MathSciNet bibliographic data MR2664615 (2011d:35107) 35H10 (35A20 35H20) Tartakoff, David S. Analytic hypoellipticity for a sum of squares of vector fields in \$\Bbb R\sp 3\$$\Bbb R\sp 3$ whose Poisson stratification consists of a single symplectic stratum of codimension four. Advances in phase space analysis of partial differential equations, 253–261, Progr. Nonlinear Differential Equations Appl., 78, Birkhäuser Boston, Inc., Boston, MA, 2009. Article

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