  We show that the t-J Hamiltonian is not in general reduced to H(S,f), where S
and f stand for independent ([S,f]=0) SU(2) (spin) generators and spinless
fermionic (hole) field, respectively. The proof is based upon an identification
of the Hubbard operators with the generators of the su(2|1) superalgebra in the
degenerate fundamental representation and ensuing SU(2|1) path integral
representation of the partition function.
