Differential equation that contains unknown multivariable functions and their partial derivatives
A typical example is diffusion equation for a function p(t, x, y, z) or 4 variables:
| \frac{\partial p}{\partial t} = \frac{\partial^2 p}{\partial x^2} + \frac{\partial^2 p}{\partial y^2} + \frac{\partial^2 p}{\partial z^2} |