Ultrahyperbolic wave equation

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In the mathematical field of partial differential equations, the ultrahyperbolic wave equation is a partial differential equation for an unknown scalar function u of 2n variables x1, ..., xn, y1, ..., yn of the form

\frac{\partial^2 u}{\partial x_1^2}+\cdots+\frac{\partial^2 u}{\partial x_n^2}-\frac{\partial^2 u}{\partial y_1^2}-\cdots-\frac{\partial^2 u}{\partial y_n^2}=0.\qquad\qquad(1)

More generally, if a is any quadratic form in 2n variables with signature (n,n), then any PDE whose principal part is a_{ij}u_{x_ix_j} is said to be ultrahyperbolic. Any such equation can be put in the form 1. above by means of a change of variables.[1]

The ultrahyperbolic wave equation has been studied from a number of viewpoints. On the one hand, it resembles the classical wave equation. This has led to a number of developments concerning its characteristics, one of which is due to Fritz John: the John equation.

The equation has also been studied from the point of view of symmetric spaces, and elliptic differential operators.[2] In particular, the ultrahyperbolic wave equation satisfies an analog of the mean value theorem for harmonic functions

Any constant-coefficient PDE whatsoever can be realized as a reduction of the ultrahyperbolic wave equation in possibly more variables.[3]

[edit] Notes

  1. ^ See Courant and Hilbert.
  2. ^ See, for instance, Helgasson.
  3. ^ See Courant and Hilbert.

[edit] References

  • David Hilbert and Richard Courant (1962). Methods of Mathematical Physics, Vol. 2. Wiley-Interscience, 744-752. 
  • Lars Hörmander (2001). "Asgeirsson's Mean Value Theorem and Related Identities". Journal of Functional Analysis (184): 377–401. 
  • Lars Hörmander (1990). The Analysis of Linear Partial Differential Operators I. Springer-Verlag, Theorem 7.3.4. 
  • Sigurdur Helgason (2000). Groups and Geometric Analysis. American Mathematical Society, 319-323. 
  • Fritz John (1938). "[he Ultrahyperbolic Differential Equation with Four Independent Variables". Duke Math. J. 4 (2): 300–322. doi:10.1215/S0012-7094-38-00423-5. 
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