Abstract
The hypoellipticity ($C^\infty$, analytic and Gevrey, both local and global) of sub-Laplacians has been addressed by several authors, but it is still a problem with many unresolved questions, since no necessary and sufficient conditions are known to characterize the hypoellipticity of a sub-Laplacian in its general form. We propose a new line of investigation: the global hypoellipticity a…