  We present an analytical treatment of a single \down atom within a Fermi sea
of \up atoms, when the interaction is strong enough to produce a bound state,
dressed by the Fermi sea. Our method makes use of a diagrammatic analysis, with
the involved diagrams taking only into account at most two particle-hole pairs
excitations. The agreement with existing Monte-Carlo results is excellent. In
the BEC limit our equation reduces exactly to the Skorniakov and
Ter-Martirosian equation. We present results when \up and \down atoms have
different masses, which is of interest for experiments in progress.

Quantum Gases;Statistical Mechanics