“…The eigentones of three-dimensional multireflecting cavities were repeatedly considered in the literature (see, e.g., [1], [2]). For a wide class of equations and boundary conditions a wave concentrated in a neighborhood of an axial ray looks like a Gaussian beam u(s, r, k) = v(s, r √ k, k −1/2 )e ik[τ (s)+(Γ(s)r,r)…”