Brief Encounters. Binary Correlations among Bosons
  A description of a large system of particles is often sought in a derivation from the detailed behaviour of just a few of the particles. The present thesis deals with the connection between such microscopic features and the nature of a collection of many particles. The study of two-body correlations yields insight into mechanisms that in the dilute limit can be accounted for by a mean field, and yields deviations especially in the presence of a two-body bound state or a resonance. Contents: Chapter 2 presents the hyperspherical frame for studying few-body correlations within a many-boson system. Chapter 3 contains a discussion of analytical and numerical solutions to the hyperangular part of the Hamiltonian. Chapter 4 contains a discussion of the hyperradial part of the description of a many-boson system, as well as a discussion of Bose-Einstein condensation in the hyperspherical model. In chapter 5 we compare results from the previous chapters to the results from the mean-field Gross-Pitaevskii equation and ranges of validity are estimated. In chapter 6 we discuss stability criteria, approaches to the dynamical evolution of the many-boson system, and relevant time scales. Chapter 7 deals with the effects of a deformed external trap. 
O. S{\o}rensen (Department of Physics and Astronomy, University of   Aarhus, Denmark)