√99以上 x^2-y^2 741608-X 2 y 2 z 2 1

WebA source of examples is u(x,y) = (x2 −y2)n where n ≥ 2 Then the x partial is n(x2 −y2)n−1 ⋅ 2x, which vanishes on ∣x∣ = ∣y∣, and similarly for the y partial (Note this was Ellipse and