Time-like Weingarten surfaces with real principal curvatures in the   three-dimensional Minkowski space and their natural partial differential   equations
  We study time-like surfaces in the three-dimensional Minkowski space with diagonalizable second fundamental form. On any time-like W-surface we introduce locally natural principal parameters and prove that such a surface is determined uniquely (up to motion) by a special invariant function, which satisfies a natural non-linear partial differential equation. This result can be interpreted as a solution of the Lund-Regge reduction problem for time-like W-surfaces with real principal curvatures in Minkowski space. We apply this theory to linear fractional time-like W-surfaces and obtain the natural partial differential equations describing them. 
Vesselka Mihova and Georgi Ganchev
math.DG