3. For the very simple (and weird, and kinda nonsensical) disease system with susceptible individuals and infected individuals, dS = S - BSI dt dI = BSI I dt We can look at the Lyapunov function V = (S+I) + This function only satisfies the postivity requirement of Lyapunov functions when S > 0 and I 0. Is this a problem? Use the Lyapunov function to determine a region of SI-space where (0,0) is stable.