Solitary waves in a Whitham equation with small surface tension

TL;DR

This paper proves the existence of small-amplitude solitary waves in a nonlocal gravity-capillary Whitham equation using advanced mathematical techniques, revealing connections to the classical water wave problem.

Contribution

It introduces a novel application of the center manifold theorem and normal form reduction to analyze solitary waves in a nonlocal setting, extending understanding of gravity-capillary wave models.

Findings

Existence of small-amplitude solitary waves established.

Reversible bifurcations identified in the parameter space.

Connections made between the Whitham equation and full water wave equations.

Abstract

Using a nonlocal version of the center manifold theorem and a normal form reduction, we prove the existence of small-amplitude generalized solitary-wave solutions and modulated solitary-wave solutions to the steady gravity-capillary Whitham equation with weak surface tension. Through the application of the center manifold theorem, the nonlocal equation for the solitary wave profiles is reduced to a four-dimensional system of ODEs inheriting reversibility. Along particular parameter curves, relating directly to the classical gravity-capillary water wave problem, the associated linear operator is seen to undergo either a reversible $0^{2+}(i k_0)$ bifurcation or a reversible $(i s)^2$ bifurcation. Through a normal form transformation, the reduced system of ODEs along each relevant parameter curve is seen to be well approximated by a truncated system retaining only second-order or third-order terms. These truncated systems relate directly to systems obtained in the study of the full gravity-capillary water wave equation and, as such, the existence of generalized and modulated solitary waves for the truncated systems is guaranteed by classical works, and they are readily seen to persist as solutions of the gravity-capillary Whitham equation due to reversibility. Consequently, this work illuminates further connections between the gravity-capillary Whitham equation and the full two-dimensional gravity-capillary water wave problem.