REVIEW 3 major objections 3 minor 22 references
Induced arithmetic removal for partition-regular patterns of complexity 1
T0 review · 3 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper proves that every partition-regular pattern of complexity 1 over a finite field can be completely eliminated from a colouring by recolouring only a tiny fraction of the space.
desk verdict New and likely true, but a fixable parameter bug in Lemma 5.4 currently blocks the proof of the main theorem; worth a serious referee and a focused revision. 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 proof's central device is a two-stage subcoset selection. A strong regularity lemma first partitions $\mathbb{F}_p^n$ into cosets of a coarse subspace $H_1$ and then picks regular cosets of a finer subspace $H_2$ inside each $H_1$-coset, as in [7]. The novel step, Lemma 5.3, selects several cosets $H_3+z_1, \dots, H_3+z_m$ inside the zero coset of $H_2$ at once, with the translations $z_j$ solving the pattern's equation $A z = 0$; this is achieved by applying a quantitative Rado theorem over finite fields to an auxiliary colouring of the coset space. Because a whole consistent family of regular subcosets is chosen, the recolouring can be decided by the colour densities on these representatives without any special handling of the zero coset; partition-regularity guarantees that such a family exists.
What would settle it
Take $p=2$, $r=2$, $\epsilon=2^{-10}$. Then Lemma 5.4 sets $D_0 = \lceil \log_2(\epsilon/8)\rceil = -13$, so $D_1$ need not be large and $p^{-D_1}$ can exceed $\epsilon/4$. Computing this inequality would show the proof does not establish the claimed recolouring bound.
Extended reading notes
Core claim
The central claim (Theorem 1.5) is: for every $\epsilon>0$, integer $r$, and every partition-regular arithmetic pattern $H$ of complexity 1, there is a $\delta > 0$ such that any $r$-colouring of $\mathbb{F}_p^n$ with $H$-instance density at most $\delta$ can be recoloured on at most an $\epsilon$-fraction of $\mathbb{F}_p^n$ to become completely $H$-free. Unlike the earlier induced removal lemma [7], the recolouring removes all instances, including those containing 0; a non-partition-regular pattern can force a monochromatic solution at 0 in every recolouring, so partition-regularity is exactly the condition that makes complete removal possible.
Load-bearing premise
The proof's guarantee that the recolouring changes only a small fraction of the space depends on the coarse subspace being chosen large enough; the bound stated in Lemma 5.4 does not, as written, ensure this for small epsilon.
Editorial extensions
If this is right
- Complete removal holds for all partition-regular patterns of complexity 1, including non-translation-invariant patterns such as rainbow solutions to $x+y+z=w$.
- Partition-regularity is exactly the dividing line: for a non-partition-regular pattern, some instances (e.g. at 0) must survive any recolouring, as shown by the paper's Example 1.4.
- Corollary 1.7: a subset $A$ of $\mathbb{F}_p^n$ with $A+A$ almost disjoint from the complement is $\epsilon$-close to a subspace.
- The theorem extends to removing any finite collection of partition-regular complexity-1 patterns simultaneously.
Reading between the lines
- A corrected parameter choice in the recolouring lemma (taking the coarse-subspace codimension large enough that the unchanged fraction is bounded by $\epsilon/4$) would repair the proof's approximation bound and leave the main theorem intact.
- The same multiple-subcoset strategy is the natural candidate for extending complete removal to partition-regular patterns of higher true complexity, although the author notes that the required rank control is not yet available.
- If the theorem survives scrutiny, it implies that partition-regularity is the exact dividing line between induced removal lemmas that leave a zero-residue and those that do not, at least in the complexity-1 range.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims a complete induced arithmetic removal lemma for partition-regular patterns of complexity 1 over F_p^n: whenever an r-colouring has pattern density at most delta, one can recolour an epsilon-fraction of the space so that no instance of the pattern remains, including instances involving 0. This would extend the Fox--Tidor--Zhao induced removal lemma, which necessarily leaves non-generic instances for non-partition-regular patterns. The proof uses a two-stage subcoset selection scheme, a quantitative Rado theorem over finite fields, a counting lemma for complexity-1 systems, and a recolouring lemma constructed from regular subcosets.
Significance. If the proof is completed, the result is a natural and significant strengthening of the Fox--Tidor--Zhao theorem: it gives fully pattern-free recolourings for all partition-regular complexity-1 patterns, not just recolourings with possible non-generic leftovers. The multiple-subcoset idea is actually interesting and adapts the Alon--Fischer--Krivelevich--Szegedy approach to the arithmetic setting. The paper also contains a self-contained proof of the needed counting lemma and a useful discussion of quantitative bounds, including the polynomial true-complexity bounds of Manners. However, as written, the central proof contains several concrete parameter errors, and the main theorem is not established by the current text. The errors appear repairable, but they are load-bearing.
major comments (3)
- [Section 5, Lemma 5.4 (first paragraph and final bound)] The proof defines D0 = ceil(log_p(epsilon/4r)) and takes H0 of codimension D0. For every nontrivial choice of parameters with epsilon < 4r, this D0 is non-positive, so no such subspace exists and the construction cannot be instantiated. Even if one ignores this and starts from D0 = 0, property (i) only gives D1 >= log_p(epsilon/4r), while the final recolouring estimate requires p^{-D1} <= epsilon/4, i.e. D1 >= log_p(4/epsilon). These two lower bounds differ substantially. For example, with p=2, r=2 and epsilon=1/100, the written lower bound is about -9 while the needed lower bound is about 9. Consequently, Lemma 5.4 does not establish that phi' differs from phi in at most epsilon|F_p^n| places, and the epsilon-approximation guarantee of Theorem 1.5 is unproved. The repair is to set D0 = ceil(log_p(4/epsilon)) and then check that the subsequent applications of Proposition 5.1 and Lemma 5.3 still go through; as written, this load-bearing parameter is wrong.
- [Section 5, Theorem 5.5 (proof, final lower bound)] Theorem 5.5 applies Lemma 5.4 with epsilon' = min(epsilon, epsilon_count(delta')), where delta' = (epsilon/8r)^m/(2m). Lemma 5.4(iv) then gives alpha_i >= epsilon'/8r, not epsilon/8r. The displayed lower bound, however, uses (epsilon/8r)^m and claims a contradiction with delta < (8 r p^{C_rcl} epsilon^{-1})^{-m}/2. If epsilon' is strictly smaller than epsilon, which is the typical situation because epsilon_count(delta') tends to 0 with delta', the product (epsilon'/8r)^m can be smaller than the error term m delta' = (epsilon/8r)^m/2, in which case the counting lemma gives no positive lower bound at all. Thus the contradiction argument in Theorem 5.5 is not justified as written. The proof needs a coherent parameter choice that keeps the recolouring threshold used in Lemma 5.4 compatible with the counting error delta', for example by choosing delta' in terms of the actual lower bound on alpha_i, or by applying Lemma 5.4 with the original epsilon and choosing delta' so that both epsilon_count(delta') >= epsilon p^{-C_zreg}/8r and m delta' < (epsilon/8r)^m/2 hold.
- [Section 5, Lemma 5.3 (proof, regularity of monochromatic solutions)] There are two quantitative gaps in the proof of Lemma 5.3. First, the auxiliary colouring psi takes values in P([r]), so it has up to 2^r colours; Theorem 4.2 should be applied with 2^r colours, not with 2r colours as written. Second, the number of monochromatic solutions that fail property (ii) is at most m epsilon' |F_p^D|^{m-l}, not epsilon' |F_p^D|^{m-l}, since a solution fails if any one of its m coordinates lies in a non-regular coset. With the current choice epsilon' <= crado(p,2r,m)/2, the bound m epsilon' |F_p^D|^{m-l} can exceed the guaranteed number crado |F_p^D|^{m-l} of monochromatic solutions when m > 1. The proof therefore does not establish the existence of a monochromatic solution with all cosets regular. Both issues are local and repairable, for instance by using 2^r colours and taking epsilon' <= crado/(4m), but as written Lemma 5.3 is not fully proved.
minor comments (3)
- [Section 5, Proposition 5.1] In the statement of Proposition 5.1, the bound in part (i) is written as D1 <= D2 <= Creg(epsilon, zeta, r, N, d), but N is not defined; this should presumably be r, as in the rest of the statement. There is also a typo in Theorem 2.3, where 'regularity lemm' should be 'regularity lemma'.
- [Section 5, Lemma 5.4 (recolouring cost accounting)] In the recolouring step for u != 0, the text says that recolouring low-density colour classes changes at most epsilon/4r + zeta <= epsilon/2r proportion of H1+u. This is the per-colour bound; the total proportion over all r colours is at most sum of the low densities, which is at most r(epsilon/4r + zeta) = epsilon/2. The subsequent line 'zeta + r epsilon/2r <= 3epsilon/4' shows that this is what was intended, but the phrasing should be clarified to avoid an apparent off-by-r error.
- [Section 1, Example 1.6] In Example 1.6, the claim that there are at most delta |F_p^n|^3 instances is off by a factor of |F_p^n|: the number of l-tuples is |F_p^n|^4, and fixing one variable in a set of size at most delta|F_p^n|/4 gives at most delta|F_p^n|^4/4 instances. The intended statement is that the density is at most delta/4, not that the number of instances is at most delta |F_p^n|^3.
Circularity Check
No significant circularity: the main theorem is derived from external regularity, Rado, and counting results, and the counting lemma is proved independently in the appendix.
full rationale
The derivation of Theorem 1.5 is self-contained relative to external, independently established ingredients: Green's arithmetic regularity lemma (Theorem 2.3), the Serra–Vena quantitative Rado theorem (Theorem 4.2), the Fox–Tidor–Zhao subcoset selection (Proposition 5.1), and the counting lemma (Lemma 2.7), the last of which is proved in Appendix A rather than imported. None of these inputs is defined in terms of the target removal statement, and no parameter is fitted to the colourings whose instances are to be removed; the recolouring in Lemmas 5.3 and 5.4 is constructed from the original colouring via regularity and Rado's theorem, not assumed to be pattern-free. The only self-citation, reference [8], concerns a lower-bound discussion in Section 6 and is not load-bearing for the main theorem. The parameter issue in Lemma 5.4 identified by the skeptical reader is a technical correctness concern, not a circularity: it does not reduce the conclusion to an input or rename a fitted quantity as a prediction.
Assumptions & free parameters
assumptions (5)
- standard math Green's arithmetic regularity lemma (Theorem 2.3) gives a subspace H of bounded codimension such that P(H) is epsilon-regular for a given list of functions.
- standard math Quantitative Rado theorem over finite fields (Theorem 4.2, Serra-Vena): if the matrix A satisfies Rado's column conditions, then every r-colouring of F_p^D (D >= n_rado) contains at least c_rado(p,r,m) p^{D(m-l)} monochromatic solutions to Az=0.
- standard math Intermediate subcoset selection (Proposition 5.1, restated from Fox-Tidor-Zhao [7, Proposition 3.2]): there are nested subspaces H2 <= H1 of H0 such that non-zero cosets of H2 are epsilon-regular and densities on most subcosets approximate densities on the H1-cosets.
- standard math True complexity of a linear system and the associated counting lemma (Definition 2.4 and Lemma 2.7): for complexity-1 systems, if cosets are consistent and epsilon-regular, Lambda_L is close to the product of the averages.
- standard math Rado's theorem in abelian groups (Deuber, Bergelson-Deuber-Hindman) characterises partition regularity of the matrix K(L) by Rado's column conditions, used to define partition-regular linear systems in Section 4.
Cite this review
Pith. "Pith review of Induced arithmetic removal for partition-regular patterns of complexity 1." pith.science (2026). https://pith.science/paper/LPUGAU4W
@misc{pith2026241215170,
author = {Pith},
title = {Pith review of: Induced arithmetic removal for partition-regular patterns of complexity 1},
year = {2026},
howpublished = {\url{https://pith.science/paper/LPUGAU4W}},
note = {Machine review of arXiv:2412.15170}
}
read the original abstract
In 2019, Fox, Tidor and Zhao (arXiv:1911.03427) proved an induced arithmetic removal lemma for linear patterns of complexity 1 in vector spaces over a fixed finite field. With no further assumptions on the pattern, this induced removal lemma cannot guarantee a fully pattern-free recolouring of the space, as some `non-generic' instances must necessarily remain. On the other hand, Bhattacharyya et al. (arXiv:1212.3849) showed that in the case of translation-invariant patterns, it is possible to obtain recolourings that eliminate the given pattern completely, with no exceptions left behind. This paper demonstrates that such complete removal can be achieved for all partition-regular arithmetic patterns of complexity 1.
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Reviewed August 11, 2026 · model on record in the stance chip above.
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