Minimax Optimal Algorithms for Fixed-Budget Best Arm Identification

We consider the fixed-budget best arm identification problem where the goal is to find the arm of the largest mean with a fixed number of samples. It is known that the probability of misidentifying the best arm is exponentially small to the number of rounds. However, limited characterizations have been discussed on the rate (exponent) of this value. In this paper, we characterize the minimax optimal rate as a result of an optimization over all possible parameters. We introduce two rates, R go and R go ∞ , corresponding to lower bounds on the probability of misidentification, each of which is associated with a proposed algorithm. The rate R go is associated with R go -tracking, which can be efficiently implemented by a neural network and is shown to outperform existing algorithms. However, this rate requires a nontrivial condition to be achievable. To address this issue, we introduce the second rate R go ∞ . We show that this rate is indeed achievable by introducing a conceptual algorithm called delayed optimal tracking (DOT). 1 We use I * = I * (P ) ⊂ [K] as the set of best arms and i * = i * (P ) ∈ I * (P ) as one of them (ties are broken in an arbitrary way). These differences do not matter much in this paper. 2 See Section 1.3 regarding the related work on BAI and R&S.