  The hyperspherical adiabatic expansion method is used to describe
correlations in a symmetric boson system rigorously confined to two spatial
dimensions. The hyperangular eigenvalue equation turns out to be almost
independent of the hyperradius, whereas the solutions are strongly varying with
the strength of the attractive two-body potentials. Instability is encountered
in hyperangular, hyperradial, and mean-field equations for almost identical
strengths inversely proportional to the particle number. The derived conditions
for stability are similar to mean-field conditions and closely related to the
possible occurrence of the Thomas and Efimov effects. Renormalization in
mean-field calculations for two spatial dimensions is probably not needed.
