Paper:
Best Response Regression
In a regression task, a predictor is given a set of instances, along with a real value for each point. Subsequently, she has to identify the value of a new instance as accurately as possible. In this work, we initiate the study of strategic predictions in machine learning. We consider a regression task tackled by two players, where the payoff of each player is the proportion of the points she predicts more accurately than the other player. We first revise the probably approximately correct learning framework to deal with the case of a duel between two predictors. We then devise an algorithm which finds a linear regression predictor that is a best response to any (not necessarily linear) regression algorithm. We show that it has linearithmic sample complexity, and polynomial time complexity when the dimension of the instances domain is fixed. We also test our approach in a high-dimensional setting, and show it significantly defeats classical regression algorithms in the prediction duel. Together, our work introduces a novel machine learning task that lends itself well to current competitive online settings, provides its theoretical foundations, and illustrates its applicability.

Review:
STRENGTHS:
On some "abstract" level, this paper pursues an interesting direction:
- I fully agree that "prediction is not done in isolation" (l24), and though this is a vague observation, it is worth pursuing this beyond established work (although I'm not an expert with a full overview over this field).
- Making explicit the tradeoff between good-often and not-bad-always in deciding on a loss function is interesting.
Overall, the writing and mathematical quality of the paper is good to very good, with some uncertainty regarding proofs though:
- Well written and structured.
- The paper is very strong in that the algorithmic and mathemtaical results are well connected (combine the VC theorems and lemmas to reach corollary 1 which shows sample bounds for their EPM algo) and cover the usual questions (consistency, runtime).
- In terms of correctness of the mathematical results, it seems that the authors know what they are doing. I only checked one proof, that of Theorem 1 (via the second proof of Lemma 5), and it seems correct although I couldn't follow all steps.
WEAKNESSES:
For me the main issue with this paper is that the relevance of the *specific* problem that they study -- maximizing the "best response" payoff (l127) on test data -- remains unclear. I don't see a substantial motivation in terms of a link to settings (real or theoretical) that are relevant:
- In which real scenarios is the objective given by the adverserial prediction accuracy they propose, in contrast to classical prediction accuracy?
- In l32-45 they pretend to give a real example but for me this is too vague. I do see that in some scenarios the loss/objective they consider (high accuracy on majority) kind of makes sense. But I imagine that such losses already have been studied, without necessarily referring to "strategic" settings. In particular, how is this related to robust statistics, Huber loss, precision, recall, etc.?
- In l50 they claim that "pershaps even in most [...] practical scenarios" predicting accurate on the majority is most important. I contradict: in many areas with safety issues such as robotics and self-driving cars (generally: control), the models are allowed to have small errors, but by no means may have large errors (imagine a self-driving car to significantly overestimate the distance to the next car in 1% of the situations).
Related to this, in my view they fall short of what they claim as their contribution in the introduction and in l79-87:
- Generally, this seems like only a very first step towards real strategic settings: in light of what they claim ("strategic predictions", l28), their setting is only partially strategic/game theoretic as the opponent doesn't behave strategically (i.e., take into account the other strategic player). 
- In particular, in the experiments, it doesn't come as a complete surprise that the opponent can be outperformed w.r.t. the multi-agent payoff proposed by the authors, because the opponent simply doesn't aim at maximizing it (e.g. in the experiments he maximizes classical SE and AE).
- Related to this, in the experiments it would be interesting to see the comparison of the classical squared/absolute error on the test set as well (since this is what LSE claims to optimize).
- I agree that "prediction is not done in isolation", but I don't see the "main" contribution of showing that the "task of prediction may have strategic aspects" yet.
REMARKS:
What's "true" payoff in Table 1? I would have expected to see the test set payoff in that column. Or is it the population (complete sample) empirical payoff?
Have you looked into the work by Vapnik about teaching a learner with side information? This looks a bit similar as having your discrapency p alongside x,y.