From External to Internal Regret
Avrim Blum and Yishay Mansour
In: COLT 2005, June 27-30, 2005, Bertinoro, Italy.

## Abstract

External regret compares the performance of an online algorithm, selecting among $N$ actions, to the performance of the best of those actions in hindsight. Internal regret compares the loss of an online algorithm to the loss of a modified online algorithm, which consistently replaces one action by another. In this paper, we give a simple generic reduction that, given an algorithm for the external regret problem, converts it to an efficient online algorithm for the internal regret problem. We provide methods that work both in the {\em full information} model, in which the loss of every action is observed at each time step, and the {\em partial information} (bandit) model, where at each time step only the loss of the selected action is observed. The importance of internal regret in game theory is due to the fact that in a general game, if each player has sublinear internal regret, then the empirical frequencies converge to a correlated equilibrium. For external regret we also derive a quantitative regret bound for a very general setting of regret, which includes an arbitrary set of modification rules (that possibly modify the online algorithm) and an arbitrary set of time selection functions (each giving different weight to each time step). The regret for a given time selection and modification rule is the difference between the cost of the online algorithm and the cost of the modified online algorithm, where the costs are weighted by the time selection function. This can be viewed as a generalization of the previously-studied {\em sleeping experts} setting.

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EPrint Type: Conference or Workshop Item (Paper) Project Keyword UNSPECIFIED Theory & Algorithms 987 Yishay Mansour 19 June 2005