REVIEW 2 major objections 5 minor 1 cited by
Computing zero-dimensional tropical varieties via projections
T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read This paper claims that a zero-dimensional tropical variety can be computed exactly from coordinate projections, in polynomial time, by gluing the projections with slim unimodular transformations.
desk verdict Solid algorithmic contribution with a proved polynomial bound for the sequential strategy; the implementation's overlap strategy and the positive-dimensional remark sit outside the proof, so the abstract overreaches slightly. 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 object is the slim unimodular transformation: a torus automorphism that replaces one variable $x_\ell$ by $x_\ell$ times a Laurent monomial in the remaining variables, with exponent vector having one $-1$ and all other entries nonnegative. Its associated linear functional $\pi_u$ projects $\mathbb{R}^n$ onto one coordinate in a direction that can separate finitely many points. The key identity (Lemma 3.2) is $\pi_u(\operatorname{Trop}(I)) = \operatorname{Trop}(\phi_u(I) \cap K[x_\ell^{\pm}])$, which turns a projection of the tropical variety into a univariate tropical variety computable from the Newton polygon of a minimal polynomial. A companion algorithm updates a lex Gröbner basis in shape position to the transformed basis in $O(d^2 \sum (1+\log u_i))$ field operations, by inverting and multiplying univariate polynomials modulo $f_n$. The gluing step then filters the candidate set $w \in \mathbb{R}^A$ whose coordinate projections are already known by testing whether $\pi_u(w)$ lies in the univariate tropical variety.
What would settle it
Compute Trop(I) for a zero-dimensional ideal in shape position over the 2-adic numbers by Algorithm 4.2 and independently by p-adic root approximation, comparing point sets with multiplicities; any mismatch would refute the correctness claim. A sharper test is to search for an ideal whose smallest injective slope $m$ exceeds $\binom{d^2}{2}$, which would contradict Lemma 7.4.
Extended reading notes
Core claim
The central claim is that Trop(I) for a zero-dimensional ideal $I \subseteq K[x^{\pm}]$ is completely determined by a small collection of univariate tropical varieties, and that these data can be assembled exactly. The paper shows that for a slim unimodular transformation $\phi_u$, the image of Trop(I) under the linear functional $\pi_u$ equals the tropical variety of the elimination ideal $\phi_u(I) \cap K[x_\ell^{\pm}]$, a univariate object read off from a Newton polygon. Using this identity, Algorithm 4.1 merges any two known projections into a larger projection by picking $u$ so that $\pi_u$ separates the finite candidate set, and Algorithm 4.2 repeatedly applies this merge until the full set is obtained. Theorem 7.6 states that the sequential strategy uses $O(n d^2 \log d \log \log d)$ coefficient-field operations and $O(n d^4)$ rational operations, given a reduced lex Gröbner basis of shape position and a valuation $\nu(K^*) \subseteq \mathbb{Q}$. The authors also argue that plugging this algorithm into tropical-link computations makes link computation polynomial-time on average, so that the Gröbner walk dominates in positive dimension.
Load-bearing premise
The polynomial-time guarantee assumes the field's valuation takes only rational values and that a reduced lex Gröbner basis in shape position is supplied; if either fails, the stated complexity bound is not established.
Editorial extensions
If this is right
- A zero-dimensional tropical variety can be computed exactly from the coordinate-axis tropical projections, with no need to solve polynomial systems or approximate roots in the valuation topology.
- For an ideal of degree $d$ presented by a shape-position lex Gröbner basis, the sequential gluing strategy keeps every candidate set at size at most $d^2$, so both work and memory stay polynomial in $d$ and $n$.
- The complexity bound makes the computation of tropical links, the local step that guides the Gröbner walk in existing tropical-variety software, a polynomial-time subtask on average for zero-dimensional links.
- In practice, the projection method scales with degree rather than with the size of the splitting field, which is what slows down p-adic root-approximation baselines.
Reading between the lines
- The projection-and-gluing paradigm suggests that zero-dimensional tropical computation is essentially univariate: the multivariate structure enters only through the choice of finitely many injective linear functionals, so randomized or heuristic choices of $u$ could reduce the exact rational-operation cost.
- If link computation really becomes polynomial-time, further progress on positive-dimensional tropical varieties will depend mainly on Gröbner basis computation and the Gröbner walk, not on tropical-specific elimination.
- The same gluing scheme might extend to positive-dimensional ideals if the finite candidate set is replaced by a polyhedral candidate, offering a route to positive-dimensional tropical varieties without traversing the full Gröbner complex.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents an algorithm for computing the tropical variety of a zero-dimensional ideal from a lexicographical Gröbner basis in shape position. The method rests on 'slim' unimodular transformations, which map the given Gröbner basis to one for the transformed ideal, and then reduce the computation to univariate minimal polynomials and Newton polygons. Projections of the tropical variety onto coordinate subspaces are computed separately and then glued via Algorithm 4.1, with Algorithm 4.2 assembling the full tropical variety. The authors prove correctness of these algorithms, give a complexity analysis for the sequential gluing strategy, describe a Singular implementation, compare timings with a p-adic root-approximation approach in Magma, and discuss implications for computing positive-dimensional tropical links.
Significance. If the proof is completed, this is a valuable contribution: it gives a polynomial bound in the degree and ambient dimension for computing zero-dimensional tropical varieties from a Gröbner basis, avoiding both elimination and root approximation to unknown precision. The paper ships an implementation and reproducible benchmark data, and the honeycomb-cubic example is a meaningful stress test. The explicit complexity theorem for the sequential strategy, the exact minimal-polynomial/Newton-polygon steps, and the preservation of shape position and saturation are all concrete strengths that make the central idea credible and potentially useful for subsequent work on tropical links.
major comments (2)
- [Section 7, Proposition 7.5] The proof of Proposition 7.5 states that ϕ_w(I) = ϕ_u(ϕ_v(I)) for u := m e_k − e_ℓ. This equality does not hold: for two slim transformations v and u concentrated at the same coordinate ℓ, one computes (ϕ_u∘ϕ_v)(x_ℓ) = x_ℓ · ∏_{i≠ℓ} x_i^{v_i-u_i}, whereas ϕ_w(x_ℓ) = x_ℓ^{-1} · ∏_{i≠ℓ} x_i^{w_i}. In particular, the exponents of x_ℓ differ by sign. Thus Algorithm 3.4 applied to u and a Gröbner basis of ϕ_v(I) produces a Gröbner basis of the composite ideal, not of ϕ_w(I). The intended invariant should be formulated on the level of elimination ideals: applying Algorithm 3.4 to u and the known basis of ϕ_v(I) yields an ideal whose elimination ideal in x_ℓ has tropical variety π_{v+m e_k}(Trop(I)), which is exactly what the gluing step requires. This repair is localized and does not change the stated operation counts, but Theorem 7.6 relies on this proposition, so the proof as written is incomplete. Additionally, for k=2 the induction assumption 'known from a previous call of Algorithm 4.1' is not satisfied; an initial slim transformation and its Gröbner basis must be supplied or computed, adding an O(d^2) cost that is within the claimed bound.
- [Sections 5.4, 6, and Theorem 7.6] The complexity guarantee is proved only for the sequential gluing strategy, whereas the implementation and the timings in Section 6 use the overlap strategy. For overlap, the candidate set T in Algorithm 4.1 is a relation with equality constraints on a common prefix, not a Cartesian product, so Lemma 7.4 does not directly apply; no analogue is proved bounding the size of the separating integer or the cost of finding it. Consequently, the abstract's claim that 'our algorithm requires only a polynomial number of arithmetic operations' is not supported for the strategy actually used in the benchmarks. The authors should either prove a complexity bound for the overlap strategy or explicitly restrict the polynomial-complexity claim to the sequential strategy and state that the implemented default strategy is not covered.
minor comments (5)
- [Lemma 3.2] The proof uses the equality p_ℓ(Trop(J)) = Trop(J ∩ K[x_ℓ^±]) for zero-dimensional ideals without comment; since this equality is not valid for arbitrary ideals, a short justification in this setting would improve readability.
- [Example 4.3] The notation ϕ_{(−1,3)}(I) omits the third coordinate of the transformation; writing ϕ_{(−1,3,0)}(I) would avoid confusion.
- [Convention 7.1 / Theorem 7.6] The assumption ν(K*) ⊆ Q is explicit in Section 7 but should be restated in the abstract or in the statement of Theorem 7.6 so that the polynomial bound is not over-read to arbitrary valued fields.
- [Remark 7.7] The extrapolation to positive-dimensional tropical links relies on the heuristic claim that zero-dimensional Gröbner bases can be computed in average polynomial time; please label this as experimental/heuristic and distinguish it from the proved Theorem 7.6.
- [Throughout] The text contains residual LaTeX markup artifacts such as '/lessnotequal' and 'bracehtipupleft' in the rendering; these should be cleaned in the journal version.
Circularity Check
No significant circularity: the algorithm derives tropical points from exact Gröbner-basis data and univariate elimination, with no fitted quantity renamed as a prediction.
full rationale
The paper's derivation chain is self-contained in the relevant sense: Algorithm 4.2 takes as input a reduced lexicographical Gröbner basis in shape position, computes univariate tropical varieties from Newton polygons of eliminants, and reconstructs Trop(I) by gluing projections. The correctness proofs of Algorithms 4.1 and 4.2 use Lemma 3.2 (πu(Trop(I)) = Trop(ϕu(I) ∩ K[xℓ^±])) and the well-known univariate Newton-polygon fact, neither of which is defined in terms of the target output. Algorithm 3.4 transforms the Gröbner basis by explicit polynomial arithmetic, and no parameter is fitted to a subset of the tropical data and then reported as a prediction. The complexity statement Theorem 7.6 is honestly scoped to the sequential strategy; Section 5.4 states that the overlap strategy is used in the timings, and the paper gives no analogue of Lemma 7.4 or Proposition 7.5 for the overlap candidate relation. This is a scope-of-guarantee mismatch rather than a circular derivation. The references to [Cha13] and [HR18] are contextual or comparative, not load-bearing for the main theorem, and the standard complexity facts are cited from [GG13]. Remark 7.7 explicitly presents the positive-dimensional complexity claim as an argument based on average-case Gröbner basis results, not as a theorem forced by the paper's own equations. No self-definitional step, fitted-input-called-prediction, or ansatz-smuggled-via-citation pattern is present.
Assumptions & free parameters
assumptions (5)
- domain assumption K has a non-trivial valuation ν: K^* → R
- domain assumption The value group satisfies ν(K^*) ⊆ Q for the complexity analysis
- domain assumption The input is a zero-dimensional ideal in shape position with a reduced lex Gröbner basis
- standard math Standard complexity bounds for univariate polynomial arithmetic over K
- standard math The algebraic closure of the completion of K carries a unique extension of the valuation
Cite this review
Pith. "Pith review of Computing zero-dimensional tropical varieties via projections." pith.science (2026). https://pith.science/paper/7Q3KKZVS
@misc{pith2026190803486,
author = {Pith},
title = {Pith review of: Computing zero-dimensional tropical varieties via projections},
year = {2026},
howpublished = {\url{https://pith.science/paper/7Q3KKZVS}},
note = {Machine review of arXiv:1908.03486}
}
read the original abstract
We present an algorithm for computing zero-dimensional tropical varieties using projections. Our main tools are fast unimodular transforms of lexicographical Gr\"obner bases. We prove that our algorithm requires only a polynomial number of arithmetic operations if given a Gr\"obner basis, and we demonstrate that our implementation compares favourably to other existing implementations. Applying it to the computation of general positive-dimensional tropical varieties, we argue that the complexity for calculating tropical links is dominated by the complexity of the Gr\"obner walk.
Figures
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Forward citations
Cited by 1 Pith paper
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