“…Let x 0 , y 0 , z 0 , w 0 > 0 and c = max{Φ(x 0 , y 0 , z 0 , w 0 ), Φ(p)}. As above we see that N = {(x, y, z, w) ∈ (0, ∞) 4 : Φ(x, y, z, w) ≤ c} is closed, invariant, and contains exactly the equilibriump. By La Salle's principle, the solution converges to p * as t → ∞.…”