Four-dimensional Osserman metrics of neutral signature
  In the algebraic context, we show that null Osserman, spacelike Osserman, and timelike Osserman are equivalent conditions for a model of signature (2,2). We also classify the null Jordan Osserman models of signature (2,2). In the geometric context, we show that a pseudo-Riemannian manifold of signature (2,2) is null Jordan Osserman if and only if either it has constant sectional curvature or it is locally a complex space form. 
E. Garcia-Rio, P. Gilkey, M. E. Vazquez-Abal, and R. Vazquez-Lorenzo