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Partial differential equation

In mathematics, and in particular calculus, a partial differential equation (PDE) is an equation involving partial derivatives of an unknown function.

Partial differential equations are ubiquitous in science, as they describe phenonena such as fluid flow, gravitational fields, and electromagnetic fields. They are important in fields such as aircraft simulation, computer graphics, and weather prediction. The central equations of general relativity and quantum mechanics are also partial differential equations.

 Notation and examples

In PDEs, it is common to write the unknown function as u and its partial derivative with respect to the variable x as ux, i.e.

$u_x = {\part u \over \part x}$
$u_{xy} = {\part^2 u \over \part x\, \part y}$

### Laplace's equation

A very important and basic PDE is Laplace's equation:-

$u_{xx} + u_{yy} + u_{zz} = 0$

for the unknown function u(x,y,z). Solutions to this equation, known as harmonic functions, serve as the potentials of vector field in physics, such as the gravitational or electrostatic fields.

A generalization of Laplace's equation is Poisson's equation:-

$u_{xx} + u_{yy} + u_{zz} = f$

where f(x,y,z) is a given function. The solutions to this equation describe potentials of gravitational and electrostatic fields in the presence of masses or electrical charges, respectively.

### Wave equation

The wave equation is an equation for an unknown function u(x,y,z,t) (where we think of t as a time variable) which reads:-

$u_{tt} = c^2( u_{xx} + u_{yy} + u_{zz} )$

Its solutions describe waves such as sound or light waves; c is a number which represents the speed of the wave. In lower dimensions, this equation describes the vibration of a string or drum. Solutions will typically be combinations of oscillating sine waves.

### Heat equation

The heat equation describes the temperature in a given region over time. It is:-

$u_t = k ( u_{xx} + u_{yy} + u_{zz} )$

Solutions will typically "even out" over time. The number k describes the heat conductivity[?] of the material.

The Schrödinger equation is a PDE at the heart of quantum mechanics.

All the above equations are linear in the sense that they can be written in the form Au = f for a given linear operator A and a given function f. Important non-linear equations include the Navier-Stokes equations describing the flow of fluids and Einstein's field equations of general relativity.

There are no generally applicable methods to solve PDEs; indeed, many PDEs cannot be solved analytically at all. Nevertheless, some techniques can be used for several types of equations. The behavior of a larger set of equations can be studied through numerical analysis techniques from simple finite difference schemes to the more mature multigrid[?] and finite element methods. Many interesting problems in science and engineering are solved in this way using high performance supercomputers.

fill in: Dirichlet and Neumann boundaries, hyperbolic/parabolic/elliptic separation of variables, Fourier analysis, Greene's functions ...