REVIEW 1 major objections 5 minor 34 references
Subset Selection Problems in Planar Point Sets
T0 review · 1 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read For planar points with bounded collinearity, the largest no-three-in-line subset is $\Theta(n/s)$ when $s$ is large, with new bounds in every remaining regime.
desk verdict Genuinely new bounds, but the container-step verification has a repairable log-factor gap. 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 3-uniform hypergraph whose vertices are grid points and whose edges are collinear triples. The new step is a balanced supersaturation lemma: any $S\subseteq[n]^3$ of size $n^{3-x}$ contains a spanning subhypergraph $H'$ with $e(H')\ge n^{6-4x}/(10^7\log n)$, maximum degree at most $n^{3-3x}/(10^3 f\log n)$, and pair-codegree at most 1. Feeding this hypergraph into the container lemma yields containers that shrink by a constant fraction, and iterating $(\log n)^2$ times produces $\exp(n^{(3+\gamma)/2+o(1)})$ containers of size $n^{3-\gamma+f}$. A random subset of $[n]^3$ with no collinear $n^\alpha$-tuple is then shown, by a first-moment estimate over the containers, to have no large general-position subset; projecting back to the plane gives the upper-bound construction.
What would settle it
Compute the edge count in Lemma 11 directly for a small example such as $S=[n]^3$ with $x=0$: because each line contributes a matching of about $|\ell\cap S|/3$ edges rather than $\binom{|\ell\cap S|}{3}$, the displayed lower bound $e(H')\ge n^6/(10^7\log n)$ should be checked against the corrected count; if it falls below the bound, the supersaturation lemma loses support. Separately, a search for $n$-point sets with at most 4 collinear points and a general-position subset larger than the claimed $O(n^{5/6+o(1)}/\sqrt{4})$ bound would refute Theorem 1(ii) in that case.
Extended reading notes
Core claim
The paper's central claim is Theorem 1: for every $n$-point planar set $P$ with at most $s$ points on any line, the largest general-position subset has size $f(n,s)=O(n/s)$ for $n^{1/3}\le s\le n$ and $f(n,s)=O(n^{5/6+o(1)}/\sqrt{s})$ for $3\le s\le n^{1/3}$; in particular $f(n,s)=\Theta(n/s)$ when $s=\Omega(\sqrt{n}\log n)$. The large-$s$ range is proved by taking a random subset of the square grid $[n]^2$ and applying the pigeonhole principle. The small-$s$ range is proved by lifting to the cube $[n]^3$, applying the hypergraph container method to a sparsified hypergraph of collinear triples with pair-codegree at most one, iterating containers until every collinear-triple-free set lies in a small family of small containers, and then projecting a random subset back to the plane. The paper additionally claims bounds for monotone general-position subsets and for subsets with pairwise distinct slopes, stated as Theorems 4 through 6.
Load-bearing premise
The proof of the small-$s$ range in Theorem 1(ii) relies on equation (33), which derives the factor $1/\sqrt{s}$ for $3\le s\le\log n$ from monotonicity alone; that division is not a consequence of monotonicity, and the container argument only establishes it for $s\ge\log n$.
Editorial extensions
If this is right
- If Theorem 1 is right, the general-position problem is resolved up to constants for $n^{1/3}\le s\le n$, and exactly $\Theta(n/s)$ for $s=\Omega(\sqrt{n}\log n)$.
- When $s=\Theta(\sqrt{n})$, monotone general-position subsets have size between $\Omega((n\log\log n/\log n)^{1/4})$ and $O(n^{1/4})$, so only logarithmic factors separate the bounds.
- Every $\sqrt{n}\times\sqrt{n}$ grid contains a monotone general-position subset of size $\Omega((n/\log n)^{2/5})$, while every such subset has size $o(n^{1/2})$.
- For distinct slopes, any $n$-point set with at most $O(\sqrt{n})$ collinear points contains a subset of size $\Omega((n/\log s)^{1/3})$, matching the best grid bound when $s=\Theta(\sqrt{n})$, with worst-case upper bounds $O(\sqrt n)$ for $s\le n^{3/8}$ and $O((n/s)^{4/5})$ after.
- As a corollary, any $n$-point set with at most $\sqrt n$ collinear points can be colored with $O(\sqrt n\log n)$ colors so each color class is in general position.
Reading between the lines
- The proof of the small-$s$ range in Theorem 1(ii) contains a monotonicity step that is not fully justified for $3\le s\le\log n$; if that step fails, the claimed $1/\sqrt{s}$ improvement for fixed small $s$ would be unsupported even though the rest of the theorem could stand.
- If the small-$s$ bound survives scrutiny, it predicts a genuine constant-factor improvement over $f(n,3)$ for every fixed $s>3$, which could be probed computationally on small grids for $s=4,5$.
- The balanced supersaturation lemma is formulated for collinear triples in $[n]^3$, but the same container iteration only needs bounded pair-codegrees; it may transfer to higher-dimensional grids or to collinear $k$-tuples.
- The distinct-slopes bounds leave a wide gap in the middle range, suggesting the extremal construction is neither a grid nor a parabolic Sidon set; finding the true extremal set for $s$ near $n^{3/8}$ is a concrete next step.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies three subset-selection problems for n-point planar sets with maximum collinearity s: (i) the largest subset in general position, (ii) the largest monotone general-position subset, and (iii) the largest subset with pairwise distinct slopes. The main results are Theorem 1, giving upper bounds f(n,s)=O(n/s) for n^{1/3} ≤ s ≤ n and f(n,s)=O(n^{5/6+o(1)}/√s) for 3 ≤ s ≤ n^{1/3}; Theorem 4, giving g(n,s)=Ω((n log n log s / s^2 / log s)^{1/4}) and O((n/s)^{1/2}); Theorem 5, giving lower and upper bounds for the grid; Theorem 6, giving h(n,s)=Ω((n/log s)^{1/3}) and upper bounds; and Theorem 22, on coloring point sets into general-position classes. The proofs combine incidence geometry, the Cooper–Mubayi coloring lemma, the Saxton–Thomason container method, Szemerédi–Trotter, Sidon sets, and random sampling.
Significance. If the proofs are correct, the paper provides the first upper bounds for f(n,s) in the non-constant range that are tight up to constants when s=Ω(√n log n), as well as new bounds for the monotone and distinct-slope variants for arbitrary planar sets. The container-based approach extends Balogh–Solymosi in a novel way, and the lower-bound constructions are clean and use a well-chosen mix of existing tools. However, the central container step contains a false inequality in §3.2, Eq. (36), which currently invalidates the proof of Theorem 1(ii); the gap appears easily repairable, so the results are likely salvageable with a modest revision.
major comments (1)
- [§3.2, Eq. (36)] The application of Lemma 10 fails as written. With d ≥ n^{3−3x}/(10^7 log n), τ = n^{x+γ/2−3/2}, and x ≤ γ − f, the second term in Δ(H′,τ) satisfies 2Δ_3/(dτ^2) ≤ 2·10^7 log n / n^{γ−x} ≤ 2·10^7 log n / n^f, so Δ(H′,τ) = O(log n / n^f), not O(n^{−f}). For the claimed ε = n^{−f}/2, the required inequality Δ(H′,τ) ≤ ε/(100·3!) = n^{−f}/1200 would force log n = O(1), which is false for large n. Consequently, Lemma 10 cannot be applied with this ε, and the container collection, the shrinking bound (38), and the container-size estimate (39) are not justified as written. A simple repair is to take ε to be a small absolute constant, e.g., ε = 1/4; then Δ(H′,τ) ≤ 1/2400 for sufficiently large n, and the later steps (37)–(39) only require that ε is bounded away from 1. The repair should be stated explicitly because the current text claims ε = n^{−f}/2.
minor comments (5)
- [§3.2, Eq. (33)] The monotonicity step in Eq. (33) is actually valid: for 3 ≤ s ≤ log n, we have √s ≤ √(log n) = n^{o(1)}, so the o(1) in the known bound f(n,3) ≤ n^{5/6+o(1)} absorbs the extra √(log n) factor. The claimed bound O(n^{5/6+o(1)}/√s) therefore follows for this range; the concern that monotonicity alone cannot create the √s improvement is unfounded, because the improvement comes from the slack in the exponent, not from a hidden monotonicity inequality.
- [Lemma 11, Eq. (31)] The displayed equality e(H'[ℓ∩S]) = binom(|ℓ∩S|,3) is incorrect: H' is defined by placing a matching on ℓ∩S, so the number of edges on a line ℓ is floor((|ℓ∩S|−2)/3), not a binomial coefficient. The subsequent lower bound T_v ≥ |S|/3 − |L_v| is consistent with the matching interpretation, so this is a presentation typo rather than a substantive error, but it should be corrected.
- [Lemma 11] The bound on Δ(H′) is written as n^{3−3x} 10^{3f} log n, which is ambiguous: it could be read as n^{3−3x}/(10^{3f} log n) or n^{3−3x}/(10^3 f log n). The proof and the subsequent use of the bound in Eq. (36) depend on the intended denominator; the authors should disambiguate the notation.
- [§4.1] In the upper-bound construction for Theorem 4, the phrase 'slightly randomly perturb each of its grid points and replace it by a collinear s-tuple' should specify that the perturbed points are chosen so that no unintended collinearities or monotonicity obstructions arise; as written, the construction is rather informal, though the intended argument is clear.
- [§5.1] In the proof of Theorem 6 lower bound, the notation L is used both for the number of collinear triples and, in Lemma 18, for a line; this is not a mathematical error but the double use of L may confuse readers.
Circularity Check
No significant circularity: the derivation chain is self-contained and any self-citations are to independent published results.
full rationale
The paper's central claims are proved by explicit constructions and by standard external theorems, not by assuming the conclusions. Theorem 1(i) uses a random grid construction and Chernoff bounds; Theorem 1(ii) uses the Saxton–Thomason container lemma, a new supersaturation/sparsification lemma, and the published Balogh–Solymosi framework. The self-citations to Balogh–Morris–Samotij [1] and Balogh–Solymosi [2] reference published mathematical results whose stated assumptions do not include the target bounds; they are external evidence rather than circular support. Equation (33) is not circular: monotonicity f(n,s) ≤ f(n,3) plus the known bound f(n,3) ≤ n^{5/6+o(1)} gives the claimed form for 3 ≤ s ≤ log n because √s ≤ √(log n) = n^{o(1)} is absorbed by the o(1) exponent. No fitted parameter is renamed as a prediction, no definitional identity forces a target quantity, and no uniqueness claim is imported from the authors' prior work to forbid alternatives. The possible technical issue raised about equation (36) concerns whether the container lemma's hypotheses are verified; that is a correctness or gap concern, not a circularity, since it does not reduce the derived bound to an input by construction. The distinct-slope upper bounds similarly rest on known Sidon-set facts and grid constructions, not on the theorem being proved. Overall, the derivation is self-contained against external benchmarks, and the honest finding is no circularity.
Assumptions & free parameters
assumptions (10)
- standard math Szemerédi-Trotter theorem and its line-count corollary
- standard math Saxton-Thomason hypergraph container theorem
- standard math Cooper-Mubayi sparse hypergraph coloring theorem
- standard math Chernoff bounds
- standard math Erdős-Szekeres monotone subsequence theorem
- standard math Prime number estimates of Rosser-Schoenfeld
- standard math Jarník's bound on lattice points on convex curves
- standard math Pomerance's collinear subset theorem for lattice point sequences
- standard math Sidon set maximum size at most 2*sqrt(n)+1
- standard math Gauss circle problem estimate
Cite this review
Pith. "Pith review of Subset Selection Problems in Planar Point Sets." pith.science (2026). https://pith.science/paper/QL3RLT5X
@misc{pith2026241214287,
author = {Pith},
title = {Pith review of: Subset Selection Problems in Planar Point Sets},
year = {2026},
howpublished = {\url{https://pith.science/paper/QL3RLT5X}},
note = {Machine review of arXiv:2412.14287}
}
abstract
Given a finite set satisfying condition $\mathcal{A}$, the subset selection problem asks, how large of a subset satisfying condition $\mathcal{B}$ can we find? We make progress on three instances of subset selection problems in planar point sets. Let $n,s\in\mathbb{N}$ with $n\geq s$, and let $P\subseteq\mathbb{R}^2$ be a set of $n$ points, where at most $s$ points lie on the same line. Firstly, we select a general position subset of $P$, i.e., a subset containing no $3$ points on the same line. This problem was proposed by Erd\H{o}s under the regime when $s$ is a constant. For $s$ being non-constant, we give new lower and upper bounds on the maximum size of such a subset. In particular, we show that in the worst case such a set can have size at most $O(n/s)$ when $n^{1/3}\leq s\leq n$ and $O(n^{5/6+o(1)}/\sqrt{s})$ when $3\leq s\leq n^{1/3}$. Secondly, we select a monotone general position subset of $P$, that is, a subset in general position where the points are ordered from left to right and their $y$-coordinates are either non-decreasing or non-increasing. We present bounds on the maximum size of such a subset. In particular, when $s=\Theta(\sqrt{n})$, our upper and lower bounds differ only by a logarithmic factor. Lastly, we select a subset of $P$ with pairwise distinct slopes. This problem was initially studied by Erd\H{o}s, Graham, Ruzsa, and Taylor on the grid. We show that for $s=O(\sqrt{n})$ such a subset of size $\Omega((n/\log{s})^{1/3})$ can always be found in $P$. When $s=\Theta(\sqrt{n})$, this matches a lower bound given by Zhang on the grid. As for the upper bound, we show that in the worst case such a subset has size at most $O(\sqrt{n})$ for $2\leq s\leq n^{3/8}$ and $O((n/s)^{4/5})$ for $n^{3/8}\leq s=O(\sqrt{n})$. The proofs use a wide range of tools such as incidence geometry, probabilistic methods, the hypergraph container method, and additive combinatorics.
Figures
Reference graph
Works this paper leans on
-
[1]
J´ ozsef Balogh, Robert Morris, and Wojciech Samotij, Independent sets in hypergraphs,Journal of the American Mathematical Society 28(3) (2015), 669–709
work page 2015
-
[2]
J´ ozsef Balogh and J´ ozsef Solymosi, On the number of points in general position in the plane,Discrete Analysis 16 (2018), 1–20
work page 2018
-
[3]
Imre B´ ar´ any and Maria Prodromou, On maximal convex lattice polygons inscribed in a plane convex set,Israel Journal of Mathematics 154(1) (2006), 337–360
work page 2006
-
[4]
Peter Braß, William Moser, and J´ anos Pach, Research Problems in Discrete Geometry , Springer, New York, 2005
work page 2005
-
[5]
Yaobin Chen, Xizhi Liu, Jiaxi Nie, and Ji Zeng, Random Tur´ an and counting results for general position sets over finite fields, Preprint arXiv:2309.07744, 2024
work page Pith review arXiv 2024
-
[6]
Felix Christian Clemen, Applications of sparse hypergraph colorings, Preprint arXiv:2406.01499, 2024
arXiv 2024
-
[7]
Jeff Cooper and Dhruv Mubayi, Coloring sparse hypergraphs, SIAM Journal on Discrete Mathematics 30(2) (2016), 1165–1180
work page 2016
-
[8]
Henry Dudeney, A puzzle with pawns, in Amusements in Mathematics , Nelson, Edinburgh, 1917, pp. 94
work page 1917
Show all 34 references
-
[9]
David Eppstein, Forbidden Configurations in Discrete Geometry, Cambridge University Press, Cambridge, 2018
2018
-
[10]
Paul Erd˝ os, On some metric and combinatorial geometric problems, Discrete Mathematics 60 (1986), 147–153
1986
-
[11]
Paul Erd˝ os, Ron Graham, Imre Ruzsa, and Herbert Taylor, Bounds for arrays of dots with distinct slopes or lengths, Combinatorica 12(1) (1992), 39–44
1992
-
[12]
Paul Erd˝ os and J´ anos Sur´ anyi,Topics in the Theory of Numbers , Springer, 2003
2003
-
[13]
Paul Erd˝ os and George Szekeres, A combinatorial problem in geometry, Compositio Mathematica 2 (1935), 463–470
1935
-
[14]
Hillel Furstenberg and Yitzhak Katznelson, A density version of the Hales-Jewett theorem for k = 3, Discrete Mathematics 75 (1989), 227–241
1989
-
[15]
Zolt´ an F¨ uredi, Maximal independent subsets in Steiner systems and in planar sets,SIAM Journal on Discrete Mathematics 4(2) (1991), 196–199
1991
-
[16]
William Timothy Gowers, A geometric Ramsey problem, available on https://mathoverflow.net/questions/ 50928/a-geometric-ramsey-problem , accessed September 2024
2024
-
[17]
William Timothy Gowers, General position subsets, available on http://www.openproblemgarden.org/op/ general_position_subsets, accessed November 2024
2024
-
[18]
P´ eter Hajnal and Endre Szemer´ edi, Two geometrical applications of the semi-random method, New Trends in Intuitive Geometry, Springer, 2018, 189–199
2018
-
[19]
Godfrey Harold Hardy, On the expression of a number as the sum of two squares, The Quarterly Journal of Mathematics 46 (1915), 263–283
1915
-
[20]
Vojtˇ ech Jarn ´ ık,¨Uber die Gitterpunkte auf konvexen Kurven, Mathematische Zeitschrift 2 (1926), 500–518
1926
-
[21]
Chaya Keller and Shakhar Smorodinsky, A new lower bound on Hadwiger-Debrunner numbers in the plane, Israel Journal of Mathematics 244 (2021), 649–680
2021
-
[22]
de/informatik/ThIS/downloads/publications/lefmann_no_three_submitted.pdf, accessed October 2024
Hanno Lefmann, Extensions of the no-three-in-line problem, Preprint available on https://www.tu-chemnitz. de/informatik/ThIS/downloads/publications/lefmann_no_three_submitted.pdf, accessed October 2024
2024
-
[23]
Michael Mitzenmacher and Eli Upfal, Probability and Computing: Randomized Algorithms and Probabilistic Analysis, Cambridge University Press, Cambridge, 2017
2017
-
[24]
Michael Payne and David Wood, On the general position subset selection problem, SIAM Journal on Discrete Mathematics 27(4) (2013), 1727–1733
2013
-
[25]
Phelps and Vojtˇ ech R¨ odl, Steiner triple systems with minimum independence number,Ars Combina- torica 21 (1986), 167–172
Kevin T. Phelps and Vojtˇ ech R¨ odl, Steiner triple systems with minimum independence number,Ars Combina- torica 21 (1986), 167–172
1986
-
[26]
Carl Pomerance, Collinear subsets of lattice point sequences–An analog of Szemer´ edi’s theorem, Journal of Combinatorial Theory, Series A 28(2) (1980), 140–149
1980
-
[27]
Oliver Roche-Newton and Krishnendu Bhowmick, Counting arcs in F2 q, Discrete & Computational Geometry 72(4) (2024), 1630–1646
2024
-
[28]
Oliver Roche-Newton and Audie Warren, Arcs in F2 q, European Journal of Combinatorics 103(103512) (2022), 1–15
2022
-
[29]
Barkley Rosser and Lowell Schoenfeld, Approximate formulas for some functions of prime numbers, Illinois Journal of Mathematics 6(1) (1962), 64–94
J. Barkley Rosser and Lowell Schoenfeld, Approximate formulas for some functions of prime numbers, Illinois Journal of Mathematics 6(1) (1962), 64–94
1962
-
[30]
David Saxton and Andrew Thomason, Hypergraph containers, Inventiones mathematicae 201 (2015), 925–992
2015
-
[31]
Andrew Suk and Ji Zeng, On higher dimensional point sets in general position, In 39th International Symposium on Computational Geometry (SoCG 2023), Leibniz International Proceedings in Informatics (LIPIcs) , 258(59) (2023), 1–13. 18
2023
-
[32]
Trotter, Extremal problems in discrete geometry, Combinatorica 3 (1983), 381–392
Endre Szemer´ edi and William T. Trotter, Extremal problems in discrete geometry, Combinatorica 3 (1983), 381–392
1983
-
[33]
Terence Tao and Van Vu, Additive Combinatorics, Cambridge University Press, Cambridge, 2006
2006
-
[34]
Zhen Zhang, A note on arrays of dots with distinct slopes, Combinatorica 13(1) (1993), 127–128. J´ozsef Balogh University of Illinois at Urbana-Champaign, USA Email address : jobal@illinois.edu Felix Christian Clemen Institute for Basic Science, South Korea Email address : fel...
1993
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