Finite-Size and Finite-Temperature Effects in the Conformally Invariant   O(N) Vector Model for 2<d<4
  We study the operator product expansion (OPE) of the auxiliary scalar field \lambda(x) with itself, in the conformally invariant O(N) Vector Model for 2<d<4, to leading order in 1/N in a strip-like geometry with one finite dimension of length L. We show that consistency of the finite-geometry OPE with bulk OPE calculations requires the physical conditions of, either finite-size scaling at criticality, or finite-temperature phase transition. 
Anastasios C. Petkou and Nicholas D. Vlachos