On the Global Convergence of Natural Actor-Critic with Two-layer Neural
  Network Parametrization
Mudit Gaur Amrit Singh Bedi Di Wang Vaneet Aggarwal
Actor-critic algorithms have shown remarkable success in solving
state-of-the-art decision-making problems. However, despite their empirical
effectiveness, their theoretical underpinnings remain relatively unexplored,
especially with neural network parametrization. In this paper, we delve into
the study of a natural actor-critic algorithm that utilizes neural networks to
represent the critic. Our aim is to establish sample complexity guarantees for
this algorithm, achieving a deeper understanding of its performance
characteristics. To achieve that, we propose a Natural Actor-Critic algorithm
with 2-Layer critic parametrization (NAC2L). Our approach involves estimating
the $Q$-function in each iteration through a convex optimization problem. We
establish that our proposed approach attains a sample complexity of
$\tilde{\mathcal{O}}\left(\frac{1}{\epsilon^{4}(1-\gamma)^{4}}\right)$. In
contrast, the existing sample complexity results in the literature only hold
for a tabular or linear MDP. Our result, on the other hand, holds for countable
state spaces and does not require a linear or low-rank structure on the MDP.