  We prove Besov space bounds of the resolvent at low energies in any dimension
for a class of potentials that are negative and obey a virial condition with
these conditions imposed at infinity only. We do not require spherical
symmetry. The class of potentials includes in dimension $\geq3$ the attractive
Coulomb potential. There are two boundary values of the resolvent at zero
energy which we characterize by radiation conditions. These radiation
conditions are zero energy versions of the well-known Sommerfeld radiation
condition.
