REVIEW 1 major objections 4 minor 26 references
Finding Planted Cycles in a Random Graph
T0 review · 1 major / 4 minor · reviewed 2026-08-03 · deepseek-v4-flash
Pith's one-line read In the planted cycles model, almost exact recovery succeeds exactly below a sharp threshold—and a polynomial-time algorithm attains it; above the threshold, recovery is impossible.
desk verdict Sharp threshold result is likely correct, but the polynomial-time claim has a missing upper bound on |H| and needs a patch. 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 difference graph between the planted 2-factor and any competing 2-factor decomposes into alternating (a,b)-circuits via a black-box theorem on alternating Eulerian circuits. The generating function g(x,y) = Σ_{k≥1} (2x/(1−x) · δλy/(1−(1−δ)λy))^k counts the expected number of such trails; convergence of this series on the curve xy=1 with 0<x<1<y decides the threshold. For the impossibility direction, a branching process built from (m*,m*)-paths with mean offspring greater than 1 constructs exponentially many balanced cycles, which are then stitched into many competing 2-factors.
What would settle it
Simulate the model at δ=2/3 with λ slightly below and above 1/3, and compute the minimum achievable symmetric difference over all 2-factors (or run the paper's algorithm). The claim predicts an abrupt transition from error o(1) to error Ω(1) exactly at λ=1/3, with polynomial-time success below the threshold. A direct count of balanced (a,a)-circuits in the observed graph should show bounded expectation below the threshold and divergence above it.
Extended reading notes
Core claim
The central discovery is a sharp phase transition for almost exact recovery of a planted 2-factor in an Erdős–Rényi background graph. The threshold is derived from a generating function that counts alternating trails between planted and unplanted edges in the difference graph H*ΔH; convergence of this generating function is equivalent to λ < λ* and guarantees few balanced circuits, while divergence forces exponentially many balanced long cycles that make recovery impossible. The same threshold is achieved by a polynomial-time algorithm, so there is no statistical–computational gap.
Load-bearing premise
The entire proof rests on a black-box theorem asserting that every connected component of the difference graph H*ΔH admits an Eulerian circuit whose edge colors alternate at every shared vertex; if that decomposition fails for the degree-2 and degree-4 colored vertices that arise, the sharp-threshold derivation loses its foundation.
Editorial extensions
If this is right
- The sharp threshold holds conditionally on the planted 2-factor being a single Hamiltonian cycle, so the result covers the single-cycle version of the problem.
- There is no statistical–computational gap: the information-theoretic threshold and the polynomial-time threshold coincide exactly.
- Exact recovery of the planted cycles is impossible for any constant λ, so almost exact recovery is the correct target.
- The threshold is non-monotonic in δ, maximized as δ→0 and minimized at δ=2/3, and is always below the threshold one would get if the planted vertex set were known.
- The greedy trail-XOR algorithm runs in time O(n^{3+log(2+λ)}) with high probability.
- The generating-function method provides a unified way to count both balanced and imbalanced circuits, going beyond the simpler alternating-circuit analysis that suffices when δ=1.
Reading between the lines
- If the threshold formula is correct, the critical behavior just below λ* deserves attention: the generating function suggests that the expected number of balanced circuits diverges critically near the threshold, hinting at an infinite-order phase transition analogous to the planted matching problem.
- The machinery likely extends to planted k-factors for k≥3, where the transition may be driven by the appearance of competing k-factors rather than 2-factors; the k=1 planted partial matching case is known to have no sharp transition.
- The comparison with the known-vertex-set threshold suggests that the main statistical difficulty comes from discovering the support of the planted cycles; isolating this effect by conditioning on the vertex set would sharpen the intuition about why the unknown-support problem is harder.
- A natural testable extension is the partial-recovery regime above λ*: the paper leaves open whether a 'something' phase exists for δ<1, since for δ=1 partial recovery is trivially achievable.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the planted cycles model G(n, λ, δ), where a uniformly random set of δn vertices carries a uniformly random 2-factor H*, and all remaining pairs are present independently with probability λ/n. The main result, Theorem 1.1, asserts a sharp almost-exact-recovery threshold at λ_c = (√(2δ)+√(1−δ))^{−2}: below λ_c, almost exact recovery is possible and is achieved by a polynomial-time algorithm; above λ_c, no estimator can do better than constant error. The proof has three parts: a generating-function count of alternating (a,b)-trails to prove achievability (§2); a branching-process and sprinkling construction of many balanced cycles to prove impossibility (§3); and a greedy algorithm based on short trails to prove the algorithmic claim (§4).
Significance. If the results are correct, this is a substantial contribution: a sharp information-theoretic threshold for recovery of a planted 2-factor with unknown support, and—strikingly—no statistical–computational gap, in contrast to the planted clique problem. The generating-function technique for counting alternating trails is elegant and self-contained, and the impossibility argument via path trees and sprinkling is technically sophisticated. The algorithmic section is also ambitious: it gives a concrete polynomial-time procedure with a detailed structural analysis. The main theorem, however, is not yet fully established because of a gap in the proof of the algorithmic half.
major comments (1)
- [Theorem 4.1 / Lemmas 4.3–4.4] Lemma 4.4 only proves a lower bound on the output size, |H| ≥ δn − O(n/√log n), together with a bound on the number of degree-1 vertices. Lemma 4.3, which is the statistical engine for the efficient estimator, requires the stronger assumption E[|H|] = δn − o(n). No matching upper bound on |H| is supplied. The algorithm's updates search over all short trails in G, not only over trails contained in HΔH*, so after H reaches the desired size it can in principle absorb unplanted paths: a cost-free update can add an unplanted path joining two degree-1 vertices of H, increasing |H| without increasing the number of degree-1 vertices, and a cost-effective update can add a new unplanted path of length at least √log n. The degree-1 budget in Lemma 4.4 does not rule out Ω(n) overshoot, because each cost-effective update can add up to log n edges and there are up to n/√log n such updates. Thus the pr
minor comments (4)
- [Theorem B.1] The parameter order is inconsistent: the statement reads G∼G(n, δ, λ), while the model in Definition 1.1 is G(n, λ, δ).
- [Lemma 3.9] In the proof of Lemma 3.9, the text refers to “Proposition 3.8” when the statement cited is Lemma 3.8.
- [Algorithm 1] The pruning condition says “reachable by a path of length at most 2m*”, while the path layers are non-shortcutted (m*,m*)-paths of length exactly 2m*. Please clarify whether the pruning radius is intentionally larger than the path length, and reconcile with the domination argument in Lemma 3.8.
- [Definition 1.1] “choose a set of δn2 vertices” appears to be a typo for “δn vertices”; the surrounding text and the exponent in the abstract use δn.
Circularity Check
No significant circularity: the sharp threshold is derived from the planted model's generating function, and the self-citations are not load-bearing.
full rationale
The central claim (Theorem 1.1) is not circular. The threshold is obtained by solving the convergence/divergence condition of the generating function g(x,y) in (2.4); the coefficients c_{a,b} are model-derived expected trail counts from Lemma 2.2, not fit parameters. Achievability (Theorem 2.4) bounds the expected excess directly from this generating function, and impossibility (Lemma 3.4) proves c_{m*,m*}>1 from the same divergence, then builds supercritical trees with mean μ=(c_{m*,m*}+1)/2; no quantity is trained on the target. The paper cites its own prior work [13] for the δ=1 special case and technical analogies (e.g., 'similar in spirit to the proof of [13, Theorem 2.3]' and Eq. (3.4)), and [9, Lemma 7] is used as a black-box, parameter-free sprinkling lemma whose assumptions do not include the target recovery threshold; these are independent evidence, not circular inputs. The algorithmic proof has a separate gap that is not circularity: Lemma 4.4 proves only a lower bound |H| ≥ δn−O(n/√logn) and a degree-1 bound, while Lemma 4.3 needs E[|H|] = δn−o(n); no matching upper bound is supplied. This is a correctness concern, not a reduction of a prediction to its input.
Assumptions & free parameters
assumptions (3)
- standard math Kotzig's theorem [16, Thm. 1]: any connected even graph with a partition of incident edges satisfying |Q_{v,i}| ≤ deg(v)/2 admits an Eulerian circuit whose consecutive edges have different labels at every vertex.
- standard math Lemma 7 of [9] (Ding–Wu–Xu–Yang): a bi-colored bipartite graph on [k]×[k]' with a perfect red matching and independent blue edges of probability D/k (D ≥ 256 log(32e)) contains exp(k/20) alternating cycles of length ≥3k/4 with probability 1−exp(−Dk/214).
- domain assumption Generative model: background G0 ~ G(n,λ/n) independent edges; uniform δn-subset V; H* a uniform labeled 2-factor on V; λ,δ constants as n→∞.
Cite this review
Pith. "Pith review of Finding Planted Cycles in a Random Graph." pith.science (2026). https://pith.science/paper/GZWQZES4
@misc{pith2026251104058,
author = {Pith},
title = {Pith review of: Finding Planted Cycles in a Random Graph},
year = {2026},
howpublished = {\url{https://pith.science/paper/GZWQZES4}},
note = {Machine review of arXiv:2511.04058}
}
abstract
In this paper, we study the problem of finding a collection of planted cycles in an \ER random graph $G \sim \mathcal{G}(n, \lambda/n)$, in analogy to the famous Planted Clique Problem. When the cycles are planted on a uniformly random subset of $\delta n$ vertices, we show that almost-exact recovery (that is, recovering all but a vanishing fraction of planted-cycle edges as $n \to \infty$) is information-theoretically possible if $\lambda < \frac{1}{(\sqrt{2 \delta} + \sqrt{1-\delta})^2}$ and impossible if $\lambda > \frac{1}{(\sqrt{2 \delta} + \sqrt{1-\delta})^2}$. Moreover, despite the worst-case computational hardness of finding long cycles, we design a polynomial-time algorithm that attains almost exact recovery when $\lambda < \frac{1}{(\sqrt{2 \delta} + \sqrt{1-\delta})^2}$. This stands in stark contrast to the Planted Clique Problem, where a significant computational-statistical gap is widely conjectured.
Figures
Reference graph
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Reviewed August 3, 2026 · model on record in the stance chip above.
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