Bipartite graphs with close domination and k-domination numbers

Let k be a positive integer and let G be a graph with vertex set V(G). A subset D subset of V(G) is a k-dominating set if every vertex outside D is adjacent to at least k vertices in D. the k-domination number gamma(k)(G) is the minimum cardinality of a k-dominating set in G. For any graph G, we know that gamma(k)(G) >= gamma(G) + k - 2 where Delta(G) >= k >= 2 and this bound is sharp for every k >= 2. in this paper, we characterize bipartite graphs satisfying the equality for k >= 3 and present a necessary and sufficient condition for a bipartite graph to satisfy the equality hereditarily when k = 3. We also prove that the problem of deciding whether a graph satisfies the given equality is NP-hard in general.