Rényi entropy estimation has been comprehensively investigated by Acharya, Orlitsky, Suresh and Tyagi (SODA 2015; IEEE Trans. Inf. Theory 2017) and consequent works, whereas only the sample complexity of Rényi entropy estimation of integer order has been settled. In this paper, we settle the sample complexity of Rényi entropy estimation of noninteger order, thereby completing the complexity picture of Rényi entropy estimation. Specifically, we show that for any noninteger $α> 0$, it is sufficient and necessary to use \[ Θ\!\left(\frac{d^{\max\{1/α,1\}}}{\varepsilon^{1/α}\log(d)} + \frac{d^{|1-1/α|}}{\varepsilon^2}\right) \] samples to estimate the Rényi entropy of order $α$ of an unknown discrete distribution over an alphabet of size $d$ to within additive error $\varepsilon$. For the upper bound, we reduce the bias using a refined polynomial approximation estimator for large probabilities. For the lower bound, we employ a different hard instance equipped with a new moment matching construction. The constructive moment matching has constant bounded high-order moments, while attaining a fixed ratio between the $α$-th moments, which is of independent interest.

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