The absence of spontaneous symmetry breaking in d ≤ 2 dimensional infinite systems was rigorously proved by David Mermin and Herbert Wagner (1966), citing a more general unpublished proof by Pierre Hohenberg (published later in 1967) in statistical mechanics. It was also reformulated later by Sidney Coleman (1973) for quantum field theory. The theorem does not apply to discrete symmetries that can be seen in the two-dimensional Ising model.