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Some non-analytic-hypoelliptic sums of squares of vector fields

Michael Christ
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Abstract

Certain second-order partial differential operators, which are expressed as sums of squares of real-analytic vector fields in $\Bbb R^3$ and which are well known to be $C^\infty$ hypoelliptic, fail to be analytic hypoelliptic.

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Michael Christ. 1992-01-01. Some non-analytic-hypoelliptic sums of squares of vector fields. https://arxiv.org/abs/math/9201260

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