REVIEW 4 major objections 4 minor 25 references
Max-Cut with Multiple Cardinality Constraints
T0 review · 4 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read This paper proves that Max-Cut with multiple exact size quotas can be approximated within (0.858 − ε) of optimal in polynomial time whenever the number of quotas is constant.
desk verdict A solid, novel kernel + RT rounding for sparse constrained Max-Cut, but the main proof has a normalization gap that needs fixing, and the hardness reduction needs patching. 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
Two mechanisms carry the argument. The first is an approximate kernel: within each group the algorithm keeps only the top $O(k_i/\varepsilon)$ highest-degree vertices, merges the rest into a super-vertex, and shows that restricting solutions to this core costs at most a $1-4c\varepsilon$ factor of the optimum. The second is the rounding pipeline: solve a high-level Lasserre SDP (a hierarchy of semidefinite programming relaxations), condition on a few sampled vertices to make the solution $\alpha$-block independent (average mutual information inside each group at most $\alpha$), then apply the bias-preserving correlation-rounding procedure of [RT12], which cuts each edge with probability at least $\alpha_{\mathrm{cc}}$ times its SDP contribution and makes each group's selected count have variance $O(\alpha^{1/12})$. A final random correction replaces a small number of vertices per group to meet the quotas exactly with only an $O(\varepsilon)$ loss.
What would settle it
Construct a small Constrained Max-Cut instance with two groups, solve its Lasserre SDP, enforce $\varepsilon^{60}$-block independence by conditioning, apply the inherited rounding step, and measure the variance of the number of selected vertices in each group over many trials; if the variance is not $O(\varepsilon^5)$ relative to the group size, the correction step cannot restore exact quotas without an $O(\varepsilon)$ cut loss, and Theorems 4 and 5 fail.
Extended reading notes
Core claim
On the paper's own terms, the central claim is Theorem 5: for every $\varepsilon>0$, there is an algorithm that runs in $O(\min\{k/\varepsilon, n\}^{\mathrm{poly}(c/\varepsilon)} + \mathrm{poly}(n))$ time, where $k=\sum_i k_i$, and returns a feasible solution with cut value at least $(\alpha_{\mathrm{cc}}-\varepsilon)$ times the optimum, where $\alpha_{\mathrm{cc}}\approx 0.858$. The single-constraint special case $c=1$ gives the same guarantee for Max-Cut$_k$ for every $k$, including the sparse regime $k=o(n)$ that previously had only a $1/2+\varepsilon_0$ approximation. The same machinery yields a deterministic $1/2$-approximation for Max-Cut under an arbitrary matroid constraint, and an NP-hardness result for the fully general multi-constraint problem: even deciding whether a feasible cut cuts every edge is hard when the number of constraints is unbounded.
Load-bearing premise
The load-bearing premise is that a rounding procedure that provably works when the whole graph is nearly uncorrelated still works when the graph is made nearly uncorrelated separately inside each group, so that the paper can shrink the graph and round group-by-group without re-proving the procedure's guarantees.
Editorial extensions
If this is right
- The sparse regime of a single cardinality constraint gains the $0.858$ approximation, replacing the previous $1/2+\varepsilon_0$ bound for all $k$.
- When the number of groups $c$ is constant and $\varepsilon$ is fixed, the algorithm runs in polynomial time, so exact per-group quotas are approximable almost as well as ordinary Max-Cut.
- Deciding whether a feasible solution cuts every edge is polynomial-time for constant $c$, quasipolynomial for $c=\mathrm{poly}(\log n)$, and NP-hard for arbitrary $c$.
- Matroid Max-Cut, which contains the uniform and partition cases, has a deterministic $1/2$-approximation.
Reading between the lines
- A consequence the authors leave implicit: the same kernel-plus-rounding recipe may transfer to other constraint satisfaction problems with exact-size constraints, suggesting that sparse quotas need not force the approximation ratio down to $1/2$.
- The NP-hardness result hints at a phase transition in $c$—constant is easy, arbitrary is hard—so locating the exact threshold between $\mathrm{poly}(\log n)$ and $n$ is a natural next question.
- A testable extension the paper does not pursue: on low-degree graphs whose optimal cuts use vertices just below the degree cutoff, the kernel's $(1-4c\varepsilon)$ retention bound may be loose, and smaller kernels could be possible.
- For matroid Max-Cut the $1/2$ factor may be improvable for partition matroids, since the paper's multi-constraint algorithm already reaches $0.858$ for constant $c$; whether stronger SDP relaxations close the gap is left open.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies Constrained Max-Cut, in which vertices are partitioned into c parts and a solution must select exactly k_i vertices from each part while maximizing the weight of edges crossing the selected set. The main claimed contribution is an (α_cc − ε)-approximation algorithm for c = O(1), where α_cc ≈ 0.858 is the Raghavendra–Tan constant, running in time O(min{k/ε, n}^{poly(c/ε)} + poly(n)). The approach combines an approximate kernel that reduces the instance to O(k/ε) vertices with a Lasserre SDP and the Raghavendra–Tan correlation rounding technique, extended to a new notion of α-block independence. The paper also gives a 0.5-approximation for Max-Cut under an arbitrary matroid base constraint and proves NP-hardness of deciding whether a feasible solution cuts all edges for an arbitrary number of cardinality constraints. The kernel construction and several of the local lemmas are argued in detail, but the proof of the central approximation theorems contains a normalization error, and the multi-constraint algorithm has an internal inconsistency in the SDP round count; the hardness reduction also does not produce instances matching the paper's own definition of the problem.
Significance. If the main results were fully established, this would be a substantial advance: it would extend the near-optimal Raghavendra–Tan approximation from the balanced regime min{k, n−k} = Ω(n) to all cardinality values and to multiple simultaneous cardinality constraints, and the approximate kernel would be a reusable tool for converting sparse cut instances into dense ones. The paper is also careful in attributing the correlation rounding package to [RT12] and in identifying the exact hypotheses inherited from that work. However, the central approximation claim is not currently supported by the proof as written, because of a concrete algebraic error in the normalization of the SDP loss; this affects both Theorem 4 and Theorem 5. The block-independence extension is plausible, but the paper should explicitly verify that the imported Raghavendra–Tan rounding guarantees apply to solutions that are only block-independent rather than globally α-independent. The matroid 0.5-approximation is a nice contribution, and the hardness result, once repaired to fit the formal problem definition, would be a useful complement.
major comments (4)
- [Section 3.3, Step 2(b); Section 4.3, Lemma 24] The inference "]OPT − ε^60 ≥ (1−ε)]OPT" is not justified by the cited lemma. Lemma 18(2) states only that ]OPT is at least an ε-fraction of the total edge weight in the kernelized graph eE, i.e., ]OPT ≥ ε·W(eE). The desired inequality requires ]OPT ≥ ε^59, which does not follow. Concretely, take a perfect matching on n vertices with total edge weight normalized to 1, set k=1 and ε=0.1; the kernel keeps 10 vertices plus the supervertex s, so W(eE)=20/n and ]OPT=2/n. For n > 2·10^59 we have ]OPT < ε^59, and the additive loss ε^60 exceeds ]OPT, making the guarantee "SDP value ≥ ]OPT − ε^60" vacuous. The same erroneous step appears in Section 4.3 in the proof of Lemma 23 via Lemma 24. Since this step is used to claim that the conditioned SDP value is at least (1−ε)]OPT ≥ (1−5ε)OPT, the subsequent lower bound on the rounded cut value fails for instances with very small kernelized optimum. A separate argument covering the small-]OPT regime is needed for the proofs of Theorems 4 and 5 to go through.
- [Section 4.1, Algorithm 4.1 Step 2(a); Theorem 22] The Lasserre level stated in Algorithm 4.1 is inconsistent with Theorem 22. Theorem 22 with α = ε^60 and ℓ = 2 requires an initial (L+ℓ)-round solution with L = 4c^2/α^2 = 4c^2/ε^120, i.e., a total of 4c^2/ε^120 + 2 rounds. Algorithm 4.1 instead states that the algorithm solves a (3 + 4c/ε^120)-round SDP. This differs both in the factor multiplying c and in the additive constant. As written, the algorithm does not supply the level required by the conditioning theorem it invokes, so the multi-constraint analysis is not supported; this must be corrected.
- [Section 5.2, Proof of Theorem 7] The reduction from 3D matching does not produce an instance of Constrained Max-Cut as defined in Definition 1. In Definition 1, the sets V_1, ..., V_c partition all of V and every vertex is subject to exactly one cardinality constraint. In the reduction, the star centers are not assigned to any part V_i and are effectively unconstrained. The completeness and soundness arguments rely on the freedom to place each center either in S or outside S independently of the partition-matroid constraints; no assignment of the centers to parts with prescribed cardinalities reproduces this freedom. Consequently, Theorem 7 is not established for the problem defined in the paper. The proof also writes δ(S)=1 where the decision problem asks for δ(S)=|E|, which is a notational slip but should be fixed.
- [Sections 1.2 and 4, use of Raghavendra–Tan rounding] The paper states that Algorithm 5.3 of [RT12] preserves vertex biases, cuts each edge with probability at least α_cc times its SDP contribution, and satisfies the variance bound of Theorem 11, and then applies this package to solutions that are only α-block independent. The concentration part is a direct application of Theorem 11 with W uniform on each block, but the edge-cutting guarantee is imported without explicitly checking whether it holds for solutions that are not globally α-independent. The authors should either cite the exact statement from [RT12] that covers the block-independent case or prove that the rounding guarantee extends; as written, this is an unverified load-bearing assumption.
minor comments (4)
- [Appendix B, Lemma 27] In the second case of the proof, the text says "When x + y ≤ 1" but the case is actually x + y ≥ 1; the argument then substitutes x′ = 1−x, y′ = 1−y to reduce to the first case. The typo should be corrected.
- [Section 5.2, Proof of Theorem 7] The expression "u ∈ X ∩ Y ∩ Z" should read "u ∈ X ∪ Y ∪ Z", since every element of the three sets imposes a part constraint in the reduction.
- [Section 3.3 and Section 4.3] The proofs use the phrase "Assume that the constant hidden in the O-notation is 1 for simplicity". This is acceptable for exposition, but the constants in Chebyshev's inequality and in the final approximation ratio should be tracked more explicitly to avoid any hidden dependence on ε and c in the O(ε) terms.
- [Section 1.2] The paper defines α-independence with respect to a distribution W and then uses the same notation for the uniform distribution over a block V_j. This is clear in context, but a sentence explicitly stating that block independence is exactly α-independence with respect to each W_j would help the reader.
Circularity Check
No circular derivation; the central claim imports the external Raghavendra-Tan rounding theorem, and the only self-citation is background.
full rationale
The paper's derivation chain is self-contained with respect to the present authors' own claims: the (αcc − ε) approximation is not obtained by fitting or redefining αcc; αcc ≈ 0.858 is inherited as the approximation factor of Raghavendra–Tan Algorithm 5.3 [RT12], an external published theorem with stated SDP and independence assumptions. The kernel theorems (Theorem 12 and Theorem 14) are proven from cut-function submodularity and degree ordering, not from the conclusion being established. The block-independence extension (Definition 19, Theorem 22) is a new conditional-entropy argument adapted from RT12's conditioning technique, not an import of the theorem being proved. The only self-citation is [GMR+11] in a list of prior Max-Bisection approximations in the introduction; it is background and not load-bearing. The main derivational risk is a quantitative normalization gap: Step 2(b) of Section 3.3 infers ]OPT − ε^60 ≥ (1−ε)]OPT from Lemma 18(2), which only yields ]OPT ≥ ε·W(eE) and does not in general imply ]OPT ≥ ε^59; a similar gap appears in Lemma 24. That is a correctness/quantitative issue, not a circular dependency: the asserted approximation does not reduce by construction to a fitted parameter, a self-citation chain, or a renamed version of its own input.
Assumptions & free parameters
free parameters (4)
- alpha =
epsilon^60
- kernel size ratio h =
k/epsilon (per group)
- Lasserre level ell =
2
- SDP round count constants (3, 4, 4c^2) =
as stated in Algorithm steps
assumptions (4)
- standard math The Lasserre SDP hierarchy has the stated consistency, conditioning and SOS properties (Appendix A).
- domain assumption Raghavendra-Tan's rounding theorems hold: Algorithm 5.3 preserves biases, gives expected cut >= alpha_cc * SDP value, and Theorem 11 bounds balance variance.
- domain assumption Total edge weight is normalized to 1 and k_i <= |V_i|/2 in multi-constraint instances.
- ad hoc to paper The NP-hardness reduction assumes the star centers are not subject to any partition constraint, that is, they are omitted from the described parts.
invented entities (1)
-
Super vertex s (and s_i)
Cite this review
Pith. "Pith review of Max-Cut with Multiple Cardinality Constraints." pith.science (2026). https://pith.science/paper/FK273O25
@misc{pith2026250712607,
author = {Pith},
title = {Pith review of: Max-Cut with Multiple Cardinality Constraints},
year = {2026},
howpublished = {\url{https://pith.science/paper/FK273O25}},
note = {Machine review of arXiv:2507.12607}
}
abstract
We study the classic Max-Cut problem under multiple cardinality constraints, which we refer to as the Constrained Max-Cut problem. Given a graph $G=(V, E)$, a partition of the vertices into $c$ disjoint parts $V_1, \ldots, V_c$, and cardinality parameters $k_1, \ldots, k_c$, the goal is to select a set $S \subseteq V$ such that $|S \cap V_i| = k_i$ for each $i \in [c]$, maximizing the total weight of edges crossing $S$ (i.e., edges with exactly one endpoint in $S$). By designing an approximate kernel for Constrained Max-Cut and building on the correlation rounding technique of Raghavendra and Tan (2012), we present a $(0.858 - \varepsilon)$-approximation algorithm for the problem when $c = O(1)$. The algorithm runs in time $O\left(\min\{k/\varepsilon, n\}^{\poly(c/\varepsilon)} + \poly(n)\right)$, where $k = \sum_{i \in [c]} k_i$ and $n=|V|$. This improves upon the $(\frac{1}{2} + \varepsilon_0)$-approximation of Feige and Langberg (2001) for $\maxcut_k$ (the special case when $c=1, k_1 = k$), and generalizes the $(0.858 - \varepsilon)$-approximation of Raghavendra and Tan (2012), which only applies when $\min\{k,n-k\}=\Omega(n)$ and does not handle multiple constraints. We also establish that, for general values of $c$, it is NP-hard to determine whether a feasible solution exists that cuts all edges. Finally, we present a $1/2$-approximation algorithm for Max-Cut under an arbitrary matroid constraint.
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Reviewed August 6, 2026 · model on record in the stance chip above.
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