Schr\"odinger Maps and their associated Frame Systems
  In this paper we establish the equivalence of solutions between Schr\"odinger map into $\mathbb{S}^2$ or $ \mathbb{H}^2$ and their associated gauge invariant Schr\"odinger equations. We also establish the existence of global weak solutions into $\mathbb{H}^2$ in two space dimensions. We extend these ideas for maps into compact hermitian symmetric manifolds with trivial first cohomology. 
Andrea Nahmod, Jalal Shatah, Luis Vega, and Chongchun Zeng