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Implications of $Trγ_{5}=0$ in Lattice Gauge Theory

We analyze the implications of the relation $Trγ_{5}=0$, which is customarily assumed in practical lattice calculations. On the basis of the finite dimensional representations of the Ginsparg-Wilson algebra, it is shown that this relation reflects the species doubling in lattice theory; topological excitations associated with species doublers, which have eigenvalue $2/a$, contribute to $Trγ_{5}$ without any suppression. In this sense, the relation $Trγ_{5}=0$ is valid only when we allow the presence of unphysical states in the Hilbert space; this statement is also valid in the Pauli-Villars regularization. If one eliminates the contributions of the unphysical states, the trace $Trγ_{5}$ is replaced by $TrΓ_{5}\equiv Tr γ_{5}(1 -{1/2}aD)$ which gives rise to the Pontryagin index, to be consistent with the continuum analysis.