Title: Inverse Reinforcement Learning Using Just Classification and a Few Regressions
Primary area: reinforcement learning
Keywords: Inverse reinforcement learning, Maximum entropy IRL, Softmax, Fitted Q-iteration, Bellman

Abstract: Inverse reinforcement learning (IRL) aims to explain observed behavior by uncovering an underlying reward. In the maximum-entropy or Gumbel-shocks-to-reward frameworks, this amounts to fitting a reward function and a soft value function that together satisfy the soft Bellman consistency condition and maximize the likelihood of observed actions. While this perspective has had enormous impact in imitation learning for robotics and understanding dynamic choices in economics, practical learning algorithms often involve delicate inner-loop optimization, repeated dynamic programming, or adversarial training, all of which complicate the use of modern, highly expressive function approximators like neural nets and boosting. We revisit softmax IRL and show that the population maximum-likelihood solution is characterized by a *linear* fixed-point equation involving the behavior policy. This observation reduces IRL to two off-the-shelf supervised learning problems: probabilistic classification to estimate the behavior policy, and iterative regression to solve the fixed point. The resulting method is simple and modular across function approximation classes and algorithms. We provide a precise characterization of the optimal solution, a generic oracle-based algorithm, finite-sample error bounds, and empirical results showing competitive or superior performance to MaxEnt IRL.

TL;DR: We show entropy-regularized IRL can be reduced to a few supervised learning tasks (and thus be easily solved with flexible function approximation): multiclass classification to fit behavior policy and a few regressions to solve a linear fixed point