REVIEW 2 major objections 2 minor 1 cited by
There exists an absolute constant K such that any n-vertex d-regular γ-expander with d ≥ (γ^{-1} log n)^K is Hamiltonian if bipartite or γ-far from bipartite.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.3
2026-06-30 19:59 UTC pith:GJYSDDLV
load-bearing objection The paper gives a Hamiltonicity threshold for d-regular γ-expanders at d ≥ (γ^{-1} log n)^K when the graph is bipartite or γ-far from bipartite, plus a random connecting lemma for sublinear expanders. the 2 major comments →
Hamiltonicity of regular sublinear expanders
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We prove that there exists an absolute constant K such that any n-vertex d-regular γ-expander with d ≥ (γ^{-1} log n)^K is Hamiltonian, provided that it is bipartite or γ-far from bipartite. As part of the proof we establish a random connecting lemma for sublinear expanders.
What carries the argument
The random connecting lemma for sublinear expanders, which produces paths between prescribed vertices while preserving the expansion properties needed to complete a Hamiltonian cycle.
Load-bearing premise
The random connecting lemma for sublinear expanders holds and can be applied to produce the required paths and cycles under the stated degree threshold.
What would settle it
An n-vertex d-regular γ-expander with d just below (γ^{-1} log n)^K that is bipartite yet contains no Hamiltonian cycle would falsify the claim.
If this is right
- Highly robust versions of recent Hamiltonicity results for Cayley graphs follow directly from the degree threshold.
- Highly robust versions of recent Hamiltonicity results for Kneser graphs follow directly from the degree threshold.
- The random connecting lemma supplies a tool for constructing paths in other sparse expander settings.
Where Pith is reading between the lines
- The same degree threshold supplies a uniform test that can be applied to additional families of regular graphs once their expansion parameters are known.
- The connecting lemma may extend to questions of long paths or cycles in sublinear expanders that are not necessarily regular.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves that there exists an absolute constant K such that any n-vertex d-regular γ-expander with d ≥ (γ^{-1} log n)^K is Hamiltonian, provided the graph is bipartite or γ-far from bipartite. The proof introduces a random connecting lemma for sublinear expanders, which is used to construct the required paths and cycles, and applies the main theorem to obtain robust Hamiltonicity results for Cayley graphs and Kneser graphs.
Significance. If the random connecting lemma and its application to the degree threshold hold, the result substantially strengthens the known Hamiltonicity thresholds for regular expanders, moving from linear to polylogarithmic degree in the expansion parameter. The applications yield highly robust versions of recent results on Cayley and Kneser graphs, and the connecting lemma may have independent value for other connectivity problems in sparse expanders.
major comments (2)
- [§3] §3 (Random Connecting Lemma): the quantitative bound on the number of random walks needed to connect two sets appears to rely on a Chernoff-type tail bound whose failure probability must be union-bounded over all pairs of sets; it is not immediately clear from the statement whether the resulting degree threshold (γ^{-1} log n)^K absorbs all logarithmic factors arising from this union bound.
- [Theorem 1.1] Theorem 1.1 and the bipartite case: the argument that the lemma produces a Hamilton cycle when the graph is bipartite uses a perfect matching between the two parts; the proof sketch does not explicitly verify that the expansion parameter γ remains sufficient after removing the matching edges, which could affect the sublinear regime.
minor comments (2)
- [Abstract / Theorem 1.1] The definition of 'γ-far from bipartite' (at least γ e(G) edges to remove) is used in the non-bipartite case but is not restated in the statement of the main theorem; adding a parenthetical reminder would improve readability.
- [Introduction] Notation for the expansion parameter γ is overloaded between the expander definition and the far-from-bipartite condition; a brief remark distinguishing the two uses would prevent confusion.
Simulated Author's Rebuttal
We thank the referee for the positive assessment and constructive comments. Below we respond point-by-point to the major comments; all clarifications will be incorporated in the revised manuscript.
read point-by-point responses
-
Referee: [§3] §3 (Random Connecting Lemma): the quantitative bound on the number of random walks needed to connect two sets appears to rely on a Chernoff-type tail bound whose failure probability must be union-bounded over all pairs of sets; it is not immediately clear from the statement whether the resulting degree threshold (γ^{-1} log n)^K absorbs all logarithmic factors arising from this union bound.
Authors: In the proof of the Random Connecting Lemma, the number of walks per pair is chosen on the order of (log n / γ)^C for a sufficiently large C so that the Chernoff tail is exp(−Ω((log n / γ)^c)) for some c>0. The union bound is over at most n^2 pairs of sets and contributes an additional O(log n) to the exponent. The absolute constant K in the degree hypothesis d ≥ (γ^{-1} log n)^K is chosen larger than all constants appearing in these calculations (including those from the Chernoff and union-bound exponents), so that the failure probability is o(1/n^2) and the threshold absorbs every logarithmic factor. We will add an explicit remark after the statement of the lemma recording the dependence of K on these quantities. revision: yes
-
Referee: [Theorem 1.1] Theorem 1.1 and the bipartite case: the argument that the lemma produces a Hamilton cycle when the graph is bipartite uses a perfect matching between the two parts; the proof sketch does not explicitly verify that the expansion parameter γ remains sufficient after removing the matching edges, which could affect the sublinear regime.
Authors: In the bipartite case we delete a perfect matching M and work in G−M, which is (d−1)-regular. For any set S the number of edges leaving S in G−M is at least γ d |S| − |S|, hence the new expansion parameter satisfies γ′ ≥ (γ d − 1)/(d − 1). Whenever d ≥ 2/γ we have γ′ ≥ γ/2. Our degree lower bound already guarantees d ≫ 1/γ (since K ≥ 1), so the halved parameter γ/2 only multiplies the threshold by a constant factor 2^K, which is absorbed into the choice of the absolute constant K. We will insert this short calculation immediately after the invocation of the connecting lemma in the bipartite case of Theorem 1.1. revision: yes
Circularity Check
Derivation is self-contained; no circular reductions identified
full rationale
The paper establishes its Hamiltonicity result by proving a new random connecting lemma for sublinear expanders, presented as an independent contribution. The abstract and description contain no self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations that reduce the central claim to its own inputs by construction. The bipartite and γ-far-from-bipartite cases are handled directly within the proof structure, and applications to Cayley/Kneser graphs are straightforward consequences once the lemma holds. No equations or lemmas are quoted that exhibit the forbidden patterns of circularity.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Definition of γ-expander: for every not too large S there are at least γ d |S| edges leaving S.
- domain assumption Definition of γ-far from bipartite: at least γ e(G) edges must be removed to make G bipartite.
read the original abstract
We say that a $d$-regular graph is a $\gamma$-expander if for every not too large set of vertices $S$, there are at least $\gamma d |S|$ edges leaving $S$, and we say that a graph $G$ is $\gamma$-far from bipartite if at least $\gamma e(G)$ edges need to be removed to make it bipartite. We prove that there exists an absolute constant $K$ such that any $n$-vertex $d$-regular $\gamma$-expander with $d \ge (\gamma^{-1} \log n)^K$ is Hamiltonian, provided that it is bipartite or $\gamma$-far from bipartite. As applications, we obtain highly robust versions of recent important results on the Hamiltonicity of Cayley graphs and Kneser graphs. As part of our proof, we prove a random connecting lemma for sublinear expanders which might be of independent interest.
Figures
Forward citations
Cited by 1 Pith paper
-
Towards the Lov\'{a}sz conjecture via sublinear expanders
Every connected vertex-transitive graph of order n contains a cycle of length at least n^{2/3-o(1)}.
Reference graph
Works this paper leans on
-
[1]
R. Aharoni and P. Haxell. Hall’s theorem for hypergraphs.J. Graph Theory, 35(2):83–88, 2000
work page 2000
-
[2]
N. Alon, M. Bucić, L. Sauermann, D. Zakharov, and O. Zamir. Essentially tight bounds for rainbow cycles in proper edge-colourings.Proc. Lond. Math. Soc. (3), 130(4):Paper No. e70044, 37, 2025
work page 2025
-
[3]
N. Alon and V. D. Milman.λ 1,isoperimetric inequalities for graphs, and superconcentrators.J. Combin. Theory Ser. B, 38(1):73–88, 1985
work page 1985
- [4]
-
[5]
The Lov\'asz conjecture holds for moderately dense Cayley graphs
B. Bedert, N. Draganić, A. Müyesser, and M. Pavez-Signé. The Lovász conjecture holds for moderately dense Cayley graphs.arXiv preprint arXiv:2603.08675, 2026
work page internal anchor Pith review Pith/arXiv arXiv 2026
-
[6]
N. Biggs. Some odd graph theory. InSecond International Conference on Combinatorial Mathematics (New York, 1978), volume 319 ofAnn. New York Acad. Sci., pages 71–81. New York Acad. Sci., New York, 1979
work page 1978
- [7]
- [8]
-
[9]
M. Bucić and R. Montgomery. Towards the Erdős-Gallai Cycle Decomposition Conjecture.Advances in Mathematics, 437:Paper No. 109434, 40, 2024
work page 2024
-
[10]
D. Chakraborti, O. Janzer, A. Methuku, and R. Montgomery. Edge-disjoint cycles with the same vertex set. Advances in Mathematics, 469:110228, 2025
work page 2025
-
[11]
C. C. Chen and N. F. Quimpo. On strongly Hamiltonian abelian group graphs. InCombinatorial mathematics, VIII (Geelong, 1980), volume 884 ofLecture Notes in Math., pages 23–34. Springer, Berlin, 1981
work page 1980
-
[12]
Y.-C. Chen. Triangle-free Hamiltonian Kneser graphs.Journal of Combinatorial Theory, Series B, 89(1):1–16, 2003. 38
work page 2003
-
[13]
D. Christofides, J. Hladký, and A. Máthé. Hamilton cycles in dense vertex-transitive graphs.J. Combin. Theory Ser. B, 109:34–72, 2014
work page 2014
-
[14]
V. Chvátal. On Hamilton’s ideals.J. Combinatorial Theory Ser. B, 12:163–168, 1972
work page 1972
-
[15]
V. Chvátal and P. Erdős. A note on Hamiltonian circuits.Discrete Math., 2:111–113, 1972
work page 1972
- [16]
-
[17]
G. A. Dirac. Some theorems on abstract graphs.Proc. London Math. Soc. (3), 2:69–81, 1952
work page 1952
-
[18]
N. Draganić, R. Montgomery, D. M. Correia, A. Pokrovskiy, and B. Sudakov. Hamiltonicity of expanders: optimal bounds and applications.arXiv preprint arXiv:2402.06603, 2024
-
[19]
N. Draganić, A. Methuku, D. Munhá Correia, and B. Sudakov. Cycles with many chords.Random Structures & Algorithms, 65(1):3–16, 2024
work page 2024
- [20]
-
[21]
I. G. Fernández, J. Kim, Y. Kim, and H. Liu. Nested cycles with no geometric crossings.Proc. Amer. Math. Soc. Ser. B, 9:22–32, 2022
work page 2022
-
[22]
Hamiltoncyclesinpseudorandomgraphs.Adv
S.Glock, D.MunháCorreia, andB.Sudakov. Hamiltoncyclesinpseudorandomgraphs.Adv. Math., 458:Paper No. 109984, 45, 2024
work page 2024
-
[23]
C. Godsil and K. Meagher.Erdős-Ko-Rado theorems: algebraic approaches, volume 149 ofCambridge Studies in Advanced Mathematics. Cambridge University Press, Cambridge, 2016
work page 2016
-
[24]
C. Godsil and G. Royle.Algebraic graph theory, volume 207 ofGraduate Texts in Mathematics. Springer- Verlag, New York, 2001
work page 2001
-
[25]
B. Green. A Szemerédi-type regularity lemma in abelian groups, with applications.Geom. Funct. Anal., 15(2):340–376, 2005
work page 2005
-
[26]
$(\Delta-1)$-dicolouring of digraphs
A. Harutyunyan, K.-i. Kawarabayashi, L. Picasarri-Arrieta, and G. Puig i Surroca. (∆−1)-dicolouring of digraphs.arXiv preprint arXiv:2507.10266, 2025
work page Pith review arXiv 2025
-
[27]
J. Haslegrave, J. Kim, and H. Liu. Extremal density for sparse minors and subdivisions.Int. Math. Res. Not. IMRN, 2022(20):15505–15548, 2022
work page 2022
-
[28]
B. Jackson. Hamilton cycles in regular two-connected graphs. InGraph theory and related topics (Proc. Conf., Univ. Waterloo, Waterloo, Ont., 1977), pages 261–265. Academic Press, New York-London, 1979
work page 1977
- [29]
- [30]
-
[31]
J. R. Johnson. An inductive construction for Hamilton cycles in Kneser graphs.Electron. J. Combin., 18(1):Paper 189, 12, 2011
work page 2011
-
[32]
D. R. Karger. Random sampling in cut, flow, and network design problems.Math. Oper. Res., 24(2):383–413, 1999
work page 1999
-
[33]
R. M. Karp. Reducibility among combinatorial problems. InComplexity of computer computations (Proc. Sympos., IBM Thomas J. Watson Res. Center, Yorktown Heights, N.Y., 1972), The IBM Research Symposia Series, pages 85–103. Plenum, New York-London, 1972
work page 1972
-
[34]
A. Kelmans, D. Mubayi, and B. Sudakov. Asymptotically optimal tree-packings in regular graphs.Electron. J. Combin., 8(1):Research Paper 38, 8, 2001
work page 2001
-
[35]
J. Kim, H. Liu, M. Sharifzadeh, and K. Staden. Proof of Komlós’s conjecture on Hamiltonian subsets.Proc. Lond. Math. Soc. (3), 115(5):974–1013, 2017
work page 2017
-
[36]
J. Komlós and E. Szemerédi. Limit distribution for the existence of Hamiltonian cycles in a random graph. Discrete Math., 43(1):55–63, 1983
work page 1983
-
[37]
J. Komlós and E. Szemerédi. Topological cliques in graphs.Combin. Probab. Comput., 3(2):247–256, 1994
work page 1994
-
[38]
J. Komlós and E. Szemerédi. Topological cliques in graphs. II.Combin. Probab. Comput., 5(1):79–90, 1996
work page 1996
-
[39]
A. D. Koršunov. Solution of a problem of P. Erdős and A. Rényi on Hamiltonian cycles in nonoriented graphs. Diskret. Analiz, 228(31):17–56, 90, 1977. 39
work page 1977
-
[40]
M. Krivelevich and B. Sudakov. Sparse pseudo-random graphs are Hamiltonian.J. Graph Theory, 42(1):17–33, 2003
work page 2003
-
[41]
M. Krivelevich and B. Sudakov. Pseudo-random graphs. In E. Győri, G. O. H. Katona, L. Lovász, and T. Fleiner, editors,More Sets, Graphs and Numbers: A Salute to Vera Sós and András Hajnal, pages 199–
-
[42]
Springer Berlin Heidelberg, Berlin, Heidelberg, 2006
work page 2006
-
[43]
M. Krivelevich, B. Sudakov, V. H. Vu, and N. C. Wormald. Random regular graphs of high degree.Random Structures Algorithms, 18(4):346–363, 2001
work page 2001
-
[44]
H. Lee, M. Pavez-Signé, and T. Petrov. Spanning clique subdivisions in pseudorandom graphs.arXiv preprint arXiv:2504.01642, 2025
work page internal anchor Pith review arXiv 2025
-
[45]
S. Letzter. Sublinear expanders and their applications. InSurveys in combinatorics 2024, volume 493 of London Math. Soc. Lecture Note Ser., pages 89–130. Cambridge Univ. Press, Cambridge, 2024
work page 2024
-
[46]
S. Letzter, A. Methuku, and B. Sudakov. Nearly Hamilton cycles in sublinear expanders and applications.J. Lond. Math. Soc. (2), 113(2):Paper No. e70452, 42, 2026
work page 2026
- [47]
- [48]
-
[49]
L. Lovász. Problem 11. InCombinatorial Structures and Their Applications, page 497. Gordon and Breach, New York, 1970. Proceedings of the Calgary International Conference on Combinatorial Structures and Their Applications, 1969
work page 1970
-
[50]
L. Lovász. Random walks on graphs.Combinatorics, Paul Erdős is eighty, 2(1-46):4, 1993
work page 1993
-
[51]
W. Mader. Minimalen-fach kantenzusammenhängende Graphen.Math. Ann., 191:21–28, 1971
work page 1971
- [52]
-
[53]
G. H. J. Meredith and E. K. Lloyd. The footballers of Croam.J. Combinatorial Theory Ser. B, 15:161–166, 1973
work page 1973
- [54]
-
[55]
R. Montgomery. Spanning trees in random graphs.Adv. Math., 356:106793, 92, 2019
work page 2019
-
[56]
R. Montgomery, A. Müyesser, A. Pokrovskiy, and B. Sudakov. Approximate path decompositions of regular graphs.J. Lond. Math. Soc. (2), 112(2):Paper No. e70269, 40, 2025
work page 2025
-
[57]
T. Mütze. On Hamilton cycles in graphs defined by intersecting set systems.Notices Amer. Math. Soc., 71(5):583–592, 2024
work page 2024
- [58]
-
[59]
C. S. J. A. Nash-Williams. Edge-disjoint Hamiltonian circuits in graphs with vertices of large valency. In Studies in Pure Mathematics (Presented to Richard Rado), pages 157–183. Academic Press, London-New York, 1971
work page 1971
-
[60]
O. Ore. Note on Hamilton circuits.Amer. Math. Monthly, 67:55, 1960
work page 1960
-
[61]
L. Pósa. Hamiltonian circuits in random graphs.Discrete Math., 14(4):359–364, 1976
work page 1976
-
[62]
E. Rapaport-Strasser. Cayley color groups and Hamilton lines.Scripta Math., 24:51–58, 1959
work page 1959
-
[63]
V. Rödl, A. Ruciński, and E. Szemerédi. A Dirac-type theorem for 3-uniform hypergraphs.Combin. Probab. Comput., 15(1-2):229–251, 2006
work page 2006
-
[64]
B. Sudakov and I. Tomon. The extremal number of tight cycles.Int. Math. Res. Not. IMRN, 2022(13):9663– 9684, 2022
work page 2022
- [65]
-
[66]
M. E. Watkins. Connectivity of transitive graphs.J. Combinatorial Theory, 8:23–29, 1970. 40 A Connecting pairs of vertices by vertex-disjoint paths through a ran- dom vertex set In this appendix, we prove Lemma 5.2, which we now restate for the reader’s convenience. Lemma 5.2.LetGbe ann-vertex(d±d ′)-nearly-regularγ-expander, whered ′ ≤d/4andγ < 1 (logn) ...
work page 1970
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.