  The generalized gravitational entropy proposed in recent by Lewkowycz and
Maldacena [1] is extended to the systems of Boson fields, Fermion fields and
Maxwell fields which have arbitrary frequency and mode numbers on the BTZ
spacetime. We find the associated regular wave solution in each case and use it
to calculate the exact gravitational entropy. The results show that there is a
threshold frequency below which the Fermion fields could not contribute the
generalized gravitational entropy. Also, the static and zero-mode solutions
have no entropy, contrast to that in scalar fields. We also find that the
entropy of the static scalar fields and non-static fermions is an increasing
function of mode numbers and, after arriving the maximum entropy, it becomes a
deceasing function approaching to a constant value. We calculate the
gravitational entropy of Maxwell fields and use the duality between EM and
scalar fields to explain its result. The gravitational entropy from DBI action
is also studied.
