Cutoff effects in the O(N) sigma model at large N
  The computation of the step scaling function for the finite size mass-gap in the O(N) sigma model at large N is reviewed. Practically exact nonperturbative results become available for both finite and vanishing lattice spacing. We use them as a testbed to investigate standard procedures of continuum extrapolation in lattice field theory. 
Ulli Wolff, Francesco Knechtli, Bjoern Leder and Janos Balog