  Recent results, extending the Schmidt decomposition theorem to wavefunctions
of identical particles, are reviewed. They are used to give a definition of
reduced density operators in the case of two identical particles. Next, a
method is discussed to calculate time averaged entanglement. It is applied to a
pair of identical electrons in an otherwise empty band of the Hubbard model,
and to a pair of bosons in the the Bose-Hubbard model with infinite range
hopping. The effect of degeneracy of the spectrum of the Hamiltonian on the
average entanglement is emphasised.
