“…In this way, the time-dependent change of variables ξ → Ξ in (4) transforms the given highly oscillatory initial value problem into the initial value problem (5) where there is no periodic forcing. Therefore (5) provides an averaged version of (1). In addition, the coefficients κ w (θ) are such that, at θ = 2kπ, k = 0, 1, 2, .…”