On Directed 2-factors in Digraphs and 2-factors Containing Perfect Matchings in Bipartite Graphs

Abstract

In this paper, we give the following result: If $D$ is a digraph of order $n$, and if $d_{D}^{+}(u) + d_{D}^{-}(v) \ge n$ for every two distinct vertices $u$ and $v$ with $(u, v) \notin A(D)$, then $D$ has a directed 2-factor with exactly $k$ directed cycles of length at least 3, where $n \ge 12k+3$. This result is equivalent to the following result: If $G$ is a balanced bipartite graph of order 2n with partite sets $X$ and $Y$, and if $d_{G}(x)+d_{G}(y) \ge n + 2$ for every two vertices $x \in X$ and $y \in Y$ with $xy \notin E(G)$, then for every perfect matching $M$, $G$ has a 2-factor with exactly $k$ cycles of length at least 6 containing every edge of $M$, where $n \ge 12k+3$. These results are generalizations of theorems concerning Hamilton cycles due to Woodall [Proc. Lond. Math. Soc., 24 (1972), pp. 739--755] and Las Vergnas [Problemes de couplages et problemes hamiltoniens en theorie des graphes, Ph.D. thesis, University of Paris, 1972], respectively.