Generalized problem of Apollonius
  The aim of this paper is to generalize Apollonius' problem. The problem is to construct a circle that is tangent to three given circles in a plane. We find the maximum possible number of solution circles in the case of more than the three given circles. We show that if all the given circles are not tangent at the same point, then there exist at most six solutions in the case of the four given generalized circles and there exist at most four solutions in the case of the five given generalized circles. We also describe all quadruples of generalized circles with exactly six solutions. 
Egor Morozov
math.HO