Oscillations of anharmonic Fourier series and the wave equation.
In this paper we have collected some partial results on the sign of u(t,x) where u is a (sufficiently regular) solution of⎧ utt + (-1)m Δmu = 0 (t,x) ∈ R x Ω⎨⎩ u|Γ = ... = Δm-1 u|Γ = 0 t ∈ R.These results rely on the study of a sign of almost periodic functions of a special form restricted to a bounded interval J.