Plane torsion waves in quadratic gravitational theories
  The definition of the Riemann-Cartan space of the plane wave type is given. The condition under which the torsion plane waves exist is found. It is expressed in the form of the restriction imposed on the coupling constants of the 10-parametric quadratic gravitational Lagrangian. In the mathematical appendix the formula for commutator of the variation operator and Hodge operator is proved. This formula is applied for the variational procedure when the gravitational field equations are obtained in terms of the exterior differential forms. 
O. V. Babourova, B. N. Frolov, E. A. Klimova (Department of   Mathematics, Moscow State Pedagogical University)