Abstract
The free energy of the RPn-1 model is rigorously computed in the n= infinity limit. The system shows a first-order transition with latent heat, in two and three dimensions. The first correction in 1/n is also computed. Comparison of the results with numerical ones show that whereas in three dimensions the large-n limit correctly reproduces the behaviour of the system for n>or=3, it does not in two dimensions for some observables.