A Hopf Algebra Structure in Self-dual Gravity
  The two-dimensional non-linear sigma model approach to Self-dual Yang-Mills theory and to Self-dual gravity given by Q-Han Park is an example of the deep interplay between two and four dimensional physics. In particular, Husain's two-dimensional chiral model approach to Self-dual gravity is studied. We show that the infinite hierarchy of conservation laws associated to the Husain model carries implicitly a hidden infinite Hopf algebra structure. 
Hugo Garcia-Compean, Laura E. Morales and Jerzy F. Plebanski