Critical behavior of the correlation function of three-dimensional O(N)   models in the symmetric phase
  We present new strong-coupling series for O(N) spin models in three dimensions, on the cubic and diamond lattices. We analyze these series to investigate the two-point Green's function G(x) in the critical region of the symmetric phase. This analysis shows that the low-momentum behavior of G(x) is essentially Gaussian for all N from zero to infinity. This result is also supported by a large-N analysis. 
Massimo Campostrini, Paolo Rossi, Andrea Pelissetto, and Ettore Vicari