Unimodal wavetrains and solitons in convex Fermi–Pasta–Ulam chains

Abstract

We consider atomic chains with nearest neighbour interactions and study periodic travelling waves and homoclinic travelling waves, which are called wavetrains and solitons, respectively. Our main result is a new existence proof which relies on the constrained maximization of the potential energy and exploits the invariance properties of an improvement operator. The approach is restricted to convex interaction potentials but refines the standard results, as it provides the existence of travelling waves with unimodal and even profile functions. Moreover, we discuss both the numerical approximation and the complete localization of wavetrains, and show that wavetrains converge to solitons when the periodicity length tends to infinity.