REVIEW 3 major objections 4 minor 33 references
Beyond expected value: geometric mean optimization for long-term policy performance in reinforcement learning
T0 review · 3 major / 4 minor · reviewed 2026-08-05 · deepseek-v4-flash
Pith's one-line read A reinforcement learning objective that mixes expected return with a sliding-window geometric mean can improve long-term per-trajectory performance.
desk verdict The paper's heuristic regularizer might help in some tasks, but the theoretical bridge it claims — MGM estimates the time-average growth rate — does not hold up, leaving the central objective unsupported. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The engine of the method is a modified geometric mean (MGM) over an N-sliding window of rewards, Ĝ_t = sgn(R̂_t)(|R̂_t|)^{1/N}, with R̂_t the sum of the next N rewards. It is meant to estimate the time-average growth rate of a trajectory without knowing the reward dynamics. This estimator is injected as a regularizer into the multi-step Q-learning target, weighted by λ, and the resulting objective is analyzed through an N-step regularized Bellman optimality operator (TG)^N. The operator, which replaces each per-step reward with (1−λ)r_t + (1−γ)λG∞, is shown to be a γ^N-contraction with fixed point the optimal regularized Q-function; this is what licenses the practical algorithm.
What would settle it
Run two policies on the multiplicative coin-toss process from the paper, chosen so their true time-average growth rates are rank-ordered and separated. For each policy, collect long reward trajectories, compute the MGM estimator on N-windows for several N, and check whether the estimator orders the policies in the same direction as the true growth rates. If the ordering flips for any N, or if the estimator does not converge to a quantity monotone in the growth rate, the regularizer is optimizing a proxy unrelated to its stated target.
Extended reading notes
Core claim
The central claim is that the time-average growth rate—the long-run growth of rewards along one trajectory—can be estimated by a modified geometric mean of cumulative rewards over a sliding window, and that adding this estimate as a regularizer to the standard RL objective improves long-term policy performance. Concretely, the paper defines Ĝ_t = sgn(R̂_t)(|R̂_t|)^{1/N} with R̂_t = Σ_{i=0}^{N-1} r_{t+i}, embeds λĜ into the multi-step Q-learning target, and shows that under geometric Brownian motion reward increments a stationary Markov chain has a fixed growth rate ḡ = Σ_i d_i^π(μ_i − σ_i²/2). The idealized regularized Bellman operator is proved to be a γ^N-contraction with the optimal Q-
Load-bearing premise
The whole algorithm rests on the assumption that taking the N-th root of a sliding-window sum of rewards actually measures how fast a trajectory grows over time; if that estimator does not track the growth rate, the regularizer optimizes the wrong quantity.
Editorial extensions
If this is right
- If the estimator is faithful, setting λ=1 makes the objective directly maximize the invariant growth rate ḡ, connecting RL to ergodicity-theoretic results without requiring an explicit ergodicity transformation.
- Because λ=0 recovers standard multi-step Q-learning, the regularizer can be added to existing multi-step return estimators by changing only the target computation.
- The γ^N-contraction property means the idealized regularized operator has a unique fixed point and inherits the convergence behavior of standard multi-step Q-learning when the growth rate is known.
- The sliding window makes the value update depend on the last N rewards, effectively turning the MDP into a path-dependent process, so the policy can react to recent reward trends rather than only the current state.
Reading between the lines
- A testable extension is to replace the raw sliding-window sum with a discounted window, aligning the estimator's horizon with γ and making it more natural for off-policy corrections.
- The same MGM regularizer could be applied to policy-gradient methods as a path-dependent intrinsic reward; the paper only demonstrates Q-learning.
- The dynamic-awareness flag e suggests a curriculum—non-overlapping windows during exploration so estimates are consistent with one policy, then overlapping windows as the policy stabilizes—which is plausible but not analyzed.
- Because the sign function handles negative rewards by preserving direction, the estimator's scale sensitivity in environments with large negative rewards is worth testing; a scale-invariant normalization may be needed.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a reinforcement learning objective that adds a time-average growth-rate regularizer to the conventional expected discounted return. The authors define an N-step regularized Bellman optimality operator for an idealized known growth rate, introduce a 'modified geometric mean' (MGM) estimator over an N-sliding window to estimate this rate from observed rewards, and embed it in a multi-step Q-learning algorithm. They report improved median cumulative rewards on Lunar Lander and Cart-Pole compared with a multi-step Q-learning baseline for several values of the balance parameter λ.
Significance. The motivation is important: in non-ergodic reward dynamics, optimizing the ensemble mean can yield policies that perform poorly on individual trajectories, and a practical regularizer targeting long-run per-trajectory growth would be a useful contribution. The paper also ships code and evaluates on standard benchmarks, which is commendable. However, the central theoretical bridge—the claim that the MGM estimator (Eq. 8) estimates the time-average growth rate G∞ used in the Bellman operator—is not established, and the regularized Bellman operator is defined using the very quantity it is meant to compute. If these gaps cannot be repaired, the algorithm is an unexplained heuristic and the theoretical contribution is unsupported.
major comments (3)
- [Sec. IV-B2, Eq. (8)] The MGM estimator does not estimate the time-average growth rate. With r_t = R_t - R_{t-1}, the N-window sum is R̂_t = R_{t+N} - R_t. The growth rate in Prop. 1 is asymptotically (1/N) log(R_{t+N}/R_t), whereas |R_{t+N} - R_t|^{1/N} is a different functional: for large R_t it behaves like R_t^{1/N}(e^{Ng}-1)^{1/N}, not g, and when R_{t+N} < R_t it returns a large negative value rather than a negative log-ratio. Thus Eq. (8) is not an estimator of G∞, and Algorithm 1 is not optimizing objective (3) in the stated sense.
- [Definition 1 and Appendix B] The regularized Bellman optimality operator (TG)^N is defined using G∞, 'the time-average growth rate of the best policy'—the object that the operator is supposed to find. In the proof of Prop. 2(1), the authors prove a fixed point for T^π_G with a policy-dependent G^π_∞ and then substitute it into the optimality equation, treating G∞ as a fixed external parameter. The contraction proof likewise treats G∞ as independent of q. As stated, (TG)^N is not a well-defined operator on Q-functions, and the fixed-point and contraction claims do not establish convergence of the proposed regularized Q-learning algorithm.
- [Sec. III and Definition 1] Under Assumption 1 (irreducible and aperiodic chain), the time-average growth rate of a fixed policy is almost surely independent of the initial state-action pair. Hence G^π_∞(s,a) is a constant across (s,a) for each π. In Definition 1, the regularizer (1-γ)λG∞(s,a) then adds the same value to every action at every state and cancels in the max over actions. The theoretical regularizer therefore cannot change the greedy policy in the idealized setting, undermining the claimed mechanism. Any empirical benefit must come from the finite-window heuristic, not from the stated objective.
minor comments (4)
- [Sec. IV-B] The relationship between the continuous-time Δt in Eq. (7) and the discrete window size N in Eq. (6) is never made precise; the paper moves between continuous SDE notation and discrete-time RL without a clear discretization.
- [Sec. IV-C1, after Definition 1] The sentence 'For λ = 1, this operator maps any q to a fixed point, G∞' is unclear: an operator maps a Q-function to a Q-function, not to a scalar. This should be rephrased or removed.
- [Sec. III and IV-A] Assumption 2 (existence of an ergodicity transformation) is stated as essential but is never used in any proof or derivation; the paper should either use it or state that it is only motivational.
- [Sec. V] The experimental comparison reports only medians and density plots without error bars or statistical tests. The claim that the method 'outperforms conventional RL methods' would be strengthened by repeated-seed statistics and significance tests.
Circularity Check
The regularized Bellman optimality operator is defined in terms of the optimal growth rate G∞, which is exactly the quantity the operator is supposed to compute, so the fixed-point result is true by construction.
-
self definitional
[Section IV-C1, Definition 1 (Regularized Bellman optimality operator)]
"Now, let π∗ represent the optimal policy such that π∗ = arg maxπ qπ_G and q∗_G := max π qπ_G = qπ∗_G be the optimal Q-value. Then, we can define the corresponding N-step regularized Bellman optimality operator. Definition 1 ... where G∞ is the time-average growth rate of the best policy that can maximize qπ_G over π."
The operator (TG)^N embeds G∞(s,a), which is defined as the time-average growth rate of the optimal policy π* (the very policy the operator is meant to produce). Thus the fixed point q*_G is not derived; it is inserted as a parameter. Classical Bellman optimality operators construct the optimal value via max over actions without presupposing the optimal policy; here the regularization term requires G^{π*}_∞ before the operator can be written. Proposition 2(1) therefore reduces to "q*_G is fixed under an operator containing q*_G's own growth rate," true by construction. Contractivity (Prop. 2.2) still holds, but the claimed derivation of a Bellman operator for the time-average growth rate is circular.
full rationale
The paper's central theoretical fixed-point result is circular: the N-step regularized Bellman operator is parameterized by G∞, the optimal policy's time-average growth rate, so the fixed point q*_G is baked into the definition. The practical algorithm instead uses the MGM estimator Ĝ_t = sgn(R̂_t)|R̂_t|^{1/N}, but while no derivation connects this estimator to the GBM growth rate of Prop. 1, that is an unsupported leap rather than a demonstrated reduction, so it is not scored as a separate circular step under the strict rules. No load-bearing self-citation is present: reference [8] is motivational, and the existence of ergodicity transformations is attributed to external work [24]. The empirical comparison against conventional multi-step Q-learning is an independent benchmark comparison, not a fitted prediction. Overall, partial circularity: the theoretical guarantee reduces by construction, while the empirical claim retains independent content.
Assumptions & free parameters
free parameters (3)
- lambda (balance parameter) =
swept over [0,1] in both environments, no principled selection
- N (sliding window size) =
2 for Lunar Lander, 5 for Cart-Pole
- G_infinity (ideal growth rate) =
assumed known in Definition 1, replaced by MGM estimate in practice
assumptions (4)
- domain assumption Assumption 1: The Markov chain underlying the MDP is irreducible and aperiodic.
- domain assumption Assumption 2: An ergodicity transformation exists for the reward dynamics.
- ad hoc to paper Per-state reward increments follow a geometric Brownian motion (GBM).
- standard math Standard finite MDP with stationary transition kernel and discount factor gamma.
invented entities (1)
-
Modified geometric mean (MGM) estimator
Cite this review
Pith. "Pith review of Beyond expected value: geometric mean optimization for long-term policy performance in reinforcement learning." pith.science (2026). https://pith.science/paper/74TUT2VC
@misc{pith2026250821443,
author = {Pith},
title = {Pith review of: Beyond expected value: geometric mean optimization for long-term policy performance in reinforcement learning},
year = {2026},
howpublished = {\url{https://pith.science/paper/74TUT2VC}},
note = {Machine review of arXiv:2508.21443}
}
abstract
Reinforcement learning (RL) algorithms typically optimize the expected cumulative reward, i.e., the expected value of the sum of scalar rewards an agent receives over the course of a trajectory. The expected value averages the performance over an infinite number of trajectories. However, when deploying the agent in the real world, this ensemble average may be uninformative for the performance of individual trajectories. Thus, in many applications, optimizing the long-term performance of individual trajectories might be more desirable. In this work, we propose a novel RL algorithm that combines the standard ensemble average with the time-average growth rate, a measure for the long-term performance of individual trajectories. We first define the Bellman operator for the time-average growth rate. We then show that, under multiplicative reward dynamics, the geometric mean aligns with the time-average growth rate. To address more general and unknown reward dynamics, we propose a modified geometric mean with $N$-sliding window that captures the path-dependency as an estimator for the time-average growth rate. This estimator is embedded as a regularizer into the objective, forming a practical algorithm and enabling the policy to benefit from ensemble average and time-average simultaneously. We evaluate our algorithm in challenging simulations, where it outperforms conventional RL methods.
Figures
Reference graph
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