Group-invariant Soliton Equations and Bi-hamiltonian Geometric Curve Flows in Riemannian Symmetric Spaces

Universal bi-Hamiltonian hierarchies of group-invariant (multicomponent) soliton equations are derived from non-stretching geometric curve flows γ(t, x) in Riemannian symmetric spaces M = G/H, including compact semisimple Lie groups M = K for G = K×K, H = diag G. The derivation of these soliton hierarchies utilizes a moving parallel frame and connection 1-form along the curve flows, related to the Klein geometry of the Lie group G ⊃ H where H is the local frame structure group. The soliton equations arise in explicit form from the induced flow on the frame components of the principal normal vector N = ∇ x γx along each curve, and display invariance under the equivalence subgroup in H that preserves the unit tangent vector T = γx in the framing at any point x on a curve. Their bi-Hamiltonian integrability structure is shown to be geometrically encoded in the Cartan structure equations for torsion and curvature of the parallel frame and its connection 1-form in the tangent space TγM of the curve flow. The hierarchies include group-invariant versions of sine-Gordon (SG) and modified Korteweg-de Vries (mKdV) soliton equations that are found to be universally given by curve flows describing nonstretching wave maps and mKdV analogs of non-stretching Schrodinger maps on G/H. These results provide a geometric interpretation and explicit biHamiltonian formulation for many known multicomponent soliton equations. Moreover, all examples of group-invariant (multicomponent) soliton equations given by the present geometric framework can be constructed in an explicit fashion based on Cartan’s classification of symmetric spaces.