“…It may be shown that, for all endemic equilibrium points that form the upper curve, since * > * 0 , from (18) it results in the fact that Det( ) > 0; thereby, in (16) it follows that both eigenvalues 1 and 2 are negative, and therefore, these equilibrium points are asymptotically stable. On the other hand, it can also be shown that for endemic equilibrium points that are part of the lower curve, since * < * 0 , from (18) it occurs that Det( ) < 0; in this way, again from (16), it is obtained that 1 > 0 and 2 < 0, and consequently such equilibrium points are unstable. Note that in the latter case (in order that * ∈ Δ), quadratic branch is delimited by the point (1, 0) in which, by the way, Det( ) = 0 and therefore 1 < 0 and 2 = 0; that is, the said point is nonhyperbolic and in it a bifurcation could also occur.…”