Comparison between theoretical four-loop predictions and Monte Carlo calculations in the two-dimensional N-vector model for N = 3, 4, 8

We have computed the four-loop contribution to the beta-function and to the anomalous dimension of the field for the two-dimensional lattice N-vector model. This allows the determination of the second perturbative correction to various long-distance quantities like the correlation lengths and the susceptibilities. We compare these predictions with new Monte Carlo data for N = 3, 4, 8. From these data we also extract the values of various universal nonperturbative constants, which we compare with the predictions of the 1/N expansion.