Reduction for constrained variational problems on 3-dimensional null curves

We consider the optimal control problem for null curves in de Sitter 3-space defined by a functional which is linear in the curvature of the trajectory. We show how techniques based on the method of moving frames and exterior differential systems, coupled with the reduction procedure for systems with a Lie group of symmetries, lead to the integration by quadratures of the extremals. Explicit solutions are found in terms of elliptic functions and integrals.