RL Theory in Lean

Installation

See https://lean-lang.org/install/ for a detailed tutorial on setting up Lean.
Confirm that Lean is set up by lake --version.

git clone git@github.com:ShangtongZhang/rl-theory-in-lean.git
cd rl-theory-in-lean
lake exe cache get
lake build

It is recommended to use this project with the Lean 4 extension of VSCode.

Paper

Towards Formalizing Reinforcement Learning Theory

@article{zhang2026towards,
    title={Towards Formalizing Reinforcement Learning Theory: A Robbins-Siegmund Approach},
    author={Shangtong Zhang},
    journal={Reinforcement Learning Journal},
    year={2026}
}

Main Results

This project is organized by mirroring the structure of Mathlib so the counterparts can be upstreamed to Mathlib easily. Below are some main results.

• Algorithm
  • LinearTD.lean
    • [LinearTDIterates] - linear TD algorithm.
    • [DecreaseAlong] - $\ell_2$ norm qualifies a Lyapunov function for linear TD.
    • ae_tendsto_of_linearTD_iid — almost sure convergence of linear TD with i.i.d. samples.
    • ae_tendsto_of_linearTD_markov — almost sure convergence of linear TD with Markovian samples.
  • QLearning.lean
    • [QLearningIterates] — Q-learning algorithm.
    • [DecreaseAlong] - $\ell_p$ norm with a sufficiently large $p$ qualifies a Lyapunov function for Q-learning.
    • ae_tendsto_of_QLearning_iid — almost sure convergence of Q-learning with i.i.d. samples.
    • ae_tendsto_of_QLearning_markov — almost sure convergence of Q-learning with Markovian samples.
• StochasticApproximation
  • Iterates.lean - stochastic approximation (SA) iterates and non-uniform bounds.
    • [Iterates] — a general form of SA iterates with Martingale difference noise and additional noise.
    • [IteratesOfResidual] — a specific form of SA iterates driven by residual of an operator.
    • [AdaptedOnSamplePath] — SA iterates are completely determined by available information until current time step on every sample path.
  • CondExp.lean
    • condExp_iterates_update — computes the conditional expectation of SA iterates in the Ionescu-Tulcea probability space.
  • Lyapunov.lean
    • [LyapunovFunction] — desired properties for a function to be used as a Lyapunov function to analyze SA iterates.
  • LpSpace.lean - squared $\ell_p$ norm qualifies a Lyapunov function
    • smooth_half_sq_Lp — squared $\ell_p$ norm is $p - 1$ smooth.
  • Pathwise.lean
    • fundamental_inequality — recursive bounds of errors based on a Lyapunov function on an individual sample path.
  • DiscreteGronwall.lean — discrete Gronwall inequalities.
  • MartingaleDifference.lean
    • ae_tendsto_of_iterates_mds_noise — convergence of SA iterates with Martingale difference noise.
  • IIDSamples.lean
    • ae_tendsto_of_iterates_iid_samples — convergence of SA iterates with i.i.d. samples.
  • StepSize.lean
    • anchors_of_inv_poly — inverse polynomial step size qualifies the skeleton iterates technique that can convert Markov noise to Martingale difference noise.
  • MarkovSamples.lean
    • ae_tendsto_of_iterates_markov_samples — convergence of SA iterates with Markovian samples.
    • ae_tendsto_of_iterates_markov_samples_of_inv_poly — specializes to inverse polynomial step size schedules.
• Data
  • Int/GCD.lean
    • all_but_finite_of_closed_under_add_and_gcd_one — if a set has gcd 1 and is close under addition, it then contains all large enough integers.
  • Matrix/PosDef.lean — positive definiteness (p.d.) for asymmetric real matrices.
    • posDefAsymm_iff' — a lower bound for the quadratic form of a p.d. matrix.
  • Matrix/Stochastic.lean — stochastic vectors and row stochastic matrices.
    • smat_minorizable_with_large_pow — an irreducible and aperiodic stochastic matrix is Doeblin minorizable after sufficient powers.
    • smat_contraction_in_simplex - Doeblin minorization implies contraction in simplex
    • stationary_distribution_exists - produces one stationary distribution via Cesaro limit.
    • stationary_distribution_uniquely_exists — stationary distribution uniquely exists under irreducibility and aperiodicity.
    • [GeometricMixing] - geometric mixing under irreducibility and aperiodicity.
• Probability
  • MarkovChain/Defs.lean — basics of time homogeneous Markov chains.
  • MarkovChain/Finite/Defs.lean — matrix representation of finite Markov chains
  • MarkovChain/Trajectory.lean
    • traj_prob — constructs the sample path space probability measure for Markov chains by the Ionescu-Tulcea theorem.
  • Martingale/Convergence.lean
    • ae_tendsto_zero_of_almost_supermartingale — Robbins-Siegmund theorem for the convergence of almost supermartingales.
• MarkovDecisionProcess
  • MarkovRewardProcess.lean — basics of finite MRPs.
    • [NegDefAsymm MRP.K] — negative definiteness of the matrix $D(\gamma P - I)$
  • MarkovDecisionProcess.lean — basics of finite MDPs.