Laplace's equation in Bulgarian

(potential theory) The partial differential equation \frac {\partial^2 \varphi}{\partial x_1^2} + \frac{\partial^2 \varphi}{\partial x_2^2} + \cdots + \frac{\partial ^2 \varphi}{\partial x_n^2} =0, commonly written \Delta \varphi =0 or \nabla ^2\varphi =0, where \Delta \ (= \nabla^2) is the Laplace operator and \varphi is a scalar function.