A nonlinear partial differential equation is a partial differential equation that is not linear in the unknown function or its derivatives.
For example, the inviscid Burgers' equation is
The superposition principle therefore does not generally apply. Nonlinear partial differential equations are commonly classified
as semilinear, quasilinear, or fully nonlinear according to the way their highest-order
derivatives occur. Other examples include the Monge-Ampère
equation and the Navier-Stokes equations
(Evans 2010).
