“…If there is a critical value of τ such that a certain root of (9) has zero real part, then at this critical value the stability of the zero equilibrium (0, 0) of system (8) will switch, and under certain conditions a family of small amplitude periodic solutions can bifurcate from the zero equilibrium (0, 0); that is, a Hopf bifurcation occurs at the zero equilibrium (0, 0). Now, we look for the conditions under which the characteristic Equation (9) has a pair of purely imaginary roots, see [24]. Clearly, iω(ω > 0) is a root of Equation (9) …”