REVIEW 2 major objections 3 minor 28 references
Totally $\Delta$-modular IPs with two non-zeros in most rows
T0 review · 2 major / 3 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read For fixed Δ and k, this paper provides a strongly polynomial-time algorithm for integer programs whose totally Δ-modular constraint matrix has at most two non-zeros per row after deleting at most k rows and columns.
desk verdict Real extension of the two-nonzeros-per-row IP result with a nice signed-graph decomposition, but the strong-polynomiality claim rests on an unproven black-box transfer. 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 central object is the alternating weight of a tree: for a signed graph S with vertex weights w, a tree T has bipartition X_T,Y_T such that even edges stay inside a part and odd edges cross, and alt(T,w) = |Σ_{v∈X_T}w(v) − Σ_{v∈Y_T}w(v)|; alt(S,w) is the maximum over all trees. Proposition 11 shows that this quantity is always a subdeterminant of [A; w^T], hence bounded by Δ. Lemma 12 then shows that a graph containing an r-decorated tree — a tree with at least r terminal leaves — forces some weight function to have alternating weight at least r/(2k), so bounded subdeterminants forbid such trees. That absence is exactly what a graph-decomposition lemma (Theorem 13) needs to hand the paper a star-decomposition with a bounded number of terminals at the center, which the paper refines into the type-(i)/(ii) tree-decomposition of Theorem 16 that the dynamic program runs on.
What would settle it
A direct test: find a fixed-Δ, two-nonzero-per-row IP whose only algorithms require a number of arithmetic operations that grows with the bit length of the entries; because such IPs are exactly the type-(ii) leaf subproblems, the dynamic program could not be strongly polynomial unless that base case is. Concretely, checking whether Theorem 1 can be strengthened to strongly polynomial, or finding a lower bound showing it cannot, would settle Theorem 4 as stated.
Extended reading notes
Core claim
The paper's central claim is Theorem 4: for fixed constants Δ and k, every instance of Problem 2 — an integer program whose totally Δ-modular matrix has at most two non-zeros per row after removing at most k rows and columns — is solvable in strongly polynomial time. The argument works by a polynomial reduction to Problem 3, where the only messy part is a matrix with two non-zeros per row plus k extra rows; there, the bound on subdeterminants is shown to imply a structural restriction on an associated signed graph. Proposition 11 identifies the alternating weight of a tree as a subdeterminant, Lemma 12 turns bounded alternating weight into the absence of large decorated trees, and Theorem 16 converts that absence into a tree-decomposition with bounded adhesion and degree in which every leaf bag's terminals reappear in its parent. A bottom-up dynamic program on this decomposition solves the instance, with leaf bags handled either by bounded enumeration or by the black-box two-nonzero base case of Theorem 1.
Load-bearing premise
The claim of strong polynomiality is supported only if the black-box algorithm for the two-nonzero base case (Theorem 1) is itself strongly polynomial for fixed Δ, yet the paper states that theorem as merely polynomial-time.
Editorial extensions
If this is right
- For fixed Δ and k, the algorithmic boundary of bounded-subdeterminant IPs now includes all matrices that become two-nonzero per row after deleting at most k rows and columns.
- The reduction to Problem 3 means the proximity theorem of Cook et al. is used directly, avoiding auxiliary proximity or solution-decomposition results needed elsewhere.
- When the matrix is totally unimodular, Corollary 5 gives an FPT algorithm for Problem 3 parameterized by the largest subdeterminant, using a dimension-free proximity bound.
- Corollary 6 gives an FPT algorithm for the k-dimensional partially ordered knapsack problem, parameterized by the maximum total weight of a connected subgraph, a case for which few polynomial results were known.
- The structural decomposition theorem (Theorem 16) shows that the hard almost-embeddable bags of the cographic case are never needed for two-nonzero-plus-k matrices.
Reading between the lines
- If the base case of Theorem 1 is only weakly polynomial, the paper's main theorem still holds as a polynomial-time statement but not as a strongly polynomial one; the gap is local, not in the decomposition.
- The alternating-weight invariant could serve as a certificate for bounded subdeterminants in other matrix families: any graph with a tree whose alternating weight exceeds a known bound cannot be the signed graph of a totally Δ-modular matrix.
- The sharp contrast with the cographic near-totally-unimodular setting — where almost-embeddable bags are necessary — suggests testing whether allowing both almost-embeddable bags and two-nonzero structure yields a still larger tractable class.
- A concrete computational test: generate random totally Δ-modular instances with fixed Δ,k and growing coefficient sizes; if the observed runtime grows with bit length, suspect the missing strongly-polynomial base case.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies integer programs of the form min{c^T x : M x ≤ b, x ∈ Z^n} for totally Δ-modular matrices M that have at most two non-zeros per row after deleting at most k rows and columns, for fixed Δ and k (Problem 2). The main theorem, Theorem 4, claims a strongly polynomial-time algorithm for this class. The proof reduces Problem 2 to Problem 3, where a {0, ±1} matrix A with two non-zeros per row is augmented by k arbitrary rows W. The paper represents A via a signed graph, introduces the alternating weight of a tree, shows that bounded subdeterminants imply the absence of large decorated trees, applies a tree-decomposition theorem of Ding, and then uses a bottom-up dynamic program over the decomposition. Corollaries give an FPT algorithm for Problem 3 when A is totally unimodular and an FPT algorithm for the k-dimensional partially ordered knapsack problem.
Significance. If the main claim is correct, this is a meaningful extension of the tractability boundary for IPs with bounded subdeterminants, going beyond the two-nonzeros-per-row case of Fiorini et al. and the cographic nearly-totally-unimodular case of Aprile et al. The structural analysis is largely self-contained: the alternating-weight bound in Proposition 11, the decorated-tree obstruction in Lemma 12, and the tree-decomposition in Theorem 16 are clear and seem correct. The dynamic program is also reasonably explicit. However, the advertised strongly polynomial complexity is not supported by the written proof. Theorem 1, the main black box, is stated only as polynomial-time, and the reduction in Proposition 8 uses an LP relaxation solution without a strong-polynomiality argument. As written, the proof supports at most a polynomial-time algorithm under the stated black boxes. This gap affects the central claim of the paper.
major comments (2)
- [Section 6, proof of Theorem 4, runtime paragraph] The proof states that each local instance can be solved in polynomial time and that for bags of type (ii) this is done by invoking Theorem 1, which the manuscript states as a polynomial-time algorithm. Polynomial time does not imply strongly polynomial time; if the Fiorini-Joret-Weltge-Yuditsky algorithm depends on log ||b||, log ||c||, or uses an integer-programming oracle, the overall DP is only weakly polynomial. Since Theorem 4 explicitly claims a strongly polynomial-time algorithm, the proof must either give a strongly polynomial version of the type-(ii) solver or the theorem must be weakened to polynomial time. This is the central load-bearing step of the paper.
- [Section 3, Proposition 8 and its proof] The reduction from Problem 2 to Problem 3 uses an optimal solution x* of the LP relaxation to recenter the variables and then solves a polynomial number of Problem 3 instances. No strongly polynomial algorithm is identified for solving that LP relaxation for the class in Problem 2, and the extra rows W may have many non-zeros per row. Moreover, Proposition 8 is itself stated only as a polynomial-time reduction. Therefore, even a strongly polynomial algorithm for Problem 3 would not automatically imply Theorem 4 without an additional argument. The proof of Theorem 4 says 'by Proposition 8 it suffices to give a polynomial-time algorithm for Problem 3,' which confirms that the written argument establishes at most polynomial time, not strong polynomiality.
minor comments (3)
- [Section 6] The state-count expression '(2n∆f7(n,∆)+1)^k · (2f7(n,∆)+1)^{O((k∆)^2)}' is polynomial in n for fixed ∆ and k, but the paper should say this explicitly and state the arithmetic model, since the distinction between polynomial and strongly polynomial time is central to the main theorem.
- [Section 5, proof of Theorem 16] The treatment of disconnected graphs is quite compressed: the reader must infer how the bags of the component decompositions are combined with the empty bags at the c_i nodes. A short explicit construction of the bags for the comb attachments would improve readability.
- [References] Reference [12] contains a corrupted author name ('W˛egrzycki'), and the paper uses both 'parametrized' and 'parameterized'; these should be normalized.
Circularity Check
No significant circularity: the main theorem is supported by independent structural derivations and an external black-box algorithm.
full rationale
The derivation is self-contained for the structural part. Proposition 11 proves that the alternating weight of any tree is bounded by a subdeterminant of the combined matrix, using an explicit induction on the tree and elementary row operations. Given this bound, Lemma 12 shows that an r-decorated tree would force alternating weight at least r/(2k), so the condition that G has no (2k*Delta+1)-decorated tree follows from the total Delta-modularity assumption; it is not imposed by hand. Theorem 16 converts Ding's star-decomposition into the required bounded-adhesion tree-decomposition without assuming the conclusion. The dynamic program in Section 6 solves each local instance either by enumeration inside a bounded bag or by invoking Theorem 1, which is an external prior algorithm for exactly the same class of integer programs with two non-zeros per row and bounded subdeterminants. The self-citations to the author's thesis and to the coauthored SODA paper are used only for corollaries and not as the load-bearing premise of Theorem 4. The only substantive concern is that Theorem 4 claims strong polynomiality but the proof states only that base-case instances are solved in polynomial time; this is a running-time gap, not circularity.
Assumptions & free parameters
assumptions (5)
- standard math Cook, Gerards, Schrijver and Tardos proximity bound: for a totally Δ-modular IP there is an optimal integer solution within f7(n,Δ) of an LP optimum.
- standard math Every {0,±1} matrix with at most two non-zeros per row is the edge-vertex incidence matrix of a signed graph.
- standard math Ding's structural lemma, Theorem 13: graphs with no r-decorated tree admit the described star-decomposition with O(r^2) center-terminal bound.
- domain assumption Fiorini, Joret, Weltge and Yuditsky, Theorem 1: polynomial-time algorithm for IPs that are totally Δ-modular and have at most two non-zeros per row.
- standard math Fiorini et al. Lemma 4: in the relevant matrices only O(log Δ) columns have entries exceeding 1 in absolute value.
Cite this review
Pith. "Pith review of Totally $\Delta$-modular IPs with two non-zeros in most rows." pith.science (2026). https://pith.science/paper/3NKFR37O
@misc{pith2026241115282,
author = {Pith},
title = {Pith review of: Totally $\Delta$-modular IPs with two non-zeros in most rows},
year = {2026},
howpublished = {\url{https://pith.science/paper/3NKFR37O}},
note = {Machine review of arXiv:2411.15282}
}
read the original abstract
Integer programs (IPs) on constraint matrices with bounded subdeterminants are conjectured to be solvable in polynomial time. We give a strongly polynomial time algorithm to solve IPs where the constraint matrix has bounded subdeterminants and at most two non-zeros per row after removing a constant number of rows and columns. This result extends the work by Fiorini, Joret, Weltge \& Yuditsky (J. ACM 72(1), 1-50 (2025)) by allowing for additional, unifying constraints and variables.
Figures
Reference graph
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