“…Note that when x > 0 and y < 0, we have z(x, y) > 0, ∂ 2 z/∂ x 2 > 0, ∂ 2 z/∂ y 2 > 0 and ∂ 2 z/∂ x 2 · ∂ 2 z/∂ y 2 − (∂ 2 z/∂ x∂ y) 2 on the positive coordinate axis x = 0, that is, p > 0, q ≥ 0 and r ≥ 0, there cannot be any tangent plane of the envelope S which also passes through this point, and when the point ( p, q, r ) is vertically above the envelope S in the forth octant, that is, p > 0, q < 0 and r > (−1) τ +1 τ τ q τ +1 /(τ + 1) τ +1 p τ , there cannot exist any tangent plane of the envelope S which also passes through this point, while the point ( p, q, r ) is situated elsewhere, such a tangent plane can be drawn. Since (15) is identical with (17) for the existence of positive solutions, it follows from Lemma 4 that (15) does not have any positive roots if and only if p > 0, q ≥ 0 and r ≥ 0 or p > 0, q < 0 and…”