Rigorous asymptotics of a KdV soliton gas

Abstract

We analytically study the long time and large space asymptotics of a new broad class of solutions of the KdV equation introduced by Dyachenko, Zakharov, and Zakharov. These solutions are characterized by a Riemann--Hilbert problem which we show arises as the limit $N\to \infty$ of a gas of $N$-solitons. We show that this gas of solitons in the limit $N\to \infty$ is slowly approaching a cnoidal wave solution for $x\to -\infty$ (up to terms of order $\mathcal{O}(1/x)$), while approaching zero exponentially fast for $x\to +\infty$. We establish an asymptotic description of the gas of solitons for large times that is valid over the entire spatial domain, in terms of Jacobi elliptic functions.

42 pages, 7 figures. To appear in Comm. Math. Physics