Differential equations with general highly oscillatory forcing terms

The concern of this paper is in expanding and computing initial-value problems of the form y' = f(y) + hw(t) where the function hw oscillates rapidly for w >> 1. Asymptotic expansions for such equations are well understood in the case of modulated Fourier oscillators hw(t) = Σm am(t)eim!t and they can be used as an organising principle for very accurate and aordable numerical solvers. However, there is no similar theory for more general oscillators and there are\udsound reasons to believe that approximations of this kind are unsuitable in that setting. We follow in this paper an alternative route, demonstrating that, for a much more general family of oscillators, e.g. linear combinations of functions of the form ei!gk(t), it is possible to expand y(t) in a different manner. Each rth term in the expansion is for some & > 0 and it can be represented as an r-dimensional highly oscillatory integral. Since computation of multivariate highly oscillatory integrals is fairly well understood, this provides a powerful method for an effective discretisation of a numerical solution for a large family of highly oscillatory ordinary differential equations