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 lin…
Hyperbolic partial differential equation - Wikipedia
en.wikipedia.org › wiki › Hyperbolic_partialIn mathematics, a hyperbolic partial differential equation of order n {\displaystyle n} is a partial differential equation that, roughly speaking, has a well-posed initial value problem for the first n − 1 {\displaystyle n-1} derivatives. More precisely, the Cauchy problem can be locally solved for arbitrary initial data along any non-characteristic hypersurface. Many of the equations of mechanics are hyperbolic, and so the study of hyperbolic equations is of substantial contemporary ...