REVIEW 3 major objections 6 minor 46 references
Strong Detection Threshold for Correlated Erd\H{o}s-R\'enyi Graphs with Constant Average Degree
T0 review · 3 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read This paper proves that strong detection between two graphs subsampled from a common parent and two independent graphs with the same marginals is information-theoretically possible exactly when the subsampling probability exceeds…
desk verdict A sharp and essentially correct resolution of the strong detection threshold for sparse correlated Erdős-Rényi graphs, held back only by two typos that must be fixed. 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 proof is carried by the permutation-averaged likelihood ratio $L(A,B)=\frac{1}{n!}\sum_{\pi}\prod_e \ell(A_e,B_{\pi(e)})$, where $\ell$ is the per-edge likelihood ratio. Fixing the latent alignment $\pi^*$ and writing $\sigma=(\pi^*)^{-1}\circ\pi$, the product splits over the edge orbits of $\sigma$; the short orbits, of length one, require a sharper estimate than previous work, which removes the earlier constant slack. On the event that the intersection graph is acyclic, the truncated second moment of the likelihood ratio collapses to a weighted forest count: each forest $J$ contributes $s^{2|E(J)|}$. Otter's constant $\alpha\approx 0.338$ is the radius of convergence of the unlabeled-tree generating function, and the classical asymptotic $f_k=O(\alpha^{-k}/k^{1.5})$ makes the forest series converge precisely for $s^2<\alpha$; the $1/\sqrt{\lambda}$ term in the threshold comes from requiring $\lambda s^2<1$ so the intersection graph is subcritical and the acyclic event has positive probability.
What would settle it
For a fixed constant $\lambda$ such as $\lambda=2$, simulate graph pairs at $s$ just below and just above $\sqrt{\alpha}\approx 0.581$ and estimate the total variation distance; the theorem predicts $\mathrm{TV}(P,Q)$ stays bounded away from $1$ below the boundary and tends to $1$ above it. A more direct check is to estimate the truncated second moment $\mathbb{E}_P[(P/Q)\mathbf{1}_A]$: the proof requires it to stay $O(1)$ for $s<\min\{1/\sqrt{\lambda},\sqrt{\alpha}\}$ and to diverge above, so a single parameter value that violates this would refute the claimed threshold.
Extended reading notes
Core claim
The central discovery is a sharp phase transition for the total variation distance between the correlated model $P$ and the independent model $Q$ with the same marginals. For any constant $\epsilon>0$, if $s<\min\{1/\sqrt{\lambda},\sqrt{\alpha}\}-\epsilon$ then $\mathrm{TV}(P,Q)=1-\Omega(1)$, so no test can strongly distinguish the two distributions; if $s>\min\{1/\sqrt{\lambda},\sqrt{\alpha}\}+\epsilon$ then $\mathrm{TV}(P,Q)=1-o(1)$, so strong detection is possible. The upper direction is assembled from existing statistics, densest subgraphs when $\lambda s^2>1$ and tree counting when $s^2>\alpha$. The lower direction is new: conditioning on the event $A$ that the intersection graph, the edges present in both copies under the latent alignment, is a forest, which occurs with probability bounded below by a constant, the proof shows $\mathbb{E}_P[(P/Q)\mathbf{1}_A]=O(1)$, which forces $\mathrm{TV}(P,Q)=1-\Omega(1)$. The calculation reduces to a weighted sum over forests $J$ of $s^{2|E(J)|}$, and Otter's asymptotic $f_k=O(\alpha^{-k}/k^{1.5})$ for unlabeled trees makes this series converge exactly when $s^2<\alpha$; the condition $s<1/\sqrt{\lambda}$ enters through the positivity of the acyclic event.
Load-bearing premise
The proof imports the classical asymptotic $f_k=O(\alpha^{-k}/k^{1.5})$ for the number of unlabeled trees, and the entire lower bound rests on the forest generating function converging at the radius this asymptotic dictates; if that asymptotic were inaccurate, the threshold $\min\{1/\sqrt{\lambda},\sqrt{\alpha}\}$ would shift.
Editorial extensions
If this is right
- The constant-factor gap between the previous best upper and lower bounds is closed: the sharp threshold is the minimum of $1/\sqrt{\lambda}$ and $\sqrt{\alpha}$.
- For $\lambda>1/\alpha\approx 2.96$, the detection threshold $1/\sqrt{\lambda}$ is smaller than $\sqrt{\alpha}$, so strong detection becomes possible exactly when partial recovery of the latent matching becomes possible.
- For $\lambda<1/\alpha$, detection is possible once $s>\sqrt{\alpha}$ while recovery of the matching remains impossible until $s>1/\sqrt{\lambda}$, leaving a constant-factor detection-recovery gap.
- Above the threshold, known efficient algorithms succeed (densest subgraphs when $\lambda s^2>1$, tree counting when $s^2>\alpha$); below it, no algorithm, efficient or not, can strongly detect the correlation.
- In the regime $n^{-1+o(1)}\le \lambda=o(1)$, the threshold reduces to $\sqrt{\alpha}$, matching the tree-counting upper bound.
Reading between the lines
- The conditional-second-moment technique, which works on a positive-probability acyclic event rather than a high-probability one, is likely transferable to other alignment problems on sparse random graphs, such as partially correlated Erdős-Rényi graphs where only constant-factor bounds are currently known.
- The appearance of Otter's constant in the statistical limit for all $\lambda<1/\alpha$ suggests that tree counting is not just an algorithmic heuristic but captures the fundamental information-theoretic boundary whenever the intersection graph is subcritical; a testable extension is whether decorated-tree statistics can close the remaining gap for $\lambda>1/\alpha$.
- Because the paper shows the impossibility region extends to $s<\sqrt{\alpha}$ for $\lambda<1/\alpha$, it implies that any future robust detector that survives $o(n)$ edge corruptions would have to beat the tree-counting statistic in the window $1/\sqrt{\lambda}<s<\sqrt{\alpha}$, or the robust and non-robust thresholds must separate there.
- The same min-formula might generalize to other correlated latent-permutation models with binomial edges, such as sparse correlated stochastic block models, where the analogous threshold would be determined by the subcriticality of the shared-edge graph and the tree-counting radius.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the information-theoretic detection problem between a pair of correlated Erdős-Rényi graphs formed by subsampling a common parent G(n, λ/n) with probability s, and a pair of independent Erdős-Rényi graphs with matching marginals. The main result, Theorem 1.1, asserts a sharp threshold: strong detection is possible if and only if s > min{1/√λ, √α}, where α ≈ 0.338 is Otter's constant. The paper proves the lower bound (impossibility below threshold) by a conditional second-moment calculation: it conditions on the acyclic event A in the intersection graph, decomposes the likelihood ratio over permutation orbits, and reduces the second moment to a forest-counting sum that converges below the Otter threshold. The upper bound (achievability) is not proved in the paper but is cited to [11] and [31].
Significance. If the proof is repaired, the result closes a constant gap in the detection threshold for constant-degree correlated Erdős-Rényi graphs, improving the previous lower bound of [44] (s < min{1/√λ, 0.1}) to the conjectured sharp value. The lower-bound argument is self-contained and parameter-free, using FKG, Cauchy-Schwarz, and Otter's tree enumeration, and it exposes the detection-recovery and information-computation gaps discussed in Section 1.1. However, as submitted the proof contains a load-bearing typo in the running assumption and a few incorrect intermediate estimates, so the theorem is not yet verified as written.
major comments (3)
- [§2, Eq. (2.1)] The proof assumes λ²s < 1−ε and s < √α − ε, but the lower-bound hypothesis of Theorem 1.1 is s < min{1/√λ, √α} − ε. For λ > 1 these conditions are not equivalent: taking λ = 2 and s = 0.5 gives s < √α and s < 1/√λ, yet λ²s = 2. The only smallness condition actually needed by the proof is λs² < 1−ε, which appears in Lemma 2.1 (the event A requires cycle probability λs²/n), in Lemma 2.4, and in the short-orbit factor of (2.16)–(2.17). As printed, the proof leaves the interval s ∈ (1/λ², min{1/√λ, √α}) uncovered for every λ > 1, so the claimed impossibility range is unproved. The assumption should be corrected to λs² < 1−ε, and the proof re-verified under that condition.
- [§2, Lemma 2.3, Eq. (2.11)] The bound for |O| ≥ 2 is stated as 1 + O(n^{-2}). Expanding (2.10) to first order gives 1 − 2|O| s^{2|O|} λ(1−s)/n + O(n^{-2}), which is strictly smaller than 1 for large n, not 1 + O(n^{-2}). The displayed inequality is therefore false as written. Since the exact formula (2.10) actually implies the factor is < 1 for all |O| ≥ 2, the subsequent step can be repaired by bounding the product over non-full long orbits by 1, but the current text needs correction.
- [§2, Eqs. (2.16)–(2.17)] The passage from the conditional expectation to (2.16) is not valid as written. For a permutation σ with many fixed edges (e.g., σ = identity), |O1(J)| can be Θ(n²), and the product ∏_{O1}(1 + λs²(2s−1)/n) in (2.16) can be exponential in n when s > 1/2; this cannot be absorbed into the leading O(1) factor. The proof should multiply the O1 factor by the corresponding (1 − λs²/n)^{|O1|} from Lemma 2.4 before discarding it; the combined per-orbit factor is 1 − 2λs²(1−s)/n + O(n^{-2}) ≤ 1, so the final bound (2.17) is still within reach. As written, however, the intermediate inequality (2.16) does not follow from (2.15) and Lemma 2.4.
minor comments (6)
- [Abstract] The null model is described as two independent Erdős-Rényi graphs G(n, λ/n), but the body correctly defines Q as two independent G(n, λs/n) graphs; the abstract should be corrected.
- [Introduction, first paragraph] The parameters are introduced as λ, s ∈ (0, 1), but Theorem 1.1 allows λ > 1; the notation should read λ > 0, s ∈ (0, 1).
- [§2, just before Eq. (2.13)] The phrase "in the second inequality" should be "in the second equality", since the passage is an equality.
- [Lemma 2.5] The indexing in the list of unlabeled trees is written as "1 ≤ i ≤ k"; it should be "1 ≤ i ≤ f_k".
- [Lemma 2.5] Otter's asymptotic for unlabeled free trees is usually quoted with exponent k^{-5/2}; the displayed k^{-3/2} is harmless for the convergence argument, but the reference should be checked for accuracy.
- [Throughout] "Cauchy-Schwartz" should be "Cauchy-Schwarz".
Circularity Check
No circularity found: the lower-bound proof is self-contained and the upper bound is cited from independent prior work.
full rationale
The paper's central lower-bound result is proved by a conditional second-moment calculation with no fitted parameters and no quantity defined in terms of the target threshold. The only external input in the lower bound is Otter's classical asymptotic f_k = O(alpha^{-k}/k^{1.5}) for unlabeled trees, cited as [36]; this is an independent classical result, not derived from the theorem and not authored by the present author. The upper bound is explicitly assembled in Remark 1.2 from [11] and [31], which are works by other author groups, so there is no self-citation load-bearing chain. The apparent mismatch between assumption (2.1), which states lambda^2 s < 1 - epsilon, and the theorem's lower regime s < min{1/sqrt(lambda), sqrt(alpha)} is a correctness or proof-gap concern rather than circularity: even if the printed assumption is stronger than the theorem's hypothesis, the proof does not reduce any output to an input by construction. No fitted parameter is renamed as a prediction, and no self-cited uniqueness theorem is used to force the form of the threshold. Therefore the derivation chain is not circular.
Assumptions & free parameters
assumptions (3)
- standard math Otter's asymptotic formula for the number of unlabeled trees: f_k = Θ(α^{-k}/k^{1.5})
- standard math FKG inequality for decreasing events on a product probability space
- domain assumption The correlated Erdős-Rényi model with uniform random permutation π*
Cite this review
Pith. "Pith review of Strong Detection Threshold for Correlated Erd\H{o}s-R\'enyi Graphs with Constant Average Degree." pith.science (2026). https://pith.science/paper/3JXTBXWU
@misc{pith2026250612752,
author = {Pith},
title = {Pith review of: Strong Detection Threshold for Correlated Erd\Hos-R\'enyi Graphs with Constant Average Degree},
year = {2026},
howpublished = {\url{https://pith.science/paper/3JXTBXWU}},
note = {Machine review of arXiv:2506.12752}
}
abstract
Consider a pair of correlated Erd\H{o}s-R\'enyi graphs $\mathcal G(n,\tfrac{\lambda}{n};s)$ that are subsampled from a common parent Erd\H{o}s-R\'enyi graph with average degree $\lambda$ and subsampling probability $s$. We establish a sharp information-theoretic threshold for the detection problem between this model and two independent Erd\H{o}s-R\'enyi graphs $\mathcal G(n,\tfrac{\lambda}{n})$, showing that strong detection is information-theoretically possible if and only if $s>\min\{ \tfrac{1}{\sqrt{\lambda}}, \sqrt{\alpha} \}$ where $\alpha\approx 0.338$ is the Otter's constant. Our result resolves a constant gap between arXiv:2203.14573 and arXiv:2008.10097.
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