REVIEW 4 major objections 5 minor 1 cited by
Biased random walk on the critical curve of dynamical percolation
T0 review · 4 major / 5 minor · reviewed 2026-08-08 · deepseek-v4-flash
Pith's one-line read For $d \ge 2$, the speed of a biased random walk on dynamical percolation with $\mu = \sqrt{p(1-p)}$ is eventually strictly increasing in the bias $\lambda$, closing the critical-curve case left open in [1].
desk verdict Important gap-filling result with a solid d=1 expansion, but the d≥2 proof has a sign inconsistency in the central second-order coefficient that must be fixed before the monotonicity claim is established. 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 central object is the environment seen from the walker, the configuration of open and closed edges shifted so that the walker sits at the origin. For the totally asymmetric walk that only attempts right jumps, the paper constructs an invariant measure $Q$ on this environment, shows it is extremal invariant and equivalent to the Bernoulli-$p$ product measure, and computes the stationary law of the projection onto the two edges adjacent to the walker. This stationary law gives the acceptance probability $\mu p/(\mu+1-p)$ and the expected time shift $C_{\mu,p}$ in one dimension. For $d \ge 2$, the argument decomposes jump attempts into forward, backward, and orthogonal points, couples the biased walk to reduced walkers and one-dimensional totally asymmetric walkers, and uses a monotone coupling (Proposition 2.4) to show that each backward or orthogonal excursion costs a fixed positive number of forward steps. Lemma 2.2, the exponential Taylor expansion of $v'(\lambda)$, then converts the second-order speed expansion into a sign for the derivative.
What would settle it
Compute or simulate the coefficient of $e^{-2\lambda}$ in (61) for $d=2$, $p=1/2$, $\mu=1/2$: if the displayed constant $(2d-2)^2p^2/(\mu+1-p) + \mu^2 p/(\mu+1-p)^2 + C_{1b\cup 2o}$ were zero or negative, the speed would approach its limit from above and eventual increase would fail. More directly, a numerical evaluation of $v'(\lambda)$ at large $\lambda$, for example by finite differences of regeneration-time averages, showing a non-positive derivative would refute Theorem 1.4.
Extended reading notes
Core claim
On its own terms, the paper's central claim is Theorem 1.4: for $d \ge 2$ and $p \in (0,1)$, with $\mu = \sqrt{p(1-p)}$, there exists $\lambda_0(p,d)$ such that the speed $v_{\mu,p}(\lambda)$ of the $\lambda$-biased random walk on dynamical percolation is strictly increasing for all $\lambda \ge \lambda_0$. The quantitative content behind it is Theorem 1.3, an asymptotic expansion $$v(\$\lambda$) = \frac{\mu p}{\mu+1-p} - C_{\mu,p,d} $e^{{-2\lambda}}$ + O($e^{{-3\lambda}}$)$$ with $C_{\mu,p,d} > 0$, valid for $\lambda \ge \lambda_0$. Combined with a regularity lemma stating that the derivative $v'(\lambda)$ admits an exponential Taylor expansion $v'(\lambda) = \sum_{i=1}^n c_i e^{-i\lambda} + O(e^{-(n+1)\lambda})$, the vanishing of the $e^{-\lambda}$ coefficient exactly on the critical curve and the strict positivity of the $e^{-2\lambda}$ coefficient force $v'(\lambda) = 2c_2 e^{-2\lambda} + O(e^{-3\lambda}) > 0$ for large $\lambda$. In one dimension the paper also derives an explicit expansion with the closed-form constant $C_{\mu,p}$ given in (2).
Load-bearing premise
The whole monotonicity result rests on an unproved regularity lemma, stated as Lemma 2.3 and cited as following from [1, Lemma 4.10], asserting that certain conditional expected crossing times are independent of the bias; if that lemma or the strict positivity of the $e^{-2\lambda}$ coefficient in the derivative expansion failed, the speed could still fail to be eventually increasing.
Editorial extensions
If this is right
- The critical curve $\mu^2 = p(1-p)$ joins the increasing regime: for $d \ge 2$, $v(\lambda)$ is eventually strictly increasing, so Question 1.6 of [1] is resolved.
- For large $\lambda$ on the critical curve, $v(\lambda)$ approaches its limiting value $\mu p/(\mu+1-p)$ from below at rate $e^{-2\lambda}$, and the linear-order correction vanishes exactly on this curve.
- In dimension one, the explicit expansion (1) holds for every $\mu > 0$ and $p \in (0,1)$ with constant $C_{\mu,p} > 0$; since $C_{\mu,p} \to 2$ as $\mu \to \infty$, the fast-refreshment limit recovers the speed of a biased walk that accepts every move with probability $p$.
- The derivative admits an exponential Taylor expansion $v'(\lambda) = \sum_{i=1}^n c_i e^{-i\lambda} + O(e^{-(n+1)\lambda})$, which together with the second-order speed expansion upgrades asymptotic proximity into a monotonicity statement.
Reading between the lines
- Editorial extension: the same mechanism suggests that the sign of the $e^{-\lambda}$ coefficient, which vanishes precisely on the critical curve, is what flips the eventual monotonicity between $\mu^2 > p(1-p)$ and $\mu^2 < p(1-p)$; one could test whether the speed-versus-$\lambda$ curve is in fact monotone for all $\lambda$, with the large-$\lambda$ sign already fixed.
- Editorial extension: the environment-process stationary measure $Q$ and the explicit projection on the two adjacent edges could be used to derive higher-order corrections or fluctuation bounds, such as variance scaling, for the totally asymmetric walk rather than only its speed.
- Editorial extension: a direct numerical check of Theorem 1.3 for $d=2$ is feasible by simulating the speed at several large $\lambda$ values at $\mu = \sqrt{p(1-p)}$; the predicted sign of the $e^{-2\lambda}$ coefficient is robust because the constant in (61) is a sum of manifestly positive terms plus $C_{1b\cup 2o}$.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the speed of a λ-biased random walk on dynamical percolation on Z^d. For d=1 it proves an explicit second-order expansion v(λ) = μp/(μ+1-p) - μp/(μ+1-p) C_{μ,p} e^{-2λ} + O(e^{-4λ}) with an explicit positive constant C_{μ,p}. For d≥2, on the critical curve μ^2 = p(1-p), it claims a second-order expansion v(λ) = μp/(μ+1-p) - C_{μ,p,d} e^{-2λ} + O(e^{-3λ}) with C_{μ,p,d}>0, and derives from this, together with a derivative expansion, that the speed is eventually strictly increasing in λ. The methods are the environment seen from the walker, a stationary measure Q constructed via regeneration times, couplings to totally asymmetric walks, and event decompositions by backward and orthogonal jump attempts.
Significance. If completed, the paper resolves an open question from Andres et al. [1] on the critical curve and provides a quantitative two-term expansion that is stronger than the qualitative dichotomy known off the critical curve. The d=1 part is a genuine contribution: it gives an explicit constant, uses a clean invariant-measure construction, and contains a detailed mixing argument (Lemma 4.2). The paper also ships explicit formulas rather than fitted constants, which is a strength. However, the d≥2 proof currently rests on several omitted or sketched arguments and contains a sign inconsistency in a load-bearing lemma, so the central monotonicity claim is not yet secure as written.
major comments (4)
- [Section 5.3, Lemma 5.6 and Eqs. (53)-(56)] The statement and proof of Lemma 5.6 assign opposite signs to C_{↑↓}. The statement (45) says |E[X_{τ_k} - \tilde Y_{τ_k} | E_{↑↓}] - C_{↑↓}| ≤ δ + O(e^{-λ}) with C_{↑↓} > 0, which means X is ahead of \tilde Y; the proof, however, derives E[X_{T_2} - \tilde Y_{T_2}] = -C_{↑↓} + O(e^{-λ}) and maintains P[X_t ≤ \tilde Y_t] = 1, which means X is behind. The main text then uses the plus sign in Eq. (53) and the minus sign in Eqs. (54) and (56). Only one sign can be correct, and the sign of the e^{-2λ} correction in Theorem 1.3 depends on this term. As written, the proof does not determine whether the second-order coefficient is positive, negative, or zero.
- [Section 5.1, Lemma 5.3 and Eq. (57)] Lemma 5.3, which supplies the positive constant C_{1b}, has its proof omitted: the authors write 'we leave the details to the reader'. This is not a cosmetic gap, because Eq. (57) assembles the decisive positive constant C_{1b∪2o} from C_{1b}, C_{2o\↑↓}, C_{↑↓}, and the event probabilities, and Theorem 1.3 requires C_{1b∪2o} > 0. The d=1 computation in Theorem 1.2 does not directly cover the d≥2 situation where o-points interact with the backward-jump event. In addition, Eq. (50) uses the correction -C_{1b} P(E_{1b}), while Eq. (57) uses - μp/(μ+1-p) C_{μ,p} P(E_{1b}); the substitution of C_{1b} by this expression is unexplained and needs justification. Similar sketched positivity assertions appear for C_{2o\↑↓} in Proposition 5.4(3) and C_{2o''} in Lemma 5.5(2).
- [Section 5.4, Eqs. (57)-(61)] The k-dependence of C_{1b∪2o} is not tracked consistently. Eq. (57) allows C_{1b∪2o} to depend on k, but Eq. (58) writes E[X_{τ_k}] = k v_Y e^{1/μ} - C_{1b∪2o} e^{1/μ} e^{-2λ} + ... without indicating whether the constant absorbs a factor of k. After dividing by E[τ_k] = k e^{1/μ}, the correction term would vanish as k → ∞ unless C_{1b∪2o} is O(k), which is not shown. Since Lemma 4.1 permits any k, the proof must either fix a sufficiently large k and control all constants for that k, or explicitly track the k-dependence through Eqs. (58)-(61). As written, the limiting argument leading to Eq. (61) is not justified.
- [Section 2.2, Lemma 2.3 and Eq. (10)] The proof of the derivative expansion in Lemma 2.2, which is essential for Theorem 1.4, relies on Lemma 2.3. The proof of Lemma 2.3 is omitted, with only the remark that it 'follows mutatis mutandis from [1, Lemma 4.10]'. This is load-bearing: the conclusions c_1 = 0 and c_2 > 0 in the proof of Theorem 1.4 depend on the λ,ε-independence of the functions f_{I,J} and g_{I,J}. Moreover, the displayed formula (10) contains exponents such as 1-k and k-|I|-|J|-1 that appear to be typos, and the Taylor expansion in that line is hard to verify as written. The authors should either supply the proof of Lemma 2.3 or state precisely which statements in [1] cover the present setting.
minor comments (5)
- [Section 2.2, Proof of Theorem 1.4] The integral of the derivative is computed as c_1 e^{-s} + 2c_2 e^{-2s} + O(e^{-3s}), but ∫_s^{2s} v'(t) dt = c_1 e^{-s} + (c_2/2 - c_1) e^{-2s} + O(e^{-3s}). Since c_1 = 0 on the critical curve, the monotonicity conclusion is unaffected, but the displayed coefficient should be corrected.
- [Lemma 3.8] The integrals in the first proof are written over (-∞, ∞), whereas the exponential and Gamma densities vanish on the negative half-line; the integrals should be over [0, ∞).
- [Lemma 4.4] In the proof, the display 'E[X_{θ_n}]' should refer to the TARWDP Y, not X; the notation should be made consistent with the statement of the lemma.
- [Section 5.1] The sentence describing the rates of P^f, P^b, and P^o swaps the labels for backward and orthogonal jumps: the f-rate is e^λ Z_λ^{-1}, the o-rate is (2d-2)Z_λ^{-1}, and the b-rate is e^{-λ}Z_λ^{-1}.
- [Eq. (10)] The exponent '1-k' in the factor (1 - e^{λ+ε}Z_{λ+ε}^{-1} + e^λ Z_λ^{-1})^{1-k} seems suspect and may be a typo; the formula should be checked carefully, as the surrounding argument depends on the polynomial growth in k of the coefficients.
Circularity Check
No circularity: the speed expansion and monotonicity conclusion are derived from an independently constructed invariant measure, explicit couplings, and external published estimates; the noted sign and omitted-proof issues are correctness risks, not circular reductions.
full rationale
The derivation chain is not circular. The speed expansion in Theorems 1.2 and 1.3 is computed from explicit Poisson/update clocks, stationary probabilities of the environment process Q, Wald identities, and regeneration times; all constants are computed or shown positive, and none is fitted to the target monotonicity statement. Theorem 1.4 follows from the structural derivative expansion in Lemma 2.2 combined with the positive e^{-2\lambda} coefficient in Theorem 1.3, which is a legitimate comparison of two separately proved expansions rather than an assumption of the conclusion. The paper does rely on [1], including coauthor overlap, for the regeneration framework, differentiability, and some technical estimates, but this is reliance on a peer-reviewed external paper; Lemma 2.3 is explicitly promised to follow mutatis mutandis from [1, Lemma 4.10], so the citation carries independent support. The genuine weaknesses are correctness risks, not circularity: Lemma 5.3 omits its proof, Lemma 5.6 contains an apparent sign conflict (statement writes E[X_{\tau_k}-\tilde Y_{\tau_k}|E^{\uparrow\downarrow}]-C^{\uparrow\downarrow}, while the proof derives E[X_{T_2}-\tilde Y_{T_2}]=-C^{\uparrow\downarrow}+O(e^{-\lambda}), and Eqs. (53) and (56) use opposite signs), and the positivity of C_{1b\cup 2o} is asserted rather than proved. These affect the security of Theorem 1.3's conclusion but do not make the argument equivalent to its inputs. No fitted parameter is relabeled as a prediction, and no definition smuggles in the target result. Hence the circularity score is 0.
Assumptions & free parameters
assumptions (4)
- standard math Regeneration times (τ_n) have i.i.d. increments, exponential tails, and E[τ_1]=e^{1/μ}, and the speed satisfies v(λ)=E[X_{τ_1}]/E[τ_1].
- ad hoc to paper Lemma 2.3: there exist functions f_{I,J}, g_{I,J} independent of λ and ε giving conditional first-coordinate displacements; the proof is omitted.
- ad hoc to paper There exist positive constants C_{1b}, C_{2o\↑↓}, C_{↑↓} with the properties stated in Lemma 5.3, Proposition 5.4(3), Lemma 5.5(2), and Lemma 5.6; their proofs are sketched or left to the reader.
- domain assumption The stationary measure Q of the TARWDP environment process is extremal invariant and π_p S(t) converges weakly to Q.
Cite this review
Pith. "Pith review of Biased random walk on the critical curve of dynamical percolation." pith.science (2026). https://pith.science/paper/UPLFZZAT
@misc{pith2026250208568,
author = {Pith},
title = {Pith review of: Biased random walk on the critical curve of dynamical percolation},
year = {2026},
howpublished = {\url{https://pith.science/paper/UPLFZZAT}},
note = {Machine review of arXiv:2502.08568}
}
abstract
We study biased random walks on dynamical percolation in $\mathbb{Z}^d$, which were recently introduced by Andres et al. We provide a second order expansion for the asymptotic speed and show for $d \ge 2$ that the speed of the biased random walk on the critical curve is eventually monotone increasing. Our methods are based on studying the environment seen from the walker as well as a combination of ergodicity and several couplings arguments.
Figures
Forward citations
Cited by 1 Pith paper
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Mixing times of spin systems on dynamical percolation
For p below the critical percolation probability and sufficiently small λ, the mixing time of nearest-neighbor Glauber dynamics on dynamical percolation is Θ(log N / λ) on the d-dimensional torus.
Reference graph
Works this paper leans on
-
[1]
Biased random walk on dynamical percolation
Sebastian Andres, Nina Gantert, Dominik Schmid, and Perla Sousi. Biased random walk on dynamical percolation. The Annals of Probability, 52(6):2051–2078, 2024
work page 2024
-
[2]
Biased random walks on random graphs.Proba- bility and statistical physics in St
Gerard Ben Arous and Alexander Fribergh. Biased random walks on random graphs.Proba- bility and statistical physics in St. Petersburg, 91:99–153, 2016
work page 2016
-
[3]
Random walk in dynamic Markovian random en- vironment
Antar Bandyopadhyay and Ofer Zeitouni. Random walk in dynamic Markovian random en- vironment. ALEA Lat. Am. J. Probab. Math. Stat., 1:205–224, 2006
work page 2006
-
[4]
Mustansir Barma and Deepak Dhar. Directed diffusion in a percolation network.Journal of Physics C: Solid State Physics, 16(8):1451, 1983
work page 1983
-
[5]
The speed of biased random walk on percolation clusters
Noam Berger, Nina Gantert, and Yuval Peres. The speed of biased random walk on percolation clusters. Probability theory and related fields, 126(2):221–242, 2003
work page 2003
-
[6]
Adam M Bowditch and David A Croydon. Biased random walk on supercritical percolation: anomalous fluctuations in the ballistic regime.Electronic Journal of Probability, 27:1–22, 2022. 29
work page 2022
-
[7]
Limit theorems for the tagged particle in exclusion processes on regular trees
Dayue Chen, Peng Chen, Nina Gantert, and Dominik Schmid. Limit theorems for the tagged particle in exclusion processes on regular trees. Electronic Communications in Probability, 24(2):1–10, 2019
work page 2019
-
[8]
Volume I: Elementary theory and methods
Daryl John Daley and David Vere-Jones.An introduction to the theory of point processes. Volume I: Elementary theory and methods. Springer, 2003
work page 2003
Show all 26 references
-
[9]
Phase transition for the speed of the biased ran- dom walk on the supercritical percolation cluster
Alexander Fribergh and Alan Hammond. Phase transition for the speed of the biased ran- dom walk on the supercritical percolation cluster. Communications on Pure and Applied Mathematics, 67(2):173–245, 2014
2014
-
[10]
The speed of the tagged particle in the exclusion process on Galton–Watson trees.Electronic Journal of Probability, 25(71):1–27, 2020
Nina Gantert and Dominik Schmid. The speed of the tagged particle in the exclusion process on Galton–Watson trees.Electronic Journal of Probability, 25(71):1–27, 2020
2020
-
[11]
Random walk on dynamical percolation in Euclidean lattices: separating critical and supercritical regimes
Chenlin Gu, Jianping Jiang, Yuval Peres, Zhan Shi, Hao Wu, and Fan Yang. Random walk on dynamical percolation in Euclidean lattices: separating critical and supercritical regimes. arXiv preprint arXiv:2407.15162, 2024
2024 arXiv
-
[12]
Speed of random walk on dynamical percolation in nonamenable transitive graphs
Chenlin Gu, Jianping Jiang, Yuval Peres, Zhan Shi, Hao Wu, and Fan Yang. Speed of random walk on dynamical percolation in nonamenable transitive graphs. arXiv preprint arXiv:2407.15079, 2024
2024 arXiv
-
[13]
Dynamical percolation.Annales de l’Institut Henri Poincare (B) Probability and Statistics, 33(4):497–528, 1997
Olle Häggström, Yuval Peres, and Jeffrey Steif. Dynamical percolation.Annales de l’Institut Henri Poincare (B) Probability and Statistics, 33(4):497–528, 1997
1997
-
[14]
A comparison principle for random walk on dynamical percolation
Jonathan Hermon and Perla Sousi. A comparison principle for random walk on dynamical percolation. The Annals of Probability, 48(6):2952–2987, 2020
2020
-
[15]
A limit law for random walk in a random environment
Harry Kesten, Mykyta Kozlov, and Frank Spitzer. A limit law for random walk in a random environment. Compositio mathematica, 30(2):145–168, 1975
1975
-
[16]
David Levin, Yuval Peres, and Elisabeth Wilmer.Markov chains and mixing times, volume
-
[17]
Interacting particle systems, volume 2
Thomas Liggett. Interacting particle systems, volume 2. Springer, 1985
1985
-
[18]
Stochastic interacting systems: contact, voter and exclusion processes, vol- ume 324
Thomas Liggett. Stochastic interacting systems: contact, voter and exclusion processes, vol- ume 324. Springer Science&Business Media, 1999
1999
-
[19]
Ergodic theory on Galton—Watson trees: speed of random walk and dimension of harmonic measure.Ergodic Theory and Dynamical Systems, 15(3):593–619, 1995
Russell Lyons, Robin Pemantle, and Yuval Peres. Ergodic theory on Galton—Watson trees: speed of random walk and dimension of harmonic measure.Ergodic Theory and Dynamical Systems, 15(3):593–619, 1995
1995
-
[20]
Biased random walks on Galton–Watson trees
Russell Lyons, Robin Pemantle, and Yuval Peres. Biased random walks on Galton–Watson trees. Probability theory and related fields, 106:249–264, 1996
1996
-
[21]
Quenched exit times for random walk on dynamical percolation
Yuval Peres, Perla Sousi, and Jeffrey Steif. Quenched exit times for random walk on dynamical percolation. Markov processes and related fields, 24:715–731, 2018
2018
-
[22]
Mixing time for random walk on supercritical dynamical percolation
Yuval Peres, Perla Sousi, and Jeffrey Steif. Mixing time for random walk on supercritical dynamical percolation. Probability theory and related fields, 176(3):809–849, 2020
2020
-
[23]
Random walks on dynamical percolation: mixing times, mean squared displacement and hitting times.Probability Theory and Related Fields, 162(3):487–530, 2015
Yuval Peres, Alexandre Stauffer, and Jeffrey Steif. Random walks on dynamical percolation: mixing times, mean squared displacement and hitting times.Probability Theory and Related Fields, 162(3):487–530, 2015
2015
-
[24]
Cutoff for random walk on dynamical Erdős–Rényi graph
Perla Sousi and Sam Thomas. Cutoff for random walk on dynamical Erdős–Rényi graph. Annales de l’Institut Henri Poincaré-Probabilités et Statistiques, 56(4):2745–2773, 2020
2020
-
[25]
On the anisotropic walk on the supercritical percolation cluster.Com- munications in mathematical physics, 240:123–148, 2003
Alain-Sol Sznitman. On the anisotropic walk on the supercritical percolation cluster.Com- munications in mathematical physics, 240:123–148, 2003. 30
2003
-
[107]
American Mathematical Soc., 2017
2017
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