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 $lambda$ in the large N limit. I find that the entropy density monotonically decreases from the Stefan-Boltzmann limit at $lambda=0$ to exactly 4/5 of the Stefan-Boltzmann limit at $lambda=infty$. Calculating the retarded energy-momentum tensor correlator in the scalar channel at $lambda=infty$, I find that it has two logarithmic branch cuts originating at $omega=pm 4 T ln frac{1+sqrt{5}}{2}$, 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.
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