On extremal k-supereulerian graphs

A graph G is called k-supereulerian if it has a spanning even subgraph with at most k components. In this paper, we prove that any 2-edge-connected loopless graph of order n is ⌈(n−2)/3⌉-supereulerian, with only one exception. This result solves a conjecture in [Z. Niu, L. Xiong, Even factor of a graph with a bounded number of components, Australas. J. Combin. 48 (2010) 269-279]. As applications, we give a best possible size lower bound for a 2-edge-connected simple graph G with n>5k+2 vertices to be k-supereulerian, a best possible minimum degree lower bound for a 2-edge-connected simple graph G such that its line graph L(G) has a 2-factor with at most k components, for any given integer k>0, and a sufficient condition for k-supereulerian graphs.