Synopsis
Linear differential equations with periodic coefficients constitute a well developed part of the theory of ordinary differential equations [17, 94, 156, 177, 178, 272, 389]. They arise in many physical and technical applications [177, 178, 272]. A new wave of interest in this subject has been stimulated during the last two decades by the development of the inverse scattering method for integration of nonlinear differential equations. This has led to significant progress in this traditional area [27, 71, 72, 111 119, 250, 276, 277, 284, 286, 287, 312, 313, 337, 349, 354, 392, 393, 403, 404]. At the same time, many theoretical and applied problems lead to periodic partial differential equations. We can mention, for instance, quantum mechanics [14, 18, 40, 54, 60, 91, 92, 107, 123, 157-160, 192, 193, 204, 315, 367, 412, 414, 415, 417], hydrodynamics [179, 180], elasticity theory [395], the theory of guided waves [87-89, 208, 300], homogenization theory [29, 41, 348], direct and inverse scattering [175, 206, 216, 314, 388, 406-408], parametric resonance theory [122, 178], and spectral theory and spectral geometry [103 105, 381, 382, 389]. There is a sjgnificant distinction between the cases of ordinary and partial differential periodic equations. The main tool of the theory of periodic ordinary differential equations is the so-called Floquet theory [17, 94, 120, 156, 177, 267, 272, 389]. Its central result is the following theorem (sometimes called Floquet-Lyapunov theorem) [120, 267].
Synopsis
This monograph provides the first survey of Floquet theory for partial differential equations with riodic coefficients. The author investigates, among others, hypoelliptic, parabolic, elliptic and Schrbdinger equations, and boundary value problems arising in applications. In particular, results are given about completeness of the set of Floquet solutions, Floquet expansions of arbitrary solutions, the distribution of Floquet exponents and quasimomentums, the solvability of nonhomogeneous equations, the existence of decreasing or bounded solutions, and the structure of the spectrum of periodic operators. Many of the results discussed here have been available only in research papers until now. The role of the Floquet-Lyapunov theory for ordinary differential equations is well known, but for partial differential equations an analog of this theory ias been developed only recently. This theory is of great importance for the quantum theory of solids, the theory of wave guides, scattering theory and other fields of mathematical and theoretical physics. Some chapters devoted to operator theory may be of particular interest to specialists in complex analysis and functional analysis.
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