# Hypoelliptic operator

In mathematics, more specifically in the theory of partial differential equations, a partial differential operator $P$ defined on an open subset

$U \subset{\mathbb{R}}^n$

is called hypoelliptic if for every distribution $u$ defined on an open subset $V \subset U$ such that $Pu$ is $C^\infty$ (smooth), $u$ must also be $C^\infty$.

If this assertion holds with $C^\infty$ replaced by real analytic, then $P$ is said to be analytically hypoelliptic.

Every elliptic operator with $C^\infty$ coefficients is hypoelliptic. In particular, the Laplacian is an example of a hypoelliptic operator (the Laplacian is also analytically hypoelliptic). The heat equation operator

$P(u)=u_t - k\Delta u\,$

(where $k>0$) is hypoelliptic but not elliptic. The wave equation operator

$P(u)=u_{tt} - c^2\Delta u\,$

(where $c\ne 0$) is not hypoelliptic.

## References

• Shimakura, Norio (1992). Partial differential operators of elliptic type: translated by Norio Shimakura. American Mathematical Society, Providence, R.I. ISBN 0-8218-4556-X.
• Egorov, Yu. V.; Schulze, Bert-Wolfgang (1997). Pseudo-differential operators, singularities, applications. Birkhäuser. ISBN 3-7643-5484-4.
• Vladimirov, V. S. (2002). Methods of the theory of generalized functions. Taylor & Francis. ISBN 0-415-27356-0.
• Folland, G. B. (2009). Fourier Analysis and its applications. AMS. ISBN 0-8218-4790-2.