“…Perhaps surprisingly, the behavior of the two-dimensional model when n ≥ 2, so that the spin space S n−1 has a continuous symmetry, is quite different from that of the Ising model. The Mermin-Wagner theorem [89,88] asserts that in this case there is no phase with long-range order at any inverse temperature β. Quantifying the rate at which correlations decay has been the focus of much research along the years [69,75,101,36,106,107,99,109,54,73,21,90,92,72,57] and is still not completely understood. Improving on earlier bounds, McBryan and Spencer [86] showed in 1977 that the decay occurs at least at a power-law rate,…”