REVIEW 1 major objections 3 minor 1 cited by
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 →
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 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.
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
- 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.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
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)
- [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)
- [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.
- [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.
- [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
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
assumptions (4)
- standard math Chernoff bounds for binomial and hypergeometric random variables
- standard math McDiarmid's bounded differences inequality (Azuma's inequality)
- standard math Talagrand's concentration inequality for random permutations (McDiarmid's simplified form [30])
- domain assumption Identification of a random 2-factor with a random permutation conditioned on having no cycles of length 1 or 2
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
Forward citations
Cited by 1 Pith paper
-
Hamilton cycles in regular graphs perturbed by a random 2-factor
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
Works this paper leans on
- [1]
- [2]
-
[3]
B. Bollob´ as. The evolution of sparse graphs. In Graph theory and combinatorics (Cambridge, 1983) , pages 35–57, 1984
work page 1983
-
[4]
B. Bollob´ as, T. I. Fenner, and A. M. Frieze. An algorithm for finding Hamilton paths and cycles in random graphs. Combinatorica, 7:327–341, 1987
work page 1987
-
[5]
Cambridge University Press, 2001
B´ ela Bollob´ as.Random graphs (2nd ed.) . Cambridge University Press, 2001
work page 2001
-
[6]
V. Chv´ atal and P. Erd˝ os. A note on Hamiltonian circuits . Discrete Math., 2(2):111–113, 1972
work page 1972
-
[7]
On Hamilton’s ideals.Journal of Combinatorial Theory, Series B , 12(2):163–168, 1972
V´ aclav Chv´ atal. On Hamilton’s ideals.Journal of Combinatorial Theory, Series B , 12(2):163–168, 1972
work page 1972
-
[8]
Random regula r graphs of non-constant degree: con- nectivity and Hamilton cycles
Colin Cooper, Alan Frieze, and Bruce Reed. Random regula r graphs of non-constant degree: con- nectivity and Hamilton cycles. Combinatorics, Probability and Computing , 11(3):249–262, 2002
work page 2002
Show all 35 references
-
[9]
Csaba, D
B. Csaba, D. K¨ uhn, A. Lo, D. Osthus, and A. Treglown. Proo f of the 1-factorization and Hamilton decomposition conjectures. Mem. Amer. Math. Soc. , 244(monograph 1154):164, 2016
2016
-
[10]
Cuckler and J
B. Cuckler and J. Kahn. Hamiltonian cycles in Dirac grap hs. Combinatorica, 29:299–326, 2009
2009
-
[11]
G. A. Dirac. Some theorems on abstract graphs. Proc. Lond. Math. Soc. , 3(1):69–81, 1952
1952
-
[12]
Pancyclicity of Hamiltonian graphs
Nemanja Dragani´ c, David Munh´ a Correia, and Benny Sudakov. Pancyclicity of Hamiltonian graphs. arXiv preprint arXiv:2209.03325 , 2022
2022 arXiv
-
[13]
Hamiltonicity of expanders: optimal bounds and ap plications
Nemanja Dragani´ c, Richard Montgomery, David Munh´ a C orreia, Alexey Pokrovskiy, and Benny Sudakov. Hamiltonicity of expanders: optimal bounds and ap plications. arXiv preprint arXiv:2402.06603, 2024
2024 arXiv
-
[14]
Open problem section
Alberto Espuny D ´ ıaz. Open problem section. Novi Sad FOCS Workshop , 2024
2024
-
[15]
Hamiltonicity of graphs perturbed by a random regular graph
Alberto Espuny D ´ ıaz and Ant´ onio Gir˜ ao. Hamiltonicity of graphs perturbed by a random regular graph. Random Structures & Algorithms , 62(4):857–886, 2023
2023
-
[16]
Ferber, E
A. Ferber, E. Long, and B. Sudakov. Counting Hamilton de compositions of oriented graphs. Int. Math. Res. Not. IMRN , 2018:6908–6933, 2017
2018
-
[17]
Hamilton cycles in the union of random perm utations
Alan Frieze. Hamilton cycles in the union of random perm utations. Random Structures & Algorithms , 18(1):83–94, 2001
2001
-
[18]
R. J. Gould. Recent advances on the Hamiltonian problem : Survey iii. Graphs Combin. , 30:1–46, 2014
2014
-
[19]
Ham ilton cycles in highly connected and expand- ing graphs
Dan Hefetz, Michael Krivelevich, and Tibor Szab´ o. Ham ilton cycles in highly connected and expand- ing graphs. Combinatorica, 29(5):547–568, 2009
2009
-
[20]
Hamilton cycles in regular 2-connected g raphs
Bill Jackson. Hamilton cycles in regular 2-connected g raphs. Journal of Combinatorial Theory, Series B, 29(1):27–46, 1980
1980
-
[21]
Random graphs
Svante Janson, Tomasz Luczak, and Andrzej Rucinski. Random graphs. John Wiley & Sons, 2011
2011
-
[22]
Koml´ os and E
J. Koml´ os and E. Szemer´ edi. Limit distribution for the existence of Hamiltonian cycles in a random graph. Discrete Math. , 43(1):55–63, 1983
1983
-
[23]
Krivelevich
M. Krivelevich. The critical bias for the Hamiltonicit y game is (1 + o(1)). J. Amer. Math. Soc. , 24(1):125–131, 2011
2011
-
[24]
Krivelevich, C
M. Krivelevich, C. Lee, and B. Sudakov. Robust Hamilton icity of Dirac graphs. Trans. Amer. Math. Soc., 366(6):3095–3130, 2014
2014
-
[25]
Krivelevich, B
M. Krivelevich, B. Sudakov, V. H. Vu, and N. C. Wormald. R andom regular graphs of high degree. Random Structures & Algorithms , 18(4):346–363, 2001. 10
2001
-
[26]
Sparse pseudo- random graphs are Hamiltonian
Michael Krivelevich and Benny Sudakov. Sparse pseudo- random graphs are Hamiltonian. Journal of Graph Theory, 42(1):17–33, 2003
2003
-
[27]
K¨ uhn and D
D. K¨ uhn and D. Osthus. Hamilton decompositions of regu lar expanders: a proof of Kelly’s conjecture for large tournaments. Adv. Math., 237:62–146, 2013
2013
-
[28]
K¨ uhn and D
D. K¨ uhn and D. Osthus. Hamilton cycles in graphs and hyp ergraphs: an extremal perspective. In Proceedings of the International Congress of Mathematicians , Seoul, Korea. , volume 4, pages 381– 406, 2014
2014
-
[29]
Proof of a conjecture of Thomassen on Hamilton cycles in highly connected tournaments
Daniela K¨ uhn, John Lapinskas, Deryk Osthus, and Vires h Patel. Proof of a conjecture of Thomassen on Hamilton cycles in highly connected tournaments. Proceedings of the London Mathematical Soci- ety, 109(3):733–762, 2014
2014
-
[30]
Concentration for independent permu tations
Colin McDiarmid. Concentration for independent permu tations. Combinatorics, Probability and Computing, 11(2):163–178, 2002
2002
-
[31]
Edge-disjoint Hamiltonian circ uits in graphs with vertices of large valency
C St JA Nash-Williams. Edge-disjoint Hamiltonian circ uits in graphs with vertices of large valency. In Studies in Pure Mathematics (Presented to Richard Rado) , pages 157–183. Academic Press London, 1971
1971
-
[32]
L. P´ osa. Hamiltonian circuits in random graphs. Discrete Math. , 14(4):359–364, 1976
1976
-
[33]
Almost all cub ic graphs are Hamiltonian
Robert W Robinson and Nicholas C Wormald. Almost all cub ic graphs are Hamiltonian. Random Structures & Algorithms , 3(2):117–125, 1992
1992
-
[34]
Almost all reg ular graphs are Hamiltonian
Robert W Robinson and Nicholas C Wormald. Almost all reg ular graphs are Hamiltonian. Random Structures & Algorithms , 5(2):363–374, 1994
1994
-
[35]
Hamilton cycl es containing randomly selected edges in random regular graphs
Robert W Robinson and Nicholas C Wormald. Hamilton cycl es containing randomly selected edges in random regular graphs. Random Structures & Algorithms , 19(2):128–147, 2001. 11
2001
Reviewed August 9, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.