Exponential asymptotics with coalescing singularities Philippe H. Trinh and S. Jonathan Chapman Oxford Centre for Industrial and Applied Mathematics, Mathematical Institute, University of Oxford E-mail:
[email protected],
[email protected] Abstract. Problems in exponential asymptotics are typically characterized by divergence of the associated asymptotic expansion in the form of a factorial divided by a power. In this paper, we demonstrate that in certain classes of problems that involve coalescing singularities, a more general type of exponential-over-power divergence must be used. As a model example, we study the water waves produced by flow past an obstruction such as a surface-piercing ship. In the low speed or low Froude limit, the resultant water waves are exponentially small, and their formation is attributed to the singularities in the geometry of the obstruction. However, in cases where the singularities are closely spaced, the usual asymptotic theory fails. We present both a general asymptotic framework for handling such problems of coalescing singularities, and provide numerical and asymptotic results for particular examples. 1. Introduction Many problems in exponential asymptotics involve the analysis of singularly perturbed differential equations where the associated solution is expressed as a divergent asymptotic expansion. It has been noted by authors such as Dingle [14] and Berry [3] that in many cases, the divergence of the sequence occurs in the form of a factorial divided by a arXiv:1403.7182v2 [math.CA] 4 Oct 2014 power. In this paper, we use a model problem from the theory of water waves and ship hydrodynamics to demonstrate how in certain classes of problems, a more general form of divergence must be used in order to perform the exponential asymptotic analysis.