell}(T)\ge n-\ell+1.$ Moreover, we suggest that if $n+3\le 2d,$ then for any given $\ell$ taken in the range $n-d+3,...,d,$ the equality $c_{\ell}(T)=c_{\ell}(T_{d,n})$ implies that $T$ is isomorphic to $T_{d,n}$ or its converse $T_{d,n}^{-}.$ For $d=n-1,$ the corresponding particular statement is nothing else than Las Vergnas' theorem. Recently, we have confirmed the posed conjecture for the case $d=n-2.$ In the present paper, we show that it is also true for $d=n-3.

Splitting a tournament into two subtournaments with given minimum outdegree

A {\it $(k_1,k_2)$-outdegree-splitting} of a digraph $D$ is a partition $(V_1,V_2)$ of its vertex set such that $D[V_1]$ and $D[V_2]$ have minimum outdegree at least $k_1$ and $k_2$, respectively. We show that there exists a minimum function $f_T$ such that every tournament of minimum outdegree at least $f_T(k_1,k_2)$ has a $(k_1,k_2)$-outdegree-splitting, and $f_T(k_1,k_2) \leq k_1^2/2+3k_1/2 +k_2+1$. We also show a polynomial-time algorithm that finds a $(k_1,k_2)$-outdegree-splitting of a tournament if one exists, and returns 'no' otherwise. We give better bound on $f_T$ and faster algorithms when $k_1=1$.Un {\it $(k_1,k_2)$-partage} d'un digraphe $D$ est une partition $(V_1,V_2)$ de son ensemble de sommets telle que $D[V_1]$ et $D[V_2]$ soient de degréß sortant minimum au moins $k_1$ et $k_2$, respectivement. Nous établissons l'existence d'une fonction (minimum) $f_T$ telle que tout tournoi de degré sortant minimum au moins $f_T(k_1,k_2)$ a un $(k_1,k_2)$-partage, et que $f_T(k_1,k_2) \leq k_1^2/2+3k_1/2 +k_2+1$. Nous donnons également un algorithme en temps polynomial qui trouve un $(k_1,k_2)$-partage d'un tournoi s'il en existe un et renvoie 'non' sinon. Nous donnons de meilleures bornes sur $f_T$ et des algorithmes plus rapides pour $k_1=1$