On the number of connected components in complements to arrangements of   submanifolds
  We consider arrangements of n connected codimensional one submanifolds in closed d-dimensional manifold M. Let f be the number of connected components of the complement in M to the union of submanifolds. We prove the sharp lower bound for f via n and homology group H_{d-1}(M). The sets of all possible f-values for given n are studied for hyperplane arrangements in real projective spaces and for subtori arrangements in d-dimensional tori. 
I.N. Shnurnikov
math.GT math.CO