The nonlinear partial differential equationarises as a model for the unidirectional propagation of shallow water waves over a flat bottom, with u(x, t) representing the water's free surface in nondimensional variables. In this paper, we are concerned with the periodic solutions of (1.1), that is, u : S × [0, T ) → R where S denotes the unit circle and T > 0 is the maximal existence time of the solution. Equation (1.1) was first obtained [12] as an abstract bi-Hamiltonian equation with infinitely many conservation laws and was subsequently derived from physical principles [2]. For a discussion of the physical relevance of (1.1) in the context of water waves, we refer to [14, 15]. Equation (1.1) also arises as a model for nonlinear waves in cylindrical axially symmetric hyperelastic rods, with u(x, t) representing the radial stretch relative to a prestressed state [11]. Moreover, (1.1) is a re-expression of the geodesic flow in the group of compressible diffeomorphisms of the circle [18], just like the Euler equation is an expression of the geodesic flow in the group of incompressible diffeomorphisms of the torus [1]. This geometric interpretation leads to a proof that equation (1.1) satisfies the least action principle [7]: a state of the system is transformed to another nearby state through a uniquely determined flow that minimizes the energy. We also point out that for a large class of initial data, equation (1.1) is an infinite-dimensional completely integrable Hamiltonian system: by means of an isospectral problem, one can convert the equation into an infinite sequence of linear ordinary differential equations which can be