Degree conditions and [a,b]-covered graphs in bipartite graphs

Let g be a graph with vertex set V(G) and edge set E(G),and let g and f be two integer-valued functions defined on V(G), such that gf for every x∈V(G). A (g,f)-factor of G is a spanning subgraph F of G, such that g(x)≤d_F(x)≤f(x) for every x∈V(G). A graph G is called a (g,f)-covered graph if every two edges belong to a (g,f)-factor. Let G=(X,Y;E) be a balance bipartite graph, where X=Y=n.In this paper, it is proved that, if δ(G)≥a+b+n-2bn-1, or δ(G)≥an+1a+b and n≥(a+b)2b-a+bb, then G is a [a,b]-covered graph .