Entanglement entropies are notoriously difficult to compute. Large-N
strongly-coupled holographic CFTs are an important exception, where the AdS/CFT
dictionary gives the entanglement entropy of a CFT region in terms of the area
of an extremal bulk surface anchored to the AdS boundary. Using this
prescription, we show -- for quite general states of (2+1)-dimensional such
CFTs -- that the renormalized entanglement entropy of any region of the CFT is
bounded from above by a weighted local energy density. The key ingredient in
this construction is the inverse mean curvature (IMC) flow, which we suitably
generalize to flows of surfaces anchored to the AdS boundary. Our bound can
then be thought of as a "subregion" Penrose inequality in asymptotically
locally AdS spacetimes, similar to the Penrose inequalities obtained from IMC
flows in asymptotically flat spacetimes. Combining the result with positivity
of relative entropy, we argue that our bound is valid perturbatively in 1/N,
and conjecture that a restricted version of it holds in any CFT.Comment: 33+7 pages, 7 figures. v2: addressed referee comment
