# Hypoelliptic operator

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

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

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

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

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

${\displaystyle P(u)=u_{t}-k\Delta u\,}$

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

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

(where ${\displaystyle c\neq 0}$) 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.