We consider the adversarial linear contextual bandit setting, which allows for the loss functions associated with each of $K$ arms to change over time without restriction. Assuming the $d$-dimensional contexts are drawn from a fixed known distribution, the worst-case expected regret over the course of $T$ rounds is known to scale as $\tilde O(\sqrt{Kd T})$. Under the additional assumption that the density of the contexts is log-concave, we obtain a second-order bound of order $\tilde O(K\sqrt{d V_T})$ in terms of the cumulative second moment of the learner's losses $V_T$, and a closely related first-order bound of order $\tilde O(K\sqrt{d L_T^*})$ in terms of the cumulative loss of the best policy $L_T^*$. Since $V_T$ or $L_T^*$ may be significantly smaller than $T$, these improve over the worst-case regret whenever the environment is relatively benign. Our results are obtained using a truncated version of the continuous exponential weights algorithm over the probability simplex, which we analyse by exploiting a novel connection to the linear bandit setting without contexts.

考虑对抗性线性上下文赌博机设置，文中给出了一种新的算法，通过利用与不需要上下文设置的线性赌博机的新联系，利用连续指数权重算法在概率单形上的一个截断版本来获得结果，并证明了其结果优于最坏情况下的后悔，特别的当环境相对温和时，考虑了上下文的密度是对数凹的情况，给出了一种同时优于二阶和一阶损失的方法。

对抗性线性情境赌博机的一阶和二阶边界