A partial differential equation containing dependent variable u is given by A ∂²u/∂x² + 2B ∂²u/∂x∂y + C ∂²u/∂y² + D ∂u/∂x + E ∂u/∂y + F u + G = 0 where A, B, C, D, E, F and G are constants or functions of independent variables x or y only. Also G ≠ 0. The nature of the equation is