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P\'osa rotation through a random permutation

T0 review · 1 major / 3 minor · reviewed 2026-08-09 · deepseek-v4-flash

Pith's one-line read Adding a random 2-factor makes any sufficiently large graph Hamiltonian once the minimum degree is at least $(1+\varepsilon)\sqrt{n \log n/2}$, and this value is asymptotically best possible.

desk verdict Resolves the asymptotic threshold for Hamiltonicity with a random 2-factor, but the Phase 2 uniform-order claim has a real gap that needs fixing. read the letter →

arxiv 2502.00489 v1 pith:BNN66V65 submitted 2025-02-01 math.CO

classification math.CO MSC 05C4505C80
keywords Hamiltoncyclesrandom2-factorrandomlyperturbedgraphsminimumdegreethresholdPósarotationpermutationsconcentrationinequalitiesnearlyregular
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper asks how large the minimum degree of a graph $G$ must be before adding a random 2-factor $F$ on the same $n$ vertices almost surely creates a Hamiltonian cycle, a cycle that visits every vertex exactly once. It proves the asymptotic threshold is $(1+o(1))\sqrt{n \log n / 2}$: every graph with minimum degree at least $(1+\varepsilon)\sqrt{n \log n/2}$ becomes Hamiltonian with high probability after adding $F$, while some graph just below that degree fails with high probability. This settles the gap between the earlier lower and upper bounds, $\tfrac{1}{5} \log n$ and $n^{3/4+o(1)}$. If $G$ is nearly regular, the paper shows that a minimum degree of only $\omega(\log^3 n)$ already suffices. The proofs develop a randomised version of classical Pósa rotation adapted to progressive exposure of the random permutation that the 2-factor represents.

What carries the argument

The engine is a randomised form of Pósa rotation, the classical technique that extends a path by rotating its endpoint along unused edges, run in multiple exposure rounds on the random permutation underlying $F$. A random 2-factor is written as $\pi(F^*)$, with $F^*$ a fixed 2-factor with the exposed cycle lengths and $\pi$ uniform. Long cycles are absorbed into the growing path by exposing a short random segment that with high probability contains a neighbour of the current endpoint in $G$. Short cycles are absorbed through 'i-good' ordered pairs $xy$ on the current path, where $x$ is a neighbour in $G$ of the previous endpoint and $y$ is a neighbour in $G$ of the cycle's attachment vertex. The delicate part is the matching $M$ of such pairs: the proof shows that transposing the endpoints of any good pair does not change the set of good pairs, so conditionally all $2^{|M|}$ orders of the pairs along the path are equally likely; this turns the greedy search for correctly ordered pairs into a binomial process with success probability $1/2$. In the nearly regular case the same exposure scheme is powered by 'special vertex sequences', chains of edges of $G$ that respect the order of random intervals, produced by expansion and concentration lemmas.

What would settle it

Run the two-phase process on a large instance, say $n = 10^5$ with $d = 1.1\sqrt{n \log n / 2}$, and count how often Phase 2 exhausts all available good pairs before every short cycle is absorbed; the proof predicts a failure probability of $o(1)$ from a binomial bound, so any non-vanishing failure rate would contradict the upper bound.

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Extended reading notes

Core claim

The central claim is that the minimum degree needed for random 2-factor completion is asymptotically $\sqrt{n \log n / 2}$. For every $\varepsilon>0$ and all sufficiently large $n$, if $G$ has $n$ vertices and minimum degree at least $(1+\varepsilon)\sqrt{n \log n/2}$, then the union $G \cup F$, with $F$ a uniformly random 2-regular graph on the same vertex set, is with high probability Hamiltonian. Conversely, for every $\varepsilon>0$ there is an $n$-vertex graph with minimum degree $(1-\varepsilon)\sqrt{n \log n/2}$ for which $G \cup F$ is with high probability not Hamiltonian, so the constant $\sqrt{1/2}$ is sharp. Under the additional condition that $G$ is nearly regular, meaning $\Delta(G)=O(\delta(G))$, the paper proves that minimum degree $\omega(\log^3 n)$ suffices. It also shows that for random $C_\ell$-factors with $\ell$ at least a large constant depending on $\alpha$, minimum degree $\alpha n$ suffices for every $\alpha>0$.

Load-bearing premise

The load-bearing premise is that a vertex pair's usefulness as a connection depends only on the two labelled vertices and the set of vertices already exposed, not on their order along the growing path; if path order ever mattered, the equally-likely-order step would fail and the final count would not be a binomial variable.

Editorial extensions

If this is right

  • For every $\varepsilon>0$ and all large $n$, every $n$-vertex graph with minimum degree at least $(1+\varepsilon)\sqrt{n \log n/2}$ becomes Hamiltonian with high probability after adding a random 2-factor.
  • The threshold is asymptotically exact: there are graphs of minimum degree $(1-\varepsilon)\sqrt{n \log n/2}$ whose union with a random 2-factor is with high probability non-Hamiltonian.
  • The same conclusions hold if the random perturbation is the graph of a uniformly random permutation instead of a random 2-factor, because loops and parallel edges cannot help in forming a Hamiltonian cycle.
  • Nearly regular graphs need only a polylogarithmic minimum degree, $\omega(\log^3 n)$, to become Hamiltonian after a random 2-factor.
  • For random $C_\ell$-factors with $\ell$ large, minimum degree $\alpha n$ suffices for every fixed $\alpha>0$, so the required linear density can be made arbitrarily small.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The transposition-uniformity mechanism is a general switching principle: any edge-acceptability predicate that depends only on vertex labels and exposed vertices inherits the same all-orderings-equally-likely property, so the multiple-exposure architecture should transfer to other spanning structures such as powers of cycles or $\ell$-factors.
  • The lower-bound construction suggests the real obstruction is component structure rather than raw edge count: a random 2-factor typically has many short cycles trapped inside the large side of a bipartite split, and the small side cannot connect them. One testable extension is that random $d$-regular perturbations with constant $d>2$ might still have threshold of order $\sqrt{n \log n}$, with a $d
  • A small computational experiment could test the proof's pivotal step directly: simulate the first exposure phase for moderate $n$, tally the conditional distribution of the $2^{|M|}$ orderings of matched good pairs, and check that it is uniform; any measurable bias would locate a flaw in the binomial step.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

1 major / 3 minor

Summary. The paper determines the asymptotic minimum-degree threshold for Hamiltonicity of the union of an n-vertex graph G and a random 2-factor F on the same vertex set: the threshold is (1+o(1))√(n log n / 2). Theorem 1.1 gives a positive result for δ(G) ≥ (1+ε)√(n log n/2), Theorem 1.2 gives a matching lower-bound construction, and Theorem 1.3 shows that for nearly regular G, a polylogarithmic minimum degree suffices. The proofs use the random-permutation representation of F, a two-phase Pósa-rotation argument with multiple exposure, and concentration inequalities including Chernoff, McDiarmid, and Talagrand-type bounds for permutations.

Significance. If correct, this resolves the problem posed by Girão and Espuny Díaz, improving the previous interval [log n / 5, n^{3/4+o(1)}] to the sharp threshold. The lower-bound construction is simple and exact, and the upper bound introduces a multiphase exposure framework that is likely to be useful in other randomly perturbed graph problems. The paper is largely self-contained, proving the needed random-permutation facts and concentration lemmas. The main caveat is a gap in the uniform-order property in Phase 2 of Theorem 1.1, which appears repairable but requires restructuring the proof.

major comments (1)
  1. [Section 4, 'Preparation for Phase 2'] The assertion that for any xy ∈ M, transposing x and y in π does not affect any Mi is not a consequence of the stated definition. If M contains two edges xy and zq at consecutive positions (j,j+1) and (j+2,j+3) of P0, then the four-vertex window surrounding zq is {y,z,q,r}; after transposing x and y it becomes {x,z,q,r}. Because i-goodness depends on the labels of the vertices, the set of i′-good ordered pairs in that window can change, so membership of zq in M can change; the matching property only excludes shared vertices, not edges at distance two. The later whp claim that all Yi = 0 does rule out such adjacent M-edges, since the endpoint z of zq would be an unexposed neighbour of b_{i′−1} or a_{i′} in the window of xy, but this is proved after the invariance assertion is used. To make the binomial model for X in Phase 2 rigorous, the proof should first define and condition on the whp event from the Yi calculation, then prove that M is 2-separated on that event, and only then derive the transposition invariance and the uniform-order property for pairs in M.
minor comments (3)
  1. [Section 4, Phase 1] The displayed estimate '∼ d(√n log n)/n = Ω(log^{3/2} n)' is arithmetically incorrect; the quantity is Θ(log n). The argument still works with a union bound over the O(log n) long cycles, but the exponent should be corrected.
  2. [Section 4, Preparation for Phase 2] In the sentence 'the conditional distribution of π given M1, . . . , Mk', the list should run to M_{k+1}, since the final Hamilton-cycle step is included in the argument.
  3. [Section 4, Preparation for Phase 2] The phrase 'the only ordered pairs that are i′-good for some i′ ∈ [k+1]' is ambiguous: it should be stated explicitly whether the local-uniqueness witness i′ may depend on the edge, and the later argument should use the same index for the adjacent-edge exclusion.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity; the derivation is self-contained and the only self-citation is a non-load-bearing definitional reference.

full rationale

The paper's central theorems are proved from explicit concentration inequalities (Chernoff, McDiarmid/Azuma, Talagrand for permutations) and from lemmas proved internally in Sections 3--5. Theorem 1.1 establishes an asymptotic minimum-degree threshold and proves a whp Hamiltonicity statement without fitting any parameter to the desired conclusion; no 'prediction' is renamed from an input. Theorem 1.2 is a separate lower-bound construction using direct moment estimates, not derived from the claim it tests. The only self-citation is [12], used in Section 5 for the terminology 'special vertices'; the definition is reproduced in the paper and Lemma 5.1 proves the needed existence using the paper's own concentration and expansion lemmas, so this citation is not load-bearing. The 'uniform-order property' in Phase 2 is asserted as a consequence of the constructed matching M and the conditional distribution of the random permutation; even if a correctness objection to that assertion were raised, it is not a circularity, because the assertion is not imported from the theorem being proved or from a fitted input. Overall, the paper is self-contained against external benchmarks and exhibits no definitional circularity, no fitted-input-as-prediction, and no load-bearing self-citation chain.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The proofs rest on standard concentration inequalities and elementary random-permutation facts; no free parameters are fitted and no new entities are postulated. The only overlap with the authors' prior work is the definition of 'special vertices' from [12], but Lemma 5.1 is proved in the paper.

assumptions (4)
  • standard math Chernoff bounds for binomial and hypergeometric random variables
    Invoked throughout Section 3 and the proofs of Theorems 1.1 and 1.3, e.g., in Lemma 3.2 and Lemma 3.4.
  • standard math McDiarmid's bounded differences inequality (Azuma's inequality)
    Used in Lemma 3.4 to prove expansion of the neighbourhood of a random set.
  • standard math Talagrand's concentration inequality for random permutations (McDiarmid's simplified form [30])
    Used in Lemma 3.6 to lower bound the number of good consecutive pairs on the random path.
  • domain assumption Identification of a random 2-factor with a random permutation conditioned on having no cycles of length 1 or 2
    Stated in the introduction and used throughout; it is a standard model fact and is partially justified by Lemma 2.1.

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Cite this review

Pith. "Pith review of P\'osa rotation through a random permutation." pith.science (2026). https://pith.science/paper/BNN66V65

@misc{pith2026250200489,
  author       = {Pith},
  title        = {Pith review of: P\'osa rotation through a random permutation},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/BNN66V65}},
  note         = {Machine review of arXiv:2502.00489}
}
abstract

What minimum degree of a graph $G$ on $n$ vertices guarantees that the union of $G$ and a random $2$-factor (or permutation) is with high probability Hamiltonian? Gir\~ao and Espuny D{\'\i}az showed that the answer lies in the interval $[\tfrac15 \log n, n^{3/4+o(1)}]$. We improve both the upper and lower bounds to resolve this problem asymptotically, showing that the answer is $(1+o(1))\sqrt{n\log n/2}$. Furthermore, if $G$ is assumed to be (nearly) regular then we obtain the much stronger bound that any degree growing at least polylogarithmically in $n$ is sufficient for Hamiltonicity. Our proofs use some insights from the rich theory of random permutations and a randomised version of the classical technique of P\'osa rotation adapted to multiple exposure arguments.

Figures

Figures reproduced from arXiv: 2502.00489 by the authors.

Figure 1
Figure 1. Using a good pair to incorporate a short cycle C in the current path P. Proof of Theorem 1.1. Let G be an n-vertex graph with δ(G) = d = (1 + ε)(p n log n/2), where ε > 0 is small and n > n0(ε) is large. Let F ∼ Gn,2 be a random 2-regular graph on the same vertex set as G. We want to show that whp G ∪ F is Hamiltonian. By Lemma 2.1, whp F has (1 + o(1)) log n cycles, of which ( 1 2 + o(1)) log n are short (of length… view at source ↗
Figure 2
Figure 2. A sequence v1 < . . . < v5 of special vertices. The following lemma will be used to incorporate cycles into the path; the intended use (see [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Incorporating the cycle C2 into the long path on V (C1). The new path starts at v − 2 , following the blue and green paths and then the intervals I2, I3, . . . , Ik. In the next step, v − 2 plays the role of v1 and we find the special vertices in I2. Proof of Theorem 1.3. Let G be an n-vertex graph with δ(G) = ω(log3 n) and ∆(G) = O(δ(G)). Let F ∼ Gn,2 be a random 2-regular graph on V (G). We condition on the length… view at source ↗

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Hamilton cycles in regular graphs perturbed by a random 2-factor

    math.CO 2025-06 conditional novelty 7.0 of 10

    For every integer d ≥ 2, the union of any d-regular graph on n vertices with a uniformly random 2-factor is Hamiltonian with high probability.

Reference graph

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