Partial differential equation - Wikipedia
https://en.wikipedia.org/wiki/Partial_differential_equationWhen writing PDEs, it is common to denote partial derivatives using subscripts. For example: The Greek letter Δ denotes the Laplace operator; if u is a function of n variables, then A PDE is called linear if it is linear in the unknown and its derivatives. For example, for a function u of x and y, a second order linear PDE is of the form Nearest to linear PDEs are semilinear PDEs, where the highest order derivatives appear only as li…