  Physical systems with a large scattering length have universal properties
independent of the details of the interaction at short distances. Such systems
can be realized in experiments with cold atoms close to a Feshbach resonance.
They also occur in many other areas of physics such as nuclear and particle
physics. The universal properties include a geometric spectrum of three-body
bound states (so-called Efimov states) and log-periodic dependence of
low-energy observables on the physical parameters of the system. This behavior
is characteristic of a renormalization group limit cycle. We discuss
universality in the three- and four-body sectors and give an overview of
applications in cold atoms.
