Poincar\'e's polyhedron theorem for cocompact groups in dimension 4
  We prove a version of Poincar\'e's polyhedron theorem whose requirements are as local as possible. New techniques such as the use of discrete groupoids of isometries are introduced. The theorem may have a wide range of applications and can be generalized to the case of higher dimension and other geometric structures. It is planned as a first step in a program of constructing compact $\mathbb C$-surfaces of general type satisfying $c_1^2=3c_2$. 
Sasha Anan'in, Carlos H. Grossi, J\'ulio C. C. da Silva
math.GT