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Finite temperature CFT results for all couplings: O(N) model in 2+1 dimensions

A famous example of gauge/gravity duality is the result that the entropy density of strongly coupled ${\cal N}=4$ SYM in four dimensions for large N is exactly 3/4 of the Stefan-Boltzmann limit. In this work, I revisit the massless O(N) model in 2+1 dimensions, which is analytically solvable at finite temperature $T$ for all couplings $λ$ in the large N limit. I find that the entropy density monotonically decreases from the Stefan-Boltzmann limit at $λ=0$ to exactly 4/5 of the Stefan-Boltzmann limit at $λ=\infty$. Calculating the retarded energy-momentum tensor correlator in the scalar channel at $λ=\infty$, I find that it has two logarithmic branch cuts originating at $ω=\pm 4 T \ln \frac{1+\sqrt{5}}{2}$, but no singularities in the whole complex frequency plane. I show that the ratio 4/5 and the location of the branch points both are universal within a large class of bosonic CFTs in 2+1 dimensions.