Browsing by Subject "Multi-armed bandit problem"

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Open Access
Adaptive ambulance redeployment via multi-armed bandits
(2019-09) Şahin, Ümitcan
Emergency Medical Services (EMS) provide the necessary resources when there is a need for immediate medical attention and play a signi cant role in saving lives in the case of a life-threatening event. Therefore, it is necessary to design an EMS system where the arrival times to calls are as short as possible. This task includes the ambulance redeployment problem that consists of the methods of deploying ambulances to certain locations in order to minimize the arrival time and increase the coverage of the demand points. As opposed to many conventional redeployment methods where the optimization is primary concern, we propose a learning-based approach in which ambulances are redeployed without any a priori knowledge on the call distributions and the travel times, and these uncertainties are learned on the way. We cast the ambulance redeployment problem as a multi-armed bandit (MAB) problem, and propose various context-free and contextual MAB algorithms that learn to optimize redeployment locations via exploration and exploitation. We investigate the concept of risk aversion in ambulance redeployment and propose a risk-averse MAB algorithm. We construct a data-driven simulator that consists of a graph-based redeployment network and Markov tra c model and compare the performances of the algorithms on this simulator. Furthermore, we also conduct more realistic simulations by modeling the city of Ankara, Turkey and running the algorithms in this new model. Our results show that given the same conditions the presented MAB algorithms perform favorably against a method based on dynamic redeployment and similarly to a static allocation method which knows the true dynamics of the simulation setup beforehand.
Open Access
An efficient bandit algorithm for general weight assignments
(IEEE, 2017) Gökçesu, Kaan; Ergen, Tolga; Çiftçi, S.; Kozat, Süleyman Serdar
In this paper, we study the adversarial multi armed bandit problem and present a generally implementable efficient bandit arm selection structure. Since we do not have any statistical assumptions on the bandit arm losses, the results in the paper are guaranteed to hold in an individual sequence manner. The introduced framework is able to achieve the optimal regret bounds by employing general weight assignments on bandit arm selection sequences. Hence, this framework can be used for a wide range of applications.
Open Access
Gambler's ruin bandit problem
(IEEE, 2017) Akbarzadeh, Nima; Tekin, Cem
In this paper, we propose a new multi-armed bandit problem called the Gambler's Ruin Bandit Problem (GRBP). In the GRBP, the learner proceeds in a sequence of rounds, where each round is a Markov Decision Process (MDP) with two actions (arms): a continuation action that moves the learner randomly over the state space around the current state; and a terminal action that moves the learner directly into one of the two terminal states (goal and dead-end state). The current round ends when a terminal state is reached, and the learner incurs a positive reward only when the goal state is reached. The objective of the learner is to maximize its long-term reward (expected number of times the goal state is reached), without having any prior knowledge on the state transition probabilities. We first prove a result on the form of the optimal policy for the GRBP. Then, we define the regret of the learner with respect to an omnipotent oracle, which acts optimally in each round, and prove that it increases logarithmically over rounds. We also identify a condition under which the learner's regret is bounded. A potential application of the GRBP is optimal medical treatment assignment, in which the continuation action corresponds to a conservative treatment and the terminal action corresponds to a risky treatment such as surgery.