A Class of Sums of Squares with a Given Poisson-Treves Stratification
  We study a class of sum of squares exhibiting the same Poisson-Treves stratification as the Oleinik-Radkevi\v{c} operator. We find three types of operators having distinct microlocal structures. For one of these we prove a Gevrey hypoellipticity theorem analogous to our recent result for the corresponding Oleinik-Radkevi\v{c} operator. 
Antonio Bove and David S. Tartakoff