Worldsheet Propagator on the Lightcone Worldsheet Lattice
  We develop new more powerful techniques, based on an almost closed form for the lattice worldsheet propagator, for analyzing planar open string worldsheets defined on a lightcone lattice. We show that results obtained in earlier work are easily reproduced with far more precision. In particular, consistency checks which required numerical analysis in the earlier work can now be confirmed exactly. 
Georgios Papathanasiou and Charles B. Thorn
hep-th