Hypoellipticity without loss of derivatives for Fedii's type operators

Timur Akhunov, Lyudmila Korobenko, Cristian Rios

We prove that second order linear operators on ${\mathbb{R}}^{n+m}$ of the form $L(x,y,D_x,D_y)=L_1(x,D_x)+g(x)L_2(y,D_y)$, where $L_1$ and $L_2$ satisfy Morimoto's super-logari…