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.

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