Vortices and phase transitions in two-dimensional non-Abelian spin models

We modify 0(n) models (n>2) in two dimensions so as to compare different theories with identical local properties and different global ones. Our 0'(3) model with a particular interaction has vortex-like configurations (π1(P2)=Z2) though it is locally equivalent to an 0(3) model (π1(S2)=0). Our results have been obtained by means of strong coupling methods. The Pade extrapolants show a critical value chisub(c) = 11.8. (Author)