Emergence of cuspons through wave breaking

Abstract

We study the emergence of cuspons through wave breaking, with the Camassa--Holm equation as a principal model. For a broad open class of smooth initial data, the first singularity occurs at a unique point and has the sharp local Hölder regularity \(C^{3/5}\). We construct a continuation of the Lagrangian flow through this singular time and reconstruct from it a conservative Eulerian weak solution. We show that the first singularity does not simply persist after breaking; instead, it bifurcates immediately into an anticuspon--cuspon pair, each having the sharp local Hölder regularity \(C^{2/3}\), with opposite one-sided slope orientations. The two singular branches move apart from the original breaking point, and the portion of the profile between them reverses its local monotonicity. The proof is based on an extended inverse-slope variable that remains regular through wave breaking. At the first singular time, this variable has a nondegenerate quadratic zero, which splits immediately afterward into two simple zeros. The corresponding degeneracy of the Lagrangian flow map converts the quadratic first-breaking geometry into the \(C^{3/5}\) profile and the post-breaking simple-zero geometry into the \(C^{2/3}\) cuspon profiles. The generalized Hunter--Saxton family provides an explicit realization of the same Lagrangian mechanism and also allows us to track the subsequent interaction and annihilation of the singular branches. Thus an anticuspon--cuspon pair emerges dynamically from smooth initial data through wave breaking, rather than being imposed through a singular traveling-wave ansatz.