Nonelliptic Partial Differential Equations Analytic Hypoellipticity and the Courage to Localize High Powers of T

From the reviews:

“The present book deals with the analytic and Gevrey local hypoellipticity of certain nonelliptic partial differential operators. … this nice book is mostly addressed to Ph.D. students and researchers in harmonic analysis and partial differential equations, the reader being supposed to be familiar with the basic facts of pseudodifferential calculus and several complex variables. It represents the first presentation, in book form, of the challenging and still open problem of analytic and Gevrey hypoellipticity of sum-of-squares operators.” (Fabio Nicola, Mathematical Reviews, Issue 2012 h)