Abstract. Asymptotic expansions are constructed for the eigenvalues of the Dirichlet problem for the biharmonic operator in a domain with highly indented and rapidly oscillating boundary (the Kirchhoff model of a thin plate). The asymptotic constructions depend heavily on the quantity γ that describes the depth O(ε γ ) of irregularity (ε is the oscillation period). The resulting formulas relate the eigenvalues in domains with close irregular boundaries and make it possible, in particular, to control the order of perturbation and to find conditions ensuring the validity (or violation) of the classical Hadamard formula. §1. Introduction
Setting of problems.Let a domain Ω on the plane R 2 be bounded by a simple, closed, and smooth (of class C ∞ ) contour Γ = ∂Ω. By scaling, we make the length of Γ equal to 1. In a neighborhood V of Γ we introduce a system of natural curvilinear coordinates (n, s), where n is the distance to Γ, taken with the minus sign inside of Ω, and s is the arclength on Γ. The rapidly oscillating boundary Γ ε of the perturbed domain Ω ε (see Figure 1) is defined by the formulawhere ε = 1/N is a small parameter, N ∈ N is a large integer, γ is a quantity measuring the "irregularity" of the boundary (the greater is γ, the smaller is the perturbation irregularity), and H is a profile function that is smooth relative to both variables, the "slow" variable s and the "fast" variable η = ε −1 s, and 1-periodic relative to η. Note that, somewhat loosely, in our notation we do not distinguish between a point s ∈ Γ and its coordinate. For the role of V it is convenient to take the -neighborhood V with an appropriate > 0. Concerning the nonregular perturbation of the boundary (see Figure 2), we assume thatIn all other respects, the perturbation of Ω is arbitrary. To avoid numerous duplication of formulas, we keep the notation Ω and Ω ε also in §4. It should be emphasized that The paper was written during S. A. Nazarov's visit to the University of Linköping, whose financial support is acknowledged gratefully. Also, V. A. Kozlov thanks the Swedish Research Council (VR), and S. A. Nazarov thanks RFBR (project no. 09-01-00759).