REVIEW 2 major objections 4 minor 37 references
The stable set problem in graphs with bounded genus and bounded odd cycle packing number
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read For graphs embedded in a fixed surface with at most k node-disjoint odd cycles, the maximum-weight stable set is polynomial-time computable.
desk verdict Strong, likely-correct algorithm for stable set in bounded-genus bounded-OCP graphs; one repairable typo in the Erdos-Posa proof needs fixing before the orientable case is airtight. 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 load-bearing objects are the affine map $\sigma(x)=1-Mx$ from node space to edge space, the alternating orientation $D$ of the dual graph, and the homology class of integer circulations. The affine map turns the stable set polytope into $Q(G)$, whose integer points are exactly non-negative integer edge vectors with alternating-sum parity conditions; using the alternating orientation of the dual, $Q(G)$ becomes the set of non-negative integer circulations homologous to the all-one circulation, i.e. whose homology class is $(1,0)$ in $H_1(S;\mathbb{Z})\cong\mathbb{Z}_2\times\mathbb{Z}^{g-1}$. Solving the resulting minimum-cost homologous circulation problem by decomposing vertices into $O(g)$ strongly connected Eulerian subgraphs and using min-cost flows on a cover graph yields the algorithm. The Erdős-Pósa-type bound for 2-sided odd cycles is the preprocessing engine that reduces the general input to one satisfying parity-consistent standard assumptions.
What would settle it
Exhibit a projective-planar graph family with $\mathrm{ocp}(G)\le k$ whose smallest vertex set meeting all 2-sided odd cycles has size greater than $114k$; this would directly contradict the projective-plane base case (Theorem 36) of the Erdős-Pósa theorem and therefore break the preprocessing step on which the polynomial-time algorithm rests.
Extended reading notes
Core claim
The paper's central claim is that bounded odd cycle packing number plus bounded Euler genus makes the weighted stable set problem polynomial: for fixed $k$ and $g$, every graph $G$ with $\mathrm{ocp}(G)\le k$ embedded in a surface of Euler genus at most $g$ has a maximum-weight stable set computable in polynomial time, and its stable set polytope has a polynomial-size extended formulation. The argument establishes this by an equivalence chain: the original problem reduces to maximizing a linear function over the unbounded polyhedron $P(G)=\operatorname{conv}\{x\in\mathbb{Z}^{V(G)}:Mx\le 1\}$, where $M$ is the edge-node incidence matrix; an affine transformation sends this to a polyhedron $Q(G)$ whose integer points are non-negative integer circulations in an alternatingly oriented dual graph that are homologous to the all-one circulation. The remaining task, minimum-cost circulation in a fixed homology class, is solved in polynomial time for fixed genus by decomposing vertices into $O(g)$ Eulerian subgraphs and using min-cost flows on a cover graph. A separate result, the Erdős-Pósa theorem for 2-sided odd cycles in fixed surfaces, supplies the preprocessing that makes the reduction to parity-consistent embeddings possible.
Load-bearing premise
The algorithm's first step assumes a computable bound $f(g,k)$ on how many vertices must be deleted to destroy all 2-sided odd cycles in any fixed-genus graph with no $k+1$ disjoint such cycles; the paper cites the bound as computable rather than explicit, so its existence is the load-bearing premise.
Editorial extensions
If this is right
- For every fixed $k$ and $g$, maximum-weight stable sets in graphs with $\mathrm{ocp}(G)\le k$ embedded in a surface of Euler genus at most $g$ can be computed in polynomial time.
- The stable set polytope of every such graph admits a polynomial-size extended formulation, so the separation problem for that polytope is also tractable.
- In fixed surfaces, 2-sided odd cycles satisfy the Erdős-Pósa property, with a linear bound $19^{g+1}k$ in the orientable case; this strengthens the earlier orientable-surface result and answers its question about linear bounds.
- The paper's reductions show that in this setting the stable set problem is equivalent to finding a minimum-cost non-negative integer circulation in a fixed homology class, making the topology of the surface the only remaining source of hardness.
- The identity $\mathrm{STAB}(G)=P(G)\cap[0,1]^{V(G)}$ holds for edge-node incidence matrices of all graphs, even though it fails for general 0/1 matrices; this makes the unbounded polyhedron $P(G)$ a valid proxy for the stable set polytope in both the algorithm and the extended formulation.
Reading between the lines
- One testable extension: because the orientable bound in the Erdős-Pósa theorem is explicit and linear in $k$, the main bottleneck for a fully self-contained proof is the non-orientable case; making the computable bound $f(g,k)$ explicit would turn the preprocessing step from an existence result into a practical procedure.
- The reduction to homologous circulations suggests that the same strategy could be tried for signed graphs or for parity constraints on walks with prescribed winding numbers, where a similar chain of dual orientations and homology classes might apply.
- If the paper's Conjecture 48 is true, the homologous-circulation step would run in $g^{O(g)}n^{3/2}$ time, upgrading the polynomial-time algorithm to one with explicit genus dependence; testing the conjecture on small genus-2 and genus-3 instances would be a tractable first step.
- The identity $\mathrm{STAB}(G)=P(G)\cap[0,1]^{V(G)}$ may characterize a broader class of 0/1 matrices beyond edge-node incidence matrices; a natural test is to classify which clutter matrices satisfy it, using the paper's counterexample as the boundary case.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the maximum-weight stable set problem in graphs whose odd cycle packing number is bounded by a fixed constant k and that are embeddable in a fixed surface of Euler genus at most g. The main result, Theorem 2, claims a polynomial-time algorithm for this problem, and Theorem 3 claims a polynomial-size extended formulation for the corresponding stable set polytopes. The proof strategy is a chain of reductions: first a preprocessing step, based on a new Erdős-Pósa-type theorem for 2-sided odd cycles in embedded graphs, reduces the problem to graphs satisfying standard parity and connectivity assumptions; then polyhedral reformulations move from node space to edge space, through an alternating orientation of the dual graph, to a minimum-cost integer circulation problem in a prescribed homology class; finally this circulation problem is solved by enumerating short decompositions and running min-cost flow in a finite cover graph. The paper also proves structural results of independent interest, including an Erdős-Pósa theorem for 2-sided odd cycles with a linear bound in the orientable case.
Significance. If the proof gap identified below is repaired, the paper settles the bounded-genus case of a long-standing open problem and provides the first polynomial-time exact algorithm for this family beyond the previously known cases ocp(G)=0 and ocp(G)=1. The algebraic and algorithmic skeleton is substantial and mostly convincing: Section 6 (Proposition 14), Section 8 (Proposition 20 and Corollary 21), and the cover-graph circulation algorithm in Section 9 are detailed and internally consistent, with no circularity or fitted parameters. The claimed linear Erdős-Pósa bound for orientable surfaces would also strengthen the earlier exponential bound of Kawarabayashi and Nakamoto. However, the proof of the Erdős-Pósa theorem in Section 10 is incomplete as written, and since that theorem underpins the preprocessing step of the main algorithm, the central claims are currently conditional on a local but load-bearing repair.
major comments (2)
- [Section 10, Theorem 45, Case 2] In Case 2 the proof defines q := max{floor((t-9)/8), floor((t-1)/4)} and then asserts the equality q = floor((t-9)/8). This equality is false: for every t >= 9, floor((t-1)/4) is at least floor((t-9)/8), and for most t it is strictly larger. Consequently, when S is orientable, the orientable hypothesis of Theorem 41, namely (t-9)/8 >= q, is not guaranteed, so the claimed existence of q pairwise node-disjoint, surface-separating, noncontractible, pairwise homotopic cycles does not follow. Since Theorem 4 is the basis of the reduction in Section 4, Assumption 1, and hence of Theorem 2 for orientable surfaces, this is a load-bearing gap. It is repairable: taking q := floor((t-9)/8), which is the minimum of the two floors, still satisfies q >= m(g,k) + c(g,k) from the Case 2 lower bound on t, and it makes both bullets of Theorem 41 applicable. The proof should be corrected accordingly.
- [Section 10, final paragraph] The concluding paragraph, labelled 'a comment about the orientable case,' is explicitly a sketch. It changes the Case 1 threshold, asserts an inequality involving q and the f(gi,k), and then claims that the contradiction follows 'quickly' from the arguments of (5), without carrying out the linkage step, the parity argument, or the application of Theorem 43. Because the orientable linear bound is part of the statement of Theorem 4, and because Theorem 4 is the theorem invoked in the preprocessing of the main algorithm, this sketch must either be expanded into a complete proof or Theorem 4 must be weakened to the existential bound after the correction described in the previous comment.
minor comments (4)
- [Section 9 and Section 12.3] The recession cone of the unit-flow polytopes in the cover graph is described as the 'space of circulations'; it should be the cone of non-negative circulations, since the flows are required to be non-negative. The terminology should be adjusted to avoid ambiguity.
- [Section 10, proof of Lemma 24] The proof cites 'Lemma 23' to bound the number of pairwise node-disjoint cycles with pairwise distinct homology classes; the correct reference appears to be Lemma 22, which states exactly this bound, with Lemma 23 being used inside the proof of Lemma 22.
- [Sections 3 and 4] The notation f(g)*k is used in the algorithm overview and in Assumption 1, while Theorem 4 states a function f(g,k). The notation should be aligned to avoid suggesting a product that has not been defined.
- [Section 5, Lemma 16] The sentence 'We leave to the reader to check that Lemma 16 is actually true for all graphs' is an unproved assertion. Since only connected non-bipartite graphs are needed later, the statement should either be proved in full generality or explicitly restricted to the case used in the paper.
Circularity Check
No circularity: the derivation is a chain of independent polyhedral equivalences plus an externally grounded Erdős-Pósa theorem.
full rationale
The paper's main algorithm is built from explicit polyhedral equivalences (Sections 5–9) that connect the stable set problem to a minimum-cost homologous circulation problem; these are proofs, not fitted predictions, and none of the intermediate objects (P(G), Q(G), dual representations, cover graphs) are defined in terms of the target algorithm's output. The only preprocessing step that could raise a concern is Assumption 1, which invokes Theorem 4 to delete a small set X hitting all 2-sided odd cycles. Theorem 4 is proved in Section 10 from independent external results (planar odd-cycle min-max theorems, toroidal results, Brunet–Mohar–Richter, Robertson–Seymour disjoint paths with computable bounds from [15,23]); it is not derived from, nor equivalent to, the stable-set theorem. The self-citations to [14] and [15] are ordinary references to independent prior work and are not load-bearing in the sense of importing the target conclusion. The mathematical issue noted by the reviewer—the misdefinition of q as a maximum in Theorem 45, Case 2, and the sketchy orientable-case comment—would be a correctness gap if sustained, not a circularity: it concerns whether an external lemma supplies the required separator, not whether the paper's derivation assumes what it proves. Since no equation or fitted parameter reintroduces the conclusion as an input, the circularity score is 0.
Assumptions & free parameters
assumptions (10)
- standard math Classification of surfaces: every surface is homeomorphic to S(h,c) with Euler genus 2h+c.
- standard math Euler's formula for cellular embeddings: g = 2 + |E| - |V| - |F|.
- standard math H1(S; Z) is isomorphic to Z_2 × Z^{g-1} for non-orientable surfaces of Euler genus g.
- domain assumption Nemhauser-Trotter theorem and LP duality.
- domain assumption Odd cycle transversal and packing in planar graphs: Theorem 30 (Kráľ-Sereni-Stacho), |X| ≤ 6|C|.
- domain assumption Robertson-Seymour disjoint paths theorems on surfaces (Theorems 42 and 43).
- domain assumption Brunet-Mohar-Richter Theorem 41 giving many disjoint noncontractible separating cycles when facewidth is large.
- domain assumption de Graaf-Schrijver grid minor theorem and Schrijver's torus facewidth theorem (Theorems 38 and 39).
- domain assumption Malnič-Mohar bound on the number of homotopy classes of disjoint noncontractible curves (Proposition 3.6).
- standard math Balas' union-of-polyhedra extended formulation theorem and Chvátal's clique cutset lemma.
Cite this review
Pith. "Pith review of The stable set problem in graphs with bounded genus and bounded odd cycle packing number." pith.science (2026). https://pith.science/paper/THOBR2QR
@misc{pith2026190806300,
author = {Pith},
title = {Pith review of: The stable set problem in graphs with bounded genus and bounded odd cycle packing number},
year = {2026},
howpublished = {\url{https://pith.science/paper/THOBR2QR}},
note = {Machine review of arXiv:1908.06300}
}
abstract
Consider the family of graphs without $ k $ node-disjoint odd cycles, where $ k $ is a constant. Determining the complexity of the stable set problem for such graphs $ G $ is a long-standing problem. We give a polynomial-time algorithm for the case that $ G $ can be further embedded in a (possibly non-orientable) surface of bounded genus. Moreover, we obtain polynomial-size extended formulations for the respective stable set polytopes. To this end, we show that $2$-sided odd cycles satisfy the Erd\H{o}s-P\'osa property in graphs embedded in a fixed surface. This extends the fact that odd cycles satisfy the Erd\H{o}s-P\'osa property in graphs embedded in a fixed orientable surface (Kawarabayashi & Nakamoto, 2007). Eventually, our findings allow us to reduce the original problem to the problem of finding a minimum-cost non-negative integer circulation of a certain homology class, which turns out to be efficiently solvable in our case.
Figures
Reference graph
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