  According to one-loop perturbation theory, fermions whose masses are totally
generated from Yukawa couplings do not decouple in the heavy mass limit. We
investigate this issue nonperturbatively in the strong coupling regime of the
chiral $U(1)$ mirror-fermion model in four dimensions. Our numerical results,
obtained on $6^3\cdot 16$, $6^3\cdot 24$ and $8^3\cdot 16$ lattices, indicate
nondecoupling of heavy fermion and mirror-fermion, thus supporting the one-loop
picture.
