# Huygens principle

The statement that the propagation of oscillations, described by the wave equation, in a space of odd dimension involves the re-appearance of a sharply-localized initial state at a later moment of time at another point as a phenomenon just as sharply localized. If the number of space variables is even, Huygens' principle is absent, and the signal from the localized initial perturbation received at the observation point will be washed away. The principle was first formulated by Chr. Huygens in 1678 [1] and was subsequently developed by A. Fresnel in 1818 in his studies on the problems of diffraction (cf. also Diffraction, mathematical theory of).

Huygens' principle results from the mathematical fact that the solution of the wave equation at a point $M$ of the three-dimensional space at the moment of time $t$ is expressed in terms of the values of the solution and its derivatives on an arbitrary closed surface inside which $M$ is contained, at preceding moments of time. In particular, the solution of the Cauchy problem at the point $( M, t)$ for the wave equation is determined only by the initial data at the intersection of the initial manifold with the characteristic cone of $( M, t)$ and does not depend on the initial data inside and outside the characteristic cone (cf. also Characteristic surface). A rigorous mathematical formulation of Huygens' principle was first given by H. Helmholtz (1859) and by G. Kirchhoff (1882) for the stationary and non-stationary cases, respectively.

The results of J. Hadamard [2], according to which the solution of the Cauchy problem for the second-order linear hyperbolic equation

$$\tag{* } \sum _ {i, j = 1 } ^ { {n } - 1 } g ^ {ij} ( x) u _ {ij} + \sum _ {i = 1 } ^ { n } b ^ {i} ( x) u _ {i} + c ( x) u = 0$$

for even $n \geq 4$ depends only on the initial data at the intersection of the initial manifold with the characteristic conoid if and only if the fundamental solution of (*) has no logarithmic terms, is a generalization of Huygens' principle to include the linear hyperbolic equation (*). For an account of the entire class of equations of the form (*) for which Huygens' principle is valid, see [4].

#### References

 [1] C. Huygens, "Tractise on light" , Moscow-Leningrad (1935) (In Russian; translated from French) [2] J. Hadamard, "Lectures on Cauchy's problem in linear partial differential equations" , Dover, reprint (1952) (Translated from French) [3] R. Courant, D. Hilbert, "Methods of mathematical physics. Partial differential equations" , 2 , Interscience (1965) (Translated from German) MR0195654 [4] N.Kh. Ibragimov, "Huygens' principle" , Some problems in mathematics and mechanics , Leningrad (1970) pp. 159–170 (In Russian) MR0299961 MR0269988