The techniques developed here are tailored to proving real analytic regularity to solutions of sums of squares of vector fields with symplectic characteristic variety and others, real and complex. The motivation came from the field of several complex variables and the seminal work of J. J. Kohn. It has found application in non-degenerate (strictly pseudo-convex) and degenerate situations alike, linear and non-linear, partial and pseudo-differential equations, real and complex analysis. The technique is utterly elementary, involving powers of vector fields and carefully chosen localizing functions. No knowledge of advanced techniques, such as the FBI transform or the theory of hyperfunctions is required. In fact analyticity is proved using only $C^\infty$ techniques.
The book is intended for mathematicians from graduate students up, whether in analysis or not, who are curious which non-elliptic partial differential operators have the property that all solutions must be real analytic. Enough background is provided to prepare the reader with it for a clear understanding of the text, although this is not, and does not need to be, very extensive. In fact, it is very nearly true that if the reader iswilling to accept the fact that pointwise bounds on the derivatives of a function are equivalent to bounds on the $L^2$ norms of its derivatives locally, the book should read easily.
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"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)