Conservation laws for surfaces of constant mean curvature in 3-dimensional space forms
The exterior differential system for constant mean curvature (CMC) surfaces in a 3-dimensional space form is an elliptic Monge-Ampere system defined on the unit tangent bundle. We determine the infinite sequence of higher-order symmetries and conservation laws via an enhanced prolongation modelled on a loop algebra valued formal Killing field. As a consequence we establish Noether's theorem for the CMC system and there is a canonical isomorphism between the symmetries and conservation laws. A geometric interpretation of the $\mathbb{S}^{1}$-family of associate surfaces leads to an integrable extension for a non-local symmetry called spectral symmetry. We show that the corresponding spectral conservation law exists as a secondary characteristic cohomology class. For a compact linear finite type CMC surface of arbitrary genus, we observe that the monodromies of the associated flat $\mathfrak{sl}(2,\mathbb{C})$-connection commute with each other. It follows that a single spectral curve is defined as the completion of the set of eigenvalues of the entire monodromies. This agrees with the recent result that a compact high genus linear finite type CMC surface necessarily factors through a branched covering of a torus. We introduce a sequence of Abel-Jacobi maps defined by the periods of conservation laws. For the class of deformations of CMC surfaces which scale the Hopf differential by a real parameter, we compute the first order truncated Picard-Fuchs equation. The resulting formulae for the first few terms exhibit a similarity with the Griffiths transversality theorem for variation of Hodge structures.