Planar Symmetric Concave Central Configurations in Four-body Problem
  In this paper, we consider the problem: given a symmetric concave configuration of four bodies, under what conditions is it possible to choose positive masses which make it central. We show that there are some regions in which no central configuration is possible for positive masses. Conversely, for any configuration in the complement of the union of these regions, it is always possible to choose positive masses to make the configuration central. 
Chunhua Deng and Shiqing Zhang
math-ph math.MP