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# Adversarially Robust Multi-Armed Bandit Algorithm with Variance-Dependent Regret Bounds

*Proceedings of Thirty Fifth Conference on Learning Theory*, PMLR 178:1421-1422, 2022.

#### Abstract

This paper considers the multi-armed bandit (MAB) problem and provides a new best-of-both-worlds (BOBW) algorithm that works nearly optimally in both stochastic and adversarial settings. In stochastic settings, some existing BOBW algorithms achieve tight gap-dependent regret bounds of $O(\sum_{i: \Delta_i>0} \frac{\log T}{\Delta_i})$ for suboptimality gap $\Delta_i$ of arm $i$ and time horizon $T$. On the other hand, it is shown in Audibert et al. (2007) that the regret bound can be tightened to $O(\sum_{i: \Delta_i>0} (\frac{\sigma_i^2}{\Delta_i} + 1) \log T )$ using the loss variance $\sigma_i^2$ of each arm $i$ in the stochastic environments. In this paper, we propose an algorithm based on the follow-the-regularized-leader method, which employs adaptive learning rates that depend on the empirical prediction error of the loss. This is the first BOBW algorithm with gap-variance-dependent bounds, showing that the variance information can be used even in the possibly adversarial environment. Further, the leading constant factor in our gap-variance dependent bound is only (almost) twice the value for the lower bound. In addition, the proposed algorithm enjoys multiple data-dependent regret bounds in adversarial settings and works well in stochastic settings with adversarial corruptions. Table 1 summarizes the achievable bounds in comparison with UCB-V Audibert et al. (2007), Tsallis-INF (Zimmert and Seldin, 2021) and LB-INF (Ito, 2021).