Extended Derdzinski-Shen theorem for the Riemann tensor
  We extend a classical result by Derdzinski and Shen, on the restrictions imposed on the Riemann tensor by the existence of a nontrivial Codazzi tensor. The new conditions of the theorem include Codazzi tensors (i.e. closed 1-forms) as well as tensors with gauged Codazzi condition (i.e. "recurrent 1-forms"), typical of some well known differential structures. 
Carlo Alberto Mantica and Luca Guido Molinari
math.DG math-ph math.MP