Nonexistence of Petrov type III Space-Times on which Weyl's Neutrino   Equation or Maxwell's Equations satisfy Huygens' Principle
  Extending previous results we show that there are no Petrov type III space-times on which either the Weyl neutrino equation or Maxwell's equations satisfy Huygens' principle. We prove the result by using Maple's NPspinor package to convert the five-index necessary condition obtained by Alvarez and Wunsch to dyad form. The integrability conditions of the problem lead to a system of polynomial equations. We then apply Maple's grobner package to show that this system has no admissible solutions. 
R. G. McLenaghan and F. D. Sasse