Introduction
Recent advances in playing the game of Go Silver et al. (2017) and Starcraft Vinyals et al. (2019) has proved the capability of selfplay in achieving superhuman performance in competitive reinforcement learning (competitive RL) Crandall and Goodrich (2005), a special case of multiagent RL where each player tries to maximize its own reward. These selfplay algorithms are able to learn through repeatedly playing against themselves and update their policy based on the observed trajectory in the absence of human supervision. Despite the empirical success, the theoretical understanding of these algorithms is limited and is significantly more challenging than the singleagent RL due to its multiagent nature.
Selfplay can be considered as a special category of offline competitive RL where the learning algorithm can control both the agent and the opponent during the learning process Bai and Jin (2020); Bai et al. (2020). In the more general and sophisticated online learning case, the opponent can take arbitrary historydependent strategies and the agent has no control on the opponent during the learning process Wei et al. (2017); Xie et al. (2020); Tian et al. (2021).
In this paper, the online learning case is considered where the agent learns against an arbitrary opponent who can follow a timevariant historydependent policy and can switch its policy at any time. We consider infinitehorizon twoplayer zerosum stochastic games (SGs) with the averagereward criterion. At each time step, both players determine their actions simultaneously upon observing the state of the environment. The reward and the probability distribution of the next state is then determined by the chosen actions and the current state. The players’ payoffs sum to zero, i.e., the reward of one player (agent) is exactly the loss of the other player (opponent). The agent’s goal is to maximize its cumulative reward while the opponent tries to minimize the total loss.
We propose Posterior Samling Reinforcement Learning algorithm for Zerosum Stochastic Games (PSRLZSG), a learning algorithm that achieves Bayesian regret bound. Here is an upper bound on the biasspan, is the number of states, is the size of all possible action pairs for both players, is the horizon, and hides logarithmic factors. The best existing result in this setting is achieved by UCSG algorithm Wei et al. (2017) which obtains a regret bound of where
is the diameter of the SG. As stochastic games generalize Markov Decision Processes (MDPs), our regret bound is optimal (except for logarithmic factors) in
and due to the lower bound provided by Jaksch et al. (2010).Related Literature
SG was first formulated by Shapley (1953). A large body of work focuses on finding the Nash equilibria in SGs with known transition kernel Littman (2001); Hu and Wellman (2003); Hansen et al. (2013), or learning with a generative model Jia et al. (2019); Sidford et al. (2020); Zhang et al. (2020) to simulate the transition for an arbitrary stateaction pair. In these cases no exploration is needed.
There is a long line of research on exploration and regret analysis in singleagent RL (see e.g. Jaksch et al. (2010); Osband et al. (2013); Gopalan and Mannor (2015); Azar et al. (2017); Ouyang et al. (2017); Jin et al. (2018); Zhang and Ji (2019); Zanette and Brunskill (2019); Wei et al. (2020, 2021); Chen et al. (2021a); JafarniaJahromi et al. (2021b, a) and references therein). Extending these results to the SGs is nontrivial since the actions of the opponent also affects the state transition and can not be controlled by the agent. We review the literature on exploration in SGs and refer the interested reader to Zhang et al. (2021); Yang and Wang (2020) for an extensive literature review on multiagent RL in various settings.
Stochastic Games.
A few recent works use selfplay as a method to learn stochastic games Bai and Jin (2020); Bai et al. (2020); Liu et al. (2021); Chen et al. (2021b). However, selfplay requires controlling both the agent and the opponent and cannot be applied in the online setting where the agent plays against an arbitrary opponent. All of these works consider the setting of finitehorizon SG where the interaction of the players and the environment terminates after a fixed number of steps.
In the online setting where the opponent is arbitrary, Xie et al. (2020); Jin et al. (2021) achieve a regret bound of in the finitehorizon SGs with linear and general function approximation, respectively. However, in the applications where the interaction between the players and the environment is nonstopping (e.g., stock trading), the infinitehorizon SG is more suitable. Lack of a fixed horizon in this setting makes the problem more challenging. This is since the backward induction, a technique that is widely used in the finitehorizon, is not applicable in the infinitehorizon setting.
In the infinitehorizon setting, the primary work of Brafman and Tennenholtz (2002) who proposes Rmax does not consider regret. A special case of online learning in generalsum games is studied by DiGiovanni and Tewari (2021) where the opponent is allowed to switch its stationary policy a limited number of times. They achieve a regret bound of via posterior sampling, where is the number of switches. Their result is not directly comparable to ours because their definition of regret is different. Moreover, they assume the transition kernel is known and the opponent adopts stationary policies. To the best of our knowledge, the only existing algorithm that considers online learning against an arbitrary opponent in the infinitehorizon averagereward SG is UCSG Wei et al. (2017).
Comparison with Ucsg Wei et al. (2017).
Our work is closely related to UCSG, however clear distinctions exist in the result, the algorithm, and the technical contribution:

UCSG achieves a regret bound of under the finitediameter assumption (i.e., for any two states and every stationary randomized policy of the opponent, there exists a stationary randomized policy for the agent to move from one state to the other in finite expected time). Under the much stronger ergodicity assumption (i.e., for any two states and every stationary randomized policy of the agent and the opponent, it is possible to move from one state to the other in finite expected time), UCSG obtains a regret bound of . Note that the ergodicity assumption greatly alleviates the challenge in exploration. Our algorithm significantly improves this result and achieves a regret bound of under the finitediameter assumption.

UCSG is an optimismbased algorithm inspired by Jaksch et al. (2010) and requires the complicated maximin extended value iteration. Our algorithm, however, is the first posterior samplingbased algorithm in SGs, leveraging the ideas of Ouyang et al. (2017) in MDPs, and is much simpler both in the algorithm and the analysis.

From the analysis perspective, under the finitediameter assumption, UCSG uses a sequence of finitehorizon SGs to approximate the averagereward SG and that leads to the suboptimal regret bound of . Our analysis avoids the finitehorizon approximation by directly using the Bellman equation in the infinitehorizon SG and achieves nearoptimal regret bound.
Preliminaries
Let be a stochastic zerosum game where is the state space, is the joint action space, is the reward function and represents the transition kernel such that where are the state, the agent and the opponent’s actions at time , respectively. We assume that are finite sets with size .
The game starts at some initial state . At time , the players observe state and take actions . The agent (maximizer) receives reward from the opponent (minimizer). Then, the state evolves to
according to the probability distribution
. The goal of the agent is to maximize its cumulative reward while the opponent tries to minimize it. For the ease of notation, we denote and and accordingly will be denoted by and , respectively.The players’ actions are assumed to depend on the history. Namely, denote by (resp. ) the mappings from the history to the probability distributions over (resp. ). Let (resp. ) be the sequence of historydependent randomized policies whose class is denoted by . In the case that (resp. ) is independent of time (stationary randomized policies), we remove the subscript and with abuse of notation denote (resp. ). The class of stationary randomized policies is denoted by .
For the ease of presentation, we introduce a few notations. Let , denote the size of the action spaces. For an integer , denote by the probability simplex of dimension . Let and . With abuse of notation, let and .
To achieve a low regret algorithm, it is necessary to assume that all the states are accessible by the agent under some policy. In the special case of MDPs, this is stated by the notion of “weakly communication” (or “finite diameter” Jaksch et al. (2010)) and is known to be the minimal assumption to achieve sublinear regret Bartlett and Tewari (2009). The following assumption generalizes this notion to the stochastic games.
Assumption 1.
(Finite Diameter) There exists such that for any stationary randomized policy of the opponent and any , there exists a stationary randomized policy of the agent, such that the expected time of reaching starting from under policy does not exceed , i.e.,
where is the expected time of reaching starting from under policy .
This assumption was first introduced by Federgruen (1978) and is essential to achieve low regret algorithms in the adversarial setting Wei et al. (2017). To see this, suppose that the opponent has a way to lock the agent in a “bad” state. In the initial stages of the game where the agent has limited knowledge of the environment, it may not be possible to avoid such a state and linear regret is unavoidable. Thus, this assumption states that regardless of the strategy used by the opponent, the agent has a way to recover from bad states.
For a general matrix game with matrix of size , the game value is denoted by . Moreover, the Nash equilibrium always exists Nash and others (1950). For SGs, under Assumption 1, Federgruen (1978); Wei et al. (2017) prove that there exist unique and unique (upto an additive constant) function that satisfy the Bellman equation, i.e., for all ,
(1) 
In particular, the Nash equilibrium of the right hand side for each yields maximin stationary policies such that
(2)  
(3) 
Moreover, is the maximin average reward obtained by the agent and is independent of the initial state , i.e.,
where and and . Note that because the range of the reward function is . Define the span of the stochastic game with transition kernel as the span of the corresponding value function , i.e., . We restrict our attention to stochastic games whose transition kernel satisfies Assumption 1 and where is a known scalar. Let denote the set of all such . Moreover, observe that if satisfies the Bellman equation, also satisfies the Bellman equation for any scalar . Thus, without loss of generality, we can assume that for all and .
We consider the problem of an agent playing a stochastic game against an opponent who can take timeadaptive policies. We assume that the opponent knows the history of states and actions and can play timeadaptive historydependent policies. Recall that the state of such policies is denoted by . and are completely known to the agent. However, the transition kernel is unknown. In the beginning of the game, is drawn from an initial distribution and is then fixed. We assume that the support of is a subset of . The performance of the agent is then measured with the notion of regret defined as
(4) 
where . Here the expectation is with respect to the prior distribution , randomized algorithm and the randomness in the state transition. Note that the regret guarantee is against an arbitrary opponent who can change its policy at each time step and has the perfect knowledge of the history of the states and actions. The only hidden information from the opponent is the realization of the agent’s current action (which will be revealed after both players have chosen their actions). We note that selfplay and the case when the opponent uses the same learning algorithm as the agent are two special cases of the scenario considered here.
Posterior Sampling for Stochastic Games
In this section, we propose Posterior Sampling algorithm for Zerosum SGs (PSRLZSG). The agent maintains the posterior distribution on parameter . More precisely, the learning algorithm receives an initial distribution as the input and updates the posterior distribution upon observing the new state according to
(5) 
PSRLZSG proceeds in episodes. Let denote the start time and the length of episode , respectively. In the beginning of each episode, the agent draws a sample of the transition kernel from the posterior distribution . The maximin strategy is then derived for the sampled transition kernel according to (1) and used by the agent during the episode. Let be the number of visits to stateaction pair before time , i.e.,
As described in Algorithm 1, a new episode starts if or for some . The first criterion, , states that the length of the episode grows at most by 1 if the other criterion is not triggered. This ensures that for all . The second criterion is triggered if the number of visits to a stateaction pair is doubled. These stopping criteria balance the tradeoff between exploration and exploitation. In the beginning of the game, the episodes are short to motivate exploration since the agent is uncertain about the underlying environment. As the game proceeds, the episodes grow to exploit the information gathered about the environment. These stopping criteria are the same as those used in MDPs Ouyang et al. (2017).
Analysis
In this section, we provide the proof of Theorem 1. A central observation in our analysis is that in the beginning of each episode, and are identically distributed conditioned on the history. This key property of posterior sampling relates quantities that depend on the unknown to those of the sampled which is fully observed by the agent. Posterior sampling ensures that if is a stopping time, for any measurable function and any
measurable random variable
, Ouyang et al. (2017); Osband et al. (2013).The key challenge in the analysis of stochastic games is that the opponent is also making decisions. If the opponent follows a fixed stationary policy, it can be considered as part of the environment and thus the SG reduces to an MDP. However, in the case that the opponent uses a dynamic historydependent policy during the learning phase of the agent, this reduction is not possible. The key lemma in our analysis is Lemma 3 which overcomes this difficulty through the Bellman equation for the SG.
Proof of Theorem 1
Let be the number of episodes until time and define . Recall that where
(7) 
Let be an arbitrary historydependent randomized strategy followed by the opponent. We start by decomposing the regret into two terms
(8) 
Lemma 2 uses the property of posterior sampling to bound the first term. The second term is handled by combining the Bellman equation, concentration inequalities and the property of posterior sampling as detailed in Lemma 3. Finally, Lemma 4 bounds the number of episodes and completes the proof.
Lemma 2.
The first term of (8) can be bounded by
Proof.
(9) 
where the last inequality is by the fact that and due to the first stopping criterion. Now, note that is a stopping time and and are measurable random variables. Thus, by the property of posterior sampling and monotone convergence theorem,
Taking another expectation from both sides and using the tower property, we have
Replacing this in (Proof of Theorem 1) implies that
The last inequality is by the fact that and . ∎
Lemma 3.
The second term of (8) can be bounded by
Proof.
The policy used by the agent at episode is the solution of the Nash equilibrium in (1). Thus, for and any , (3) implies that
for any distribution . Let be an arbitrary historydependent randomized strategy for the opponent. Note that for any , is measurable. Replacing by and by implies that
Adding and subtracting to the right hand side and summing over time steps within episode implies that
(10) 
The second term on the right hand side of (10) telescopes and can be bounded as
(11) 
where the last inequality is by the fact that is chosen from the posterior distribution whose support is a subset of . Substituting (11) in (10), summing over episodes, and taking expectation implies that
We proceed to bound the last term on the right hand side of the above inequality.
(12) 
To bound the inner summation, similar to Ouyang et al. (2017); Jaksch et al. (2010), we define a confidence set around the empirical transition kernel . Here is the number of visits to stateaction pair whose next state is . The confidence set is defined as
where . Weissman et al. (2003) shows that the true transition kernel belongs to with high probability. We use this fact to show concentration of around . Concentration of around is then followed by the property of posterior sampling. More precisely, we can write
Substituting the inner sum of (12) with this upper bound implies
(13) 
The first term on the right hand side of (Proof of Theorem 1) can be bounded as
(14) 
where the first inequality is by the fact that and for all and the second inequality is by the following argument:
where the last inequality is by CauchySchwarz and the last equality is by the fact that . To bound the second term on the right hand side of (Proof of Theorem 1), we can write
where the second equality is by the property of Posterior Sampling since is measurable. Note that
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