The complex geometry of weak piecewise smooth solutions of integrable nonlinear PDE's of shallow water and dym type

An extension of the algebraic-geometric method for nonlinear integrable PDE 's is shown to lead to new piecewise smooth weak solutions of a class of N- component systems of nonlinear evolution equations. This class includes, am ong others, equations from the Dym and shallow water equation hierarchies. The main goal of the paper is to give explicit theta-functional expressions for piecewise smooth weak solutions of these nonlinear PDE's, which are as sociated to nonlinear subvarieties of hyperelliptic Jacobians. The main results of the present paper are twofold. First, we exhibit some o f the special features of integrable PDE's that admit piecewise smooth weak solutions, which make them different from equations whose solutions are gl obally meromorphic, such as the KdV equation. Second, we blend the techniqu es of algebraic geometry and weak solutions of PDE's to gain further insigh t into, and explicit formulas for, piecewise-smooth finite-gap solutions. The basic technique used to achieve these aims is rather different from ear lier papers dealing with peaked solutions. First, profiles of the finite-ga p piecewise smooth solutions are linked to certain finite dimensional billi ard dynamical systems and ellipsoidal billiards. Second, after reducing the solution of certain finite dimensional Hamiltonian stems on Riemann surfac es to the solution of a nonstandard Jacobi inversion problem, this is resol ved by introducing new parametrizations. Amongst other natural consequences of the algebraic-geometric approach, we find finite dimensional integrable Hamiltonian dynamical systems describing the motion of Peaks in the finite-gap as well a. the limiting (soliton) ca ses, and noise them exactly. The dynamics of the peaks is also obtained by using Jacobi inversion problems. Finally, we relate our method to the shock wave approach for weak solutions of wave equations by determining jump con ditions at the peak location.