Harmonic Analysis and Boundary Value Problems in the Complex Domain: 65 (Operator Theory: Advances and Applications)

As is well known, the first decades of this century were a period of elaboration of new methods in complex analysis. This elaboration had, in particular, one char­ acteristic feature, consisting in the interfusion of some concepts and methods of harmonic and complex analyses. That interfusion turned out to have great advan­ tages and gave rise to a vast number of significant results, of which we want to mention especially the classical results on the theory of Fourier series in L2 ( -7r, 7r) and their continual analog - Plancherel's theorem on the Fourier transform in L2 ( -00, +00). We want to note also two important Wiener and Paley theorems on parametric integral representations of a subclass of entire functions of expo­ nential type in the Hardy space H2 over a half-plane. Being under the strong influence of these results, the author began in the fifties a series of investigations in the theory of integral representations of analytic and entire functions as well as in the theory of harmonic analysis in the com­ plex domain. These investigations were based on the remarkable properties of the asymptotics of the entire function (p, J1 > 0), which was introduced into mathematical analysis by Mittag-Leffler for the case J1 = 1. In the process of investigation, the scope of some classical results was essentially enlarged, and the results themselves were evaluated.

The present book is a valuable continuation of the large cycle of the author's investigations on harmonic analysis in the complex domain. For certain sets of segments in the complex domain, the elegant and explicit apparatus of the biorthogonal Fourier type systems (basis systems in the Rieszian sense), is constructed by purely analytic methods of classical function theory. This is done using the remarkable asymptotic properties of the Mittag-Leffler type entire functions and new interpolation theorems for the Banach spaces of entire functions. It is especially noteworthy that at the same time the constructed basis systems are eigenfunctions for quite non-ordinary boundary value problems in the complex domain for differential equations of fractional order. Such boundary value problems, the solutions of which are carried through to a logical conclusion, i.e. up to the theorems of expansions into eigenfunctions, are considered in the book for the first time.