罗荣教授是美国West Virginia University 数学系教授，国际知名图论研究专家。他主要研究兴趣在图的整数流、图的染色、图的连通性等图论重要研究领域，发表了40多篇学术论文，多发表在J. of Combin. Theory (B)、Euorp. J. Combin、SIAM J. Discret. Math等组合图论领域顶尖期刊上。
Let A be an Abelian group. It is known that an A-connected graph cannot be very sparse. We study the extremal problem: find the maximum integer k, denoted ex(n, A), such that every graph with at most k edges is not A-connected. We determine the exact values for all finite cyclic groups. As a corollary, we present a characterization of all Z_k-connected graphic sequences. It is also known that there are Z_5-connected graph that are not Z_6-connected. We prove that every Z_3-connected graph contains two edge-disjoint spanning trees, which implies that every Z_3-connected graph is also A-connected for any A with order at least 4.