Improved bound of graph energy in terms of vertex cover number

Abstract

Let $ G $ be a simple graph with the vertex cover number $ \tau $. The energy $ \mathcal{E}(G) $ of $ G $ is the sum of the absolute values of all the adjacency eigenvalues of $ G $. In this article, we establish $ \mathcal{E}(G)\geq 2\tau $ for several classes of graphs. The result significantly improves the known result $ \mathcal{E}(G)\geq 2\tau-2c$ for many classes of graphs, where $ c $ is the number of odd cycles.