Multi-objective multi-armed bandit with lexicographically ordered and satisficing objectives

We consider multi-objective multi-armed bandit with (i) lexicographically ordered and (ii) satisficing objectives. In the first problem, the goal is to select arms that are lexicographic optimal as much as possible without knowing the arm reward distributions beforehand. We capture this goal by defining a multi-dimensional form of regret that measures the loss due to not selecting lexicographic optimal arms, and then, propose an algorithm that achieves O~(T2/3) gap-free regret and prove a regret lower bound of Ω(T2/3). We also consider two additional settings where the learner has prior information on the expected arm rewards. In the first setting, the learner only knows for each objective the lexicographic optimal expected reward. In the second setting, it only knows for each objective a near-lexicographic optimal expected reward. For both settings, we prove that the learner achieves expected regret uniformly bounded in time. Then, we show that the algorithm we propose for the second setting of lexicographically ordered objectives with prior information also attains bounded regret for satisficing objectives. Finally, we experimentally evaluate the proposed algorithms in a variety of multi-objective learning problems.