  The two-body interaction in dilute solutions of polymer chains in good
solvents can be modeled by means of effective bounded potentials, the simplest
of which being that of penetrable spheres (PSs). In this paper we construct two
simple analytical theories for the structural properties of PS fluids: a
low-temperature (LT) approximation, that can be seen as an extension to PSs of
the well-known solution of the Percus-Yevick (PY) equation for hard spheres,
and a high-temperature (HT) approximation based on the exact asymptotic
behavior in the limit of infinite temperature. Monte Carlo simulations for a
wide range of temperatures and densities are performed to assess the validity
of both theories. It is found that, despite their simplicity, the HT and LT
approximations exhibit a fair agreement with the simulation data within their
respective domains of applicability, so that they complement each other. A
comparison with numerical solutions of the PY and the hypernetted-chain
approximations is also carried out, the latter showing a very good performance,
except inside the core at low temperatures.
