Variable coefficient Schr\"odinger flows for ultrahyperbolic operators
  In this paper we study the local solvability of nonlinear Schr\"odinger equations of the form $$\p_t u = i {\cal L}(x) u + \vec b_1(x)\cdot \nabla_x u + \vec b_2(x)\cdot \nabla_x \bar u + c_1(x)u+c_2(x)\bar u +P(u,\bar u,\nabla_x u, \nabla_x\bar u), where $x\in\mathbb R^n$, $t>0$, $\displaystyle{\cal L}(x) = -\sum_{j,k=1}^n\p_{x_j}(a_{jk}(x)\p_{x_k})$, $A(x)=(a_{jk}(x))_{j,k=1,..,n}$ is a real, symmetric and nondegenerate variable coefficient matrix, and $P$ is a polynomial with no linear or constant terms. Equations of the form described in with $A(x)$ merely invertible as opposed to positive definite arise in connection with water wave problems, and in higher dimensions as completely integrable models. Under appropriate assumptions on the coefficients we shall show that the associated initial value problem is local well posed. 
C.E. Kenig, G. Ponce, C. Rolvung, L. Vega