Some remarks on an environmental defensive expenditures model

In this paper, we consider the environmental defensive expenditures model with delay proposed by Russu in P. Russu, Hopf bifurcation in a environmental defensive expenditures model with time delay, Chaos, Solitons and Fractals 42, pp. 3147-3159, 2009 and obtain different results about stability of equilibria in the case of absence of delay. Moreover we provide a more detailed analysis of the stability for equilibria and Hopf bifurcation in the case with delay. Finally, we discuss possible modifications of the model in order to make it more accurate and realistic.