# Eikonal equation

A partial differential equation of the form

$$\sum_{i=1}^m\left(\frac{\partial\tau}{\partial x^i}\right)^2=\frac{1}{c^2(x^1,\dots,x^m)}.$$

Here $m$ is the dimension of the space and $c$ is a smooth function bounded away from zero. In applications $c$ is the speed of the wave, and the surfaces $\tau(x^1,\dots,x^m)=\mathrm{const}$ are the wave fronts. The rays (see Fermat principle) are the characteristics of the eikonal equation. This equation has a number of generalizations and analogues. In particular, one such generalization is

$$H\left(x^1,\dots,x^m,\frac{\partial\tau}{\partial x^1},\dots,\frac{\partial\tau}{\partial x^m}\right)=1,$$

where the function $H$ is homogeneous of degree 1 with respect to $\partial\tau/\partial x^1,\dots,\partial\tau/\partial x^m$ and satisfies some additional restrictions. Of considerable interest is the non-stationary analogue

$$-\frac{\partial\theta}{\partial t}+c(t,x^1,\dots,x^m)\sqrt{\sum_{i=1}^m\left(\frac{\partial\theta}{\partial x^i}\right)^2}=0.$$

This is a special case of the dispersion equations occurring in the theory of wave phenomena, which have the form

$$\frac{\partial\theta}{\partial t}=\omega(t,x^1,\dots,x^m,\theta_{x^1},\theta_{x^m}).$$

Here $\omega$ is a given function.

The mathematical theory of geometrical optics can be regarded as the theory of the eikonal equation. All forms of the eikonal equation are first-order partial differential equations. The solution of the eikonal equation may have singularities. Their theory is part of that of the singularities of differentiable mappings (see also Hamilton–Jacobi theory; Geometric approximation, and Ray method).

How to Cite This Entry:
Eikonal equation. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Eikonal_equation&oldid=33014
This article was adapted from an original article by V.M. Babich (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article