“…It should be pointed here that for a class of multi-group SEIR models described by ordinary differential equations, a graph-theoretic approach to the method of global Lyapunov functions was proposed and used to obtain the global stability of a unique endemic equilibrium in [6]. During the past decades, the multi-group epidemic models have been extensively studied by many authors (see, for example, [5,6,9,12,13,21,24,25,27] and the references therein). These studies have enriched our knowledge of epidemic models with heterogeneity.…”