  In the partition function of the Kondo lattice, spin matrices are exactly
replaced by bilinear combinations of Fermi operators with the purely imaginary
chemical potential lambda=-i.pi.T/2 (Popov representation). This new
representation of spin operators allows one to introduce new Green's functions
with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature
diagram technique is constructed with the path integral method. This technique
is standard and does not contain the complicated combinatoric rules
characteristic of most of the known variants of the diagram techniques for spin
systems. The effective action for the almost antiferromagnetic Kondo lattice is
derived.
