Weakly interacting Bose gas
= Weakly interacting Bose gas
{title2=$n_{\rm ex}/n\ll1$}
A dilute condensed Bose system whose interaction is sufficiently weak for a small-depletion expansion. Its low-energy excitations are described by a <Bogoliubov transformation> rather than free particles. The homogeneous quadratic theory is controlled only while noncondensate occupation remains small compared with the condensate and its excitation energies are stable.