This book deals with the symbiotic relationship between I Quarkonial decompositions of functions, on the one hand, and II Sharp inequalities and embeddings in function spaces, III Fractal elliptic operators, IV Regularity theory for some semi-linear equations, on the other hand. Accordingly, the book has four chapters. In Chapter I we present the Weier strassian approach to the theory of function spaces, which can be roughly described as follows. Let 'IjJ be a non-negative Coo function in]R. n with compact support such that {'ljJe - m) : m E zn} is a resolution of unity in ]R. n. Let 'IjJ!3(x) = x!3'IjJ(x) where x E ]R. n and {3 E N~. One may ask under which circumstances functions and distributions f in ]R. n admit expansions 00 (0. 1) f(x) = L L L ). . ~m'IjJ!3(2jx - m), x E ]R. n, n !3ENg j=O mEZ with the coefficients ). . ~m E C. This resembles, at least formally, the Weier strassian approach to holomorphic functions (in the complex plane), combined with the wavelet philosophy: translations x 1---4 x - m where m E zn and dyadic j dilations x 1---4 2 x where j E No in ]R. n. Such representations pave the way to constructive definitions offunction spaces.

Inhalt
I Decompositions of Functions.- 1 Introduction, heuristics, and preliminaries.- 2 Spaces on ?n: the regular case.- 3 Spaces on ?n: the general case.- 4 An application: the Fubini property.- 5 Spaces on domains: localization and Hardy inequalities.- 6 Spaces on domains. decompositions.- 7 Spaces on manifolds.- 8 Taylor expansions of distributions.- 9 Traces on sets, related function spaces and their decompositions.- II Sharp Inequalities.- 10 Introduction: Outline of methods and results.- 11 Classical inequalities.- 12 Envelopes.- 13 The critical case.- 14 The super-critical case.- 15 The sub-critical case.- 16 Hardy inequalities.- 17 Complements.- III Fractal Elliptic Operators.- 18 Introduction.- 19 Spectral theory for the fractal Laplacian.- 20 The fractal Dirichlet problem.- 21 Spectral theory on manifolds.- 22 Isotropic fractals and related function spaces.- 23 Isotropic fractal drums.- IV Truncations and Semi-linear Equations.- 24 Introduction.- 25 Truncations.- 26 The Q-operator.- 27 Semi-linear equations; the Q-method.- References.- Symbois.