Counting rule for Nambu-Goldstone modes in nonrelativistic systems
  The counting rule for Nambu-Goldstone modes is discussed using Mori's projection operator method in nonrelativistic systems at zero and finite temperatures. We show that the number of Nambu-Goldstone modes is equal to the number of broken charges, Q_a, minus half the rank of the expectation value of [Q_a,Q_b]. 
Yoshimasa Hidaka
hep-th cond-mat.other hep-ph