  We theoretically investigate the effects of pairing fluctuations in an
ultracold Fermi gas near a Feshbach resonance with a negative effective range.
By employing a many-body T-matrix theory with a coupled boson-fermion model, we
show that the single-particle density of states exhibits the so-called
pseudogap phenomenon which is a precursor of superfluidity induced by strong
pairing fluctuations. We clarify the region where strong pairing fluctuations
play a crucial role in single-particle properties, from the broad-resonance
region to the narrow-resonance limit at the divergent two-body scattering
length. We also extrapolate the effects of pairing fluctuations to the
positive-effective-range region from our results near the narrow Feshbach
resonance. Results shown in this paper are relevant to the connection between
ultracold Fermi gases and low-density neutron matter from the viewpoint of
finite-effective-range corrections.
