Constructive proofs of existence and stability of solitary waves in the capillary-gravity Whitham equation

In this manuscript, we present a method to prove constructively the existence and spectral stability of solitary waves (solitons) in the capillary-gravity Whitham equation. By employing Fourier series analysis and computer-aided techniques, we successfully approximate the Fourier multiplier operator in this equation, allowing the construction of an approximate inverse for the linearization around an approximate solution u₀. Then, using a Newton-Kantorovich approach, we provide a sufficient condition under which the existence of a unique solitary wave ũ in a ball centered at u₀ is obtained. The verification of such a condition is established combining analytic techniques and rigorous numerical computations. Moreover, we derive a methodology to control the spectrum of the linearization around ũ, enabling the study of spectral stability of the solution. As an illustration, we provide a (constructive) computer-assisted proof of existence of a stable soliton in both the case with capillary effects ($T>0$) and without capillary effects ($T=0$). The methodology presented in this paper can be generalized and provides a new approach for addressing the existence and spectral stability of solitary waves in nonlocal nonlinear equations. All computer-assisted proofs, including the requisite codes, are accessible on GitHub at {julia_cadiot}.