A New Entropy Puzzle in ABJM

We point out a puzzle about BPS black holes in AdS${}_4\times S^7\mathrm{/}{\mathbb{Z}}_k$: the entropy grows when $k$ increases although the number of gauge-invariant operators in the dual conformal field theory that can contribute to the entropy grows as $k$ decreases, due to the presence of monopole operators. Analyzing the superconformal index of ABJM, we show that the same puzzle exists for finite $N$ in the large charge limit. We propose a resolution based on an increasing number of trace relations as $k$ decreases, which dominates over the other effect. We argue that this mostly affects fortuitous operators and we provide an explicit example of a fortuitous operator that disappears at small $k$.