Finite gap conditions and small dispersion asymptotics for the classical periodic Benjamin–Ono equation

In this paper we characterize the Nazarov–Sklyanin hierarchy for the classical periodic Benjamin–Ono equation in two complementary degenerations: for the multiphase initial data (the periodic multisolitons) at fixed dispersion and for bounded initial data in the limit of small dispersion. First, we express this hierarchy in terms of a piecewise-linear function of an auxiliary real variable which we call a dispersive action profile and whose regions of slope $\pm 1$ we call gaps and bands, respectively. Our expression uses Kerov’s theory of profiles and Kreĭn’s spectral shift functions. Next, for multiphase initial data, we identify Baker–Akhiezer functions in Dobrokhotov–Krichever and Nazarov–Sklyanin and prove that multiphase dispersive action profiles have finitely many gaps determined by the singularities of their Dobrokhotov–Krichever spectral curves. Finally, for bounded initial data independent of the coefficient of dispersion, we show that in the small dispersion limit, the dispersive action profile concentrates weakly on a convex profile which encodes the conserved quantities of the dispersionless equation. To establish the weak limit, we reformulate Szegő’s first theorem for Toeplitz operators using spectral shift functions. To illustrate our results, we identify the dispersive action profile of sinusoidal initial data with a profile found by Nekrasov–Pestun–Shatashvili and its small dispersion limit with the convex profile found by Vershik–Kerov and Logan–Shepp.