“…Consequently, there exist ðu * , v * Þ and ðw * , z * Þ in P l which are nonnegative extremal solutions of BVP ( 5) and (6). Since f k ðt, 0, 0Þ≡0, ∀t ∈ ½0, 1, for k = 1, 2; then, zero is not a solution of problem (5). It is obvious that ðu * , v * ÞðtÞ > 0, ðw * , z * ÞðtÞ > 0.…”