ISSN:
0170-4214
Keywords:
Engineering
;
Numerical Methods and Modeling
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Mathematics
Notes:
We study initial and boundary value problems for the wave equation ∂2tu-Δu = fe-iωt with Dirichlet or Neumann boundary data in smooth domains Ω, which coincide with Ω0 = ∝2×(0,1) outside a sufficiently large sphere. The concept of a standing wave, introduced in [7], seems to be of special relevance. A standing wave of frequency ω is defined as a non-trivial solution U of the equation ΔU+ω2U = 0 in Ω, which satisfies on ∂Ω the prescribed boundary condition U = 0 or ∂U/∂n = 0, respectively, and a suitable condition at infinity. For instance, U(x) = sinπkx3 is a standing wave of frequency πk in the unperturbed domain Ω0 with Dirichlet boundary data. As shown in a series of joint papers with K. Morgenröther, u(x,t) is bounded as t→∞ if Ω does not admit standing waves of frequency ω. The main purpose of the present paper is the proof of the converse statement: If standing waves of frequency ω exist in Ω, then u(x,t) is unbounded as t→∞ for suitably chosen f∊C∞0(Ω). Thus the appearance of resonances is closely related to the presence of standing waves. The leading term of the asymptotic expansion of u(x,t) as t→∞ will be specified. In particular, it turns out that the resonance rate is either t or ln t. The rate t appears if and only if ω2 is an eigenvalue of the spatial operator, while the rate ln t can only occur if ω = πk. In the case of Neumann boundary data, U = 1 is a standing wave of frequency 0 for every local perturbation Ω of Ω0. The corresponding resonance has already been studied in [21].
Type of Medium:
Electronic Resource
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