Partial Degree Conditions and Cycle Coverings in Bipartite Graphs

Abstract

Let k be a positive integer. Let G be a graph of order $n\ge 3$ and W a subset of V(G) with $|W|\ge 3k$. Wang (J Graph Theory 78:295–304, 2015) proved that if $d(x)\ge 2n/3$ for each $x\in W$, then G contains k vertex-disjoint cycles such that each of them contains at least three vertices of W. In this paper, we obtain an analogue result of Wang’s Theorem in bipartite graph with the partial degree condition. Let $G=(V_1,V_2;E)$ be a bipartite graph with $|V_1|=|V_2|=n$, and let W be a subset of $V_1$ with $|W|\ge 2k$, where k is a positive integer. We show that if $d(x)+d(y)\ge n+k$ for every pair of nonadjacent vertices $x\in W, y\in V_2$, then G contains k vertex-disjoint cycles such that each of them contains at least two vertices of W.