Remarks highlight examples of hypoelliptic equations in scalar systems, indicating areas needing further exploration.
Many results of smooth hypoellipticity are available for scalar equations. Much remains to be done for systems and/or at different levels of regularity and in particular for L 1 -hypoellipticity. In this article we provide some examples and counter-examples.
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Banica et al. (2026) studied this question.
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