The growth of the $\frac{1}{16}$-BPS index in 4d $\mathcal{N}=4$ SYM

We study the Hamiltonian index of $\frac{1}{16}$-BPS operators in 4d $\mathcal{N}=4$ U(N) super Yang-Mills (SYM) theory numerically for $N=2\,,\dots,10$. We show that the large-charge asymptotics agree with analytic results in the Cardy-like limit, as consistent with the entropy of supersymmetric black hole in the dual AdS. The numerics also agree with the large-N analytic result, thus providing hints towards an exact formula for the index. We then prove, using ideas from representation theory, that for values of charges (quantized in integer units) less than 2(N+1) the index agrees precisely with the multi-graviton index, and then begins to deviate for larger charges. Thus the U(N) SYM index interpolates between multi-graviton values at small charge and black hole growth at large charges.