Space group symmetries and low lying excitations of many-body systems at   integer fillings
  We show that many-body systems with conserved particle number which have the symmetries corresponding to a nonsymmorphic space group have low lying excitations for certain integer values of the particle number per unit cell. These results may be interpreted as a generalization of some band touching theorems to interacting systems. 
Rahul Roy
cond-mat.str-el