Stability of multi-solitons for the Benjamin-Ono equation

This paper is concerned with the dynamical stability of the $m$-solitons of the Benjamin-Ono (BO) equation. This extends the work of Neves and Lopes [41], which was restricted to $m=2$ the double solitons case. By constructing a suitable Lyapunov functional, it is found that the multi-solitons are non-isolated constrained minimizers satisfying a suitable variational nonlocal elliptic equation. The stability issue is reduced to the spectral analysis of higher order nonlocal operators consist of the Hilbert transform. Such operators are isoinertial and the negative eigenvalues of which are fully classified. Our approach in the spectral analysis consists of an invariance for the multi-solitons and new operator identities motivated by the bi-Hamiltonian structure of the BO equation. Since the BO equation is more likely a two dimensional integrable system, its recursion operator is not explicit which makes our analysis more involved. The key ingredient in the spectral analysis is to employ the completeness in $L^2$ of the squared eigenfunctions of the eigenvalue problem for the BO equation. It is demonstrated here that orbital stability of soliton in $H^{\frac{1}{2}}$ implies that all $m$-solitons are dynamically stable in $H^{\frac{m}{2}}$.