REVIEW 2 major objections 5 minor 26 references
The paper proves that in K_{1,d}-free graphs, excluding the k-ladder as an induced minor forces a tree-decomposition whose bags have bounded independence number.
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 · deepseek-v4-flash
2026-08-05 10:27 UTC pith:ZVMGMHBM
load-bearing objection Genuine new width-obstruction result for ladders in K_{1,d}-free graphs, with coherent proof and useful corollaries; the only real blemish is an unproved algorithmic claim in Section 1.1. the 2 major comments →
Excluding a Ladder as an Induced Minor in Graphs Without Induced Stars
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The central claim is Theorem 1.4: there is a function tau(k,d) such that every K_{1,d}-free graph with tree-independence number at least tau(k,d) contains the k-skinny ladder as an induced minor. Since the skinny ladder contains the k-ladder as an induced minor, the analogous statement for k-ladders follows. The proof proceeds from a large tree-independence number to a strong bramble, then to two non-adjacent induced paths whose closed neighbourhoods cannot be separated by a small-independence set, then to a large shuffled rope ladder, then to a rope ladder, and finally to the skinny ladder by contraction.
What carries the argument
Shuffled rope ladders: two non-adjacent induced paths P1,P2 plus k pairwise non-adjacent induced paths Phi_i, each with neighbours on both rails. A cleaning lemma sorts the attachment orders to make a rope ladder; contracting its rungs gives the k-skinny ladder. The engine is Theorem 6.1, a Menger-like statement that turns a separator-resistance condition between a fixed subgraph H and an induced path P into an ell-H-rope ladder. Its proof greedily selects shortest mutually non-adjacent paths while managing endpoints and second-endpoints, which keeps the final rail path induced.
Load-bearing premise
The proof of Theorem 6.1 relies on each chosen connector path attaching to the previous path through a vertex that is not an endpoint or second-endpoint; if attachments could only happen at those boundary vertices, the assembled rail path would not be induced.
What would settle it
Build a K_{1,3}-free graph with two non-adjacent induced paths P and H such that every separator between N[P] and N[H] has independence number at least eta(2,3)=432, yet no 2-H-rope ladder appears as an induced subgraph.
If this is right
- In K_{1,d}-free graphs, forbidding the k-ladder as induced minor bounds the independence number of every bag in a tree-decomposition.
- The skinny-ladder version yields an independence Erdos-Posa property for every connected induced subgraph of a k-skinny ladder.
- The method gives a sharper bound for wheel exclusion and a generalisation of the theta/prism theorem to k-long thetas and prisms.
- The proof is constructive, so for fixed k,d one can either find the ladder or construct the decomposition algorithmically.
- The same construction underlies several previously separate results, suggesting the ladder is the common obstruction.
Where Pith is reading between the lines
- Theorem 6.1 fixes one side H and allows the path P to be replaced; a symmetric, two-side-fixed version would amount to an induced Menger theorem that remains open.
- The bounds are recursive and likely far from optimal, so the theorem should be read as qualitative.
- If one could force rung paths to be distance-2 separated, the double-ladder step toward the induced grid would become visible.
- The cycle-rope-ladder structure could be the basis for a two-cycle version needed for double wheels.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proves that for every positive integer k and d≥2, every K_{1,d}-free graph either contains the k-skinny ladder as an induced minor or has tree-independence number bounded by an explicit function τ(k,d). Since the k-skinny ladder contains the k-ladder as an induced minor, this establishes the headline ladder theorem. The proof combines a strong-bramble duality theorem (Theorem 2.3), a lemma producing two non-adjacent induced paths whose closed neighborhoods cannot be separated by a set of small independence number (Theorem 5.2), and a Menger-like construction (Theorem 6.1) that builds a large shuffled rope ladder, which is then cleaned into a rope ladder (Lemma 4.3) and converted into a skinny ladder. The paper also derives applications: an improved bound for wheel exclusion, a bounded tree-independence number for graphs excluding long thetas and long prisms, and an induced Erdos-Posa-type statement for connected induced subgraphs of skinny ladders in K_{1,d}-free graphs.
Significance. If correct, the main theorem is a substantial advance toward the induced grid conjecture for K_{1,d}-free graphs: it gives the first family of planar obstructions handled beyond wheels/tripods, with explicit elementary bounds and a relatively uniform proof strategy. The paper is honest about using strong bramble duality and the prior result Lemma 2.4 from [CHMW25] as black boxes, and these uses are legitimate. The applications to wheels, long thetas/prisms, and induced Erdos-Posa are nontrivial and improve known bounds. The main proof chain from Theorem 5.2, Theorem 6.1, and Lemma 4.3 to Theorem 1.4 is internally consistent, and I found no circularity. The manuscript would be a valuable contribution after addressing the unproved algorithmic claim and a few local issues in the applications.
major comments (2)
- [§1.1] The paper states: 'our proofs are constructive, and with the result of [CHMW25], they imply an algorithm ... in time |V(G)|^{g(k,d)}'. No algorithm or complexity analysis is provided anywhere in the manuscript. Since this is advertised as a contribution, please either give a precise constructive argument (including the choice of g) or remove/qualify the claim. This does not affect the correctness of Theorem 1.4, but as written it is an unsupported statement.
- [Lemma 7.10] The statement assumes ℓ ≥ 12(d−1)^2(2k−1)+1, but the greedy argument in the proof discards up to 4(d−1)^2(2k−1) indices after each chosen index and needs four chosen indices. The proof itself later uses ℓ ≥ 16(d−1)^2(2k−1). With the stated 12(d−1)^2(2k−1)+1 the greedy selection may fail. This is a local numerical error, but it affects Theorem 1.6 and should be corrected in the statement and in Corollary 7.7.
minor comments (5)
- [§6.1] The final step of Theorem 6.1 asserts that each R_i has length at least 4 'since w_i is adjacent to a vertex in R_i that is not an endpoint or second-endpoint'. This is correct, but the one-sentence justification relies on the earlier deletion of S^i and on the definition of Qbar^i_{t_i}; please expand it for readability.
- [§6.1] In the induction step, 'ℓi' should be 'ℓ^i' (powers of ℓ), matching the later bounds involving ℓ^j. The current notation could confuse.
- [§7.1] In the proof of Theorem 7.3, 'β(s´2)' appears to be a typo for 'β(s_2)'. Please correct.
- [Throughout] There are several typos: 'grpah' (p.1), 'combarability' (p.1), 'T heorem1.4' (p.3), 'InrCHT24s' (p.21), 'doe snot' (p.19), 'we loose control' (p.25). A careful proofreading pass is needed.
- [Theorem 5.2] Theorem 5.2 is stated as a direct consequence of the proof of Theorem 5.1, but it is not formally proved. Since it is load-bearing for Theorem 1.4, please include a short proof or explicitly say it follows by the same argument with P2 = R.
Circularity Check
No significant circularity: Theorem 1.4 is derived from bramble duality and independent cleaning/construction lemmas; the only caveat is an unproved algorithmic claim in Section 1.1.
full rationale
The derivation chain for the main theorem is non-circular. Theorem 1.4 is obtained by combining Theorem 5.2 (which produces two non-adjacent induced paths whose closed neighborhoods cannot be separated by small-independence sets), Theorem 6.1 (which converts such a pair into a large shuffled rope ladder), and Lemma 4.3 (which cleans a shuffled rope ladder into a rope ladder, and hence into a skinny ladder as an induced minor). None of these intermediate statements assumes the excluded-ladder conclusion. The bramble-duality Theorem 2.3 is attributed to Adler and to the authors' prior paper [CHMW25], but it is a parameter-free structural statement about tree-independence number and strong brambles, not about ladders. Lemma 2.4 from [CHMW25] is also a self-citation, but it is a separate, parameter-free lemma about finding an induced path meeting every bramble element; it does not state or assume Theorem 1.4, and it is used as a black-box tool rather than as a renamed version of the target result. The flagged assertion in Theorem 6.1 that each selected path R_i has length at least 4 is justified by the construction: w_{i+1} belongs to Qbar^i_{t_i} = N[R_i] \setminus N[S^i], so its neighbor in R_i is not an endpoint or second-endpoint, forcing at least four vertices on R_i. The only unsupported statement found is in Section 1.1, where the paper claims that the constructive proofs imply an algorithm running in time |V(G)|^{g(k,d)} without giving an algorithm or complexity analysis. This is a missing proof/correctness-risk, not circularity; it does not make the main theorem reduce to its inputs. Accordingly, no circular step is identified and the score is 0.
Axiom & Free-Parameter Ledger
free parameters (4)
- eta(ell,d)=8(d-1)ell^(ell+1) =
8(d-1)ell^(ell+1)
- rho(k,d)=(2(d-1)^2(2d-1)^(2d-2))^(k-1) =
(2(d-1)^2(2d-1)^(2d-2))^(k-1)
- nu(k,d)=rho(rho((k-1)^2+1,d),d) =
rho(rho((k-1)^2+1,d),d)
- tau(k,d)=8*eta(nu(k,d),d)+8d-14 =
8*eta(nu(k,d),d)+8d-14
axioms (4)
- standard math All graphs are finite and simple.
- domain assumption The ambient restriction is K_{1,d}-freeness.
- standard math Strong bramble duality and the bramble-path lemma (Theorem 2.3 and Lemma 2.4 from [CHMW25]).
- standard math Erdos-Szekeres theorem [ES35].
Cite this review
Pith. "Pith review of Excluding a Ladder as an Induced Minor in Graphs Without Induced Stars." pith.science (2026). https://pith.science/paper/ZVMGMHBM
@misc{pith2026250904026,
author = {Pith},
title = {Pith review of: Excluding a Ladder as an Induced Minor in Graphs Without Induced Stars},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZVMGMHBM}},
note = {Machine review of arXiv:2509.04026}
}
read the original abstract
A $k$-ladder is the graph obtained from two disjoint paths, each with $k$ vertices, by joining the $i$th vertices of both paths with an edge for each $i\in\{ 1,\ldots,k\}$. In this paper, we show that for all positive integers $k$ and $d$, the class of all $K_{1,d}$-free graphs excluding the $k$-ladder as an induced minor has a bounded tree-independence number. We further show that our method implies a number of known results: We improve the bound on the tree-independence number for the class of $K_{1,d}$-free graphs not containing a wheel as an induced minor given by Choi, Hilaire, Milani\v{c}, and Wiederrecht. Furthermore, we show that the class of $K_{1,d}$-free graphs not containing a theta or a prism, whose paths have length at least $k$, as an induced subgraph has bounded tree-independence number. This improves a result by Chudnovsky, Hajebi, and Trotignon. Finally, we extend the induced Erd\H{o}s-P\'osa result of Ahn, Gollin, Huynh, and Kwon in $K_{1,d}$-free graphs from long induced cycles to any graph that is an induced minor of the $k$-ladder where every edge is subdivided exactly once.
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Reference graph
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