  It is argued that the derivative expansion is a suitable method to deal with
finite temperature field theory, if it is restricted to spatial derivatives
only. Using this method, a simple and direct calculation is presented for the
radiatively induced Chern-Simons--like piece of the effective action of
(2+1)-dimensional fermions at finite temperature coupled to external gauge
fields. The gauge fields are not assumed to be subjected to special
constraints, and in particular, they are not required to be stationary nor
Abelian.
