REVIEW 3 major objections 5 minor 28 references
On the Number of Vertices in a Hyperplane Section of a Polytope
T0 review · 3 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper proves that the cyclic polytope maximizes the number of vertices a hyperplane can cut from any convex polytope: $2(n-2)$ vertices in dimension 3 and $\lfloor n/2\rfloor\lceil n/2\rceil$ in higher dimensions.
desk verdict The main theorem is true and the paper has real new content, but the proof of Theorem 1.1 has a logical gap that needs a repair; the hypercube gap theorems are plausible but partly sketched. 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 argument is carried by the cyclic polytope $C_d(n)$ (the convex hull of $n$ points on the moment curve) together with a perturbation lemma, Lemma 2.1, that lets one replace any slicing hyperplane by one avoiding all vertices without decreasing the number of slice vertices. Gale's Evenness Condition then gives a complete description of how a hyperplane separates the vertices of $C_d(n)$: it leaves $a$ and $b$ vertices on the two sides and $i$ on the hyperplane, and the slice has exactly $ab + i$ vertices. For enumeration and hypercube results, the slicing poset $(P_u,\le)$ encodes all vertices and non-orthogonal edges in a direction $u$, and every slice is a maximal antichain in this poset; for the hypercube this poset is the extended hypercube poset, whose width is $\lceil d/2\rceil\binom{d}{\lfloor d/2\rfloor}$.
What would settle it
For any even dimension $d>4$, find an explicit hyperplane $H$ such that $H\cap Q_d$ has exactly $(d/2)\binom{d}{d/2}-1$ vertices; the simplest target is $d=6$, where the paper's table declares 59 a gap, so a single slice with 59 vertices would refute Theorem 1.5.
Extended reading notes
Core claim
The central claim is Theorem 1.1: among all $d$-polytopes with $n$ vertices, the maximum number $\nu(P)$ of vertices in a hyperplane section is attained by the cyclic polytope, with $\nu(C_d(n)) = 2(n-2)$ for $d=3$ and $\nu(C_d(n)) = \lfloor n/2\rfloor\lceil n/2\rceil$ for $d>3$. The proof shows, via Lemma 2.1, that a maximizing slice can be assumed to avoid all vertices of $P$, and then uses the Upper Bound Theorem and Gale's Evenness Condition to count the edges crossed by such a slice. The paper also determines the complete vertex slice sequence of $C_d(n)$: in dimension 3 it is $[2(n-2)]$, and in higher dimensions it consists exactly of values $ab + i$ with $a+b = n-i$ and $0\le i\le d$. For hypercubes, Theorem 1.4 locates the first gaps of the vertex slice sequence for every $d\ge4$, and Theorem 1.5 shows that for every even $d>4$ the number $(d/2)\binom{d}{d/2}-1$ is a gap in $VSS(Q_d)$.
Load-bearing premise
The most fragile step in the proof of Theorem 1.5 is the unproved geometric assertion that translating the unique maximizing slice of an even-dimensional hypercube back until it first passes through a vertex cannot reduce the vertex count by exactly one, because each central-level vertex has more than two incident edges in each neighboring level.
Editorial extensions
If this is right
- For any 3-polytope with $n$ vertices, no hyperplane slice can have more than $2(n-2)$ vertices, and cyclic polytopes realize every count from 1 to $2(n-2)$.
- In dimensions $d>3$, the maximum slice size grows like $n^2/4$, so a single hyperplane cannot cut exponentially many vertices even when the polytope itself has exponentially many.
- The complete description of $VSS(C_d(n))$ shows that high-dimensional cyclic polytopes have gaps exactly when the required number $i$ of vertices lying on the slicing hyperplane exceeds $d$.
- For hypercubes, the first missing vertex counts are known for all $d\ge4$, and in even dimensions the last value below the maximum is always missing, so $VSS(Q_d)$ always has a gap at the top.
- The poset-antichain theorem provides a finite combinatorial certificate for every slice of a given polytope and direction, enabling complete enumeration of combinatorially distinct sections.
Reading between the lines
- Going beyond the paper: the same extremal question can be turned around, asking how small $\nu(P)$ can be among $d$-polytopes with $n$ vertices; the paper fixes only the top end of this range.
- Going beyond the paper: because the slicing poset captures every slice as a maximal antichain, the same machinery could bound the number of $k$-dimensional faces of a slice, not just vertices, moving toward the broader open problem cited in [16].
- Going beyond the paper: the even-dimensional hypercube gap is parity-forced (vertex-free slices of $Q_d$ have even vertex counts), suggesting that other centrally symmetric polytopes may show analogous near-maximum gaps; a computational scan of cross-polytopes would test this.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies the function cv_P(H), the number of vertices in a hyperplane section of a convex polytope. Its central claim (Theorem 1.1) is that among d-polytopes with n vertices the cyclic polytope maximizes ν(P), with value 2(n−2) for d=3 and ⌊n/2⌋⌈n/2⌉ for d>3. The paper also introduces a slicing poset (Theorem 1.2), constructs infinitely many 3-polytopes with gaps in their vertex slice sequence (Theorem 1.3), and analyzes the vertex slice sequence of hypercubes up to dimension 7, including two general gap theorems (Theorems 1.4 and 1.5). The methods combine Gale evenness, the Upper Bound Theorem, graph connectivity, poset width, and computational experiments with provided code.
Significance. If the main theorem is correct, it gives a clean upper bound theorem for the number of vertices in hyperplane sections of polytopes, directly addressing part of Open Problem 7.7 in [16]. The explicit VSS description for cyclic polytopes, the poset framework for enumerating slices, and the new hypercube gap data are concrete and useful contributions. The paper is constructive, offers code for the hypercube computations, and does not rely on fitted parameters or circular reasoning; previous algorithmic work is used only as a comparison. However, two load-bearing proofs are incomplete as written: the reduction in Theorem 1.1 is a non sequitur, and the proof of Theorem 1.5 contains incorrect degree counts. The main claims may well be true, but the current version does not establish them.
major comments (3)
- [§2, Theorem 2.4 / Proof of Theorem 1.1] The step to bound ν(P) is not justified. The proof says that because the Upper Bound Theorem makes the cyclic polytope maximize the number of edges, it suffices to bound ν(Cd(n)). This is a non sequitur: UBT controls the total number of edges f1(P) ≤ f1(Cd(n)), whereas a slice counts only the edges crossed by one hyperplane. For d>3, f1(Cd(n)) = C(n,2), which is much larger than the claimed bound ⌊n/2⌋⌈n/2⌉, and for d=3, f1(Cd(n)) = 3n−6 is larger than 2(n−2). An additional argument is needed: for d>3, a vertex-free slice is a cut δ(S) in the 1-skeleton, so it has size at most |S|(n−|S|) ≤ ⌊n/2⌋⌈n/2⌉; for d=3 a separate planar dual or cycle bound is needed to obtain 2(n−2). Without such an argument, the upper-bound half of Theorem 1.1 is unsupported.
- [§5.1, Lemma 5.7 and proof of Theorem 1.5] The characterization of maximum antichains in Lemma 5.7 is stated and proved with incorrect level indices. With Ei = {e=(v1,v2) : ||v1||+1=||v2||=i}, the path count through an edge at level i is (d−i)!(i−1)!, whose minimum for even d occurs at i = d/2 and i = d/2+1; the lemma's E⌈d/2⌉ ∪ E⌊d/2⌋ collapses to E_{d/2} for even d, and the proof's 'i∈{d/2,d/2−1}' is inconsistent with that minimum. More seriously, the degree-count observation used in the proof of Theorem 1.5 is false as written: for e={u,v}∈Ei with u at level i−1, u is incident to d−i+1 edges in Ei and to i−1 edges in E_{i−1}, not to d−i and i. Consequently the assertion that endpoints of central-level edges have more than two incident edges in each of the consecutive levels is not true as stated (for d=6, a level-2 endpoint of an E3 edge has two edges in E2). Since this is exactly the step meant to exclude a drop of exactly one vertex when translating back to a vertex-containing slice, the proof of Theorem 1.5 is incomplete.
- [§5.2, Corollary 1.5.1] The exactness of Table 1 for Q6 and Q7 is certified only by the statement that all other values are realized by slices computed with the GitHub code. The paper does not specify the finite search space: which directions were used, how the authors know that these directions produce all possible values of cv_Qd, or how the enumeration was certified. Since Corollary 1.5.1 is a claim of exact VSS, the computational proof should include this information, for example a list of representative directions or an argument that the chosen set realizes every attainable slice type.
minor comments (5)
- [§4, Theorem 1.2] The statement should specify that an edge is counted only when H meets its relative interior. If edges are counted by mere intersection, then a hyperplane through a vertex makes all incident edges 'intersected' at that vertex, and the resulting set is not an antichain. The proof itself uses the relative-interior convention, so the statement should be aligned with it.
- [§5.1, Lemma 5.7] The set of central levels for even d should be E_{d/2} ∪ E_{d/2+1} in the statement, and the proof's minimum calculation should refer to the same two levels; the current E⌈d/2⌉ ∪ E⌊d/2⌋ is a typo that obscures the argument.
- [§5.1, proof of Lemma 5.5] In the case |H−∩V(Qd)| = 3, 'N(v1)∩N(v1) = v4' should read 'N(v1)∩N(v3) = v4'.
- [§2, Theorem 2.3] For d>3, the proof asserts that every triple (a,b,i) with a+b=n−i and i≤d is realizable by a suitable hyperplane, but it does not give the construction. A brief Gale-evenness argument showing how to place i points on H and split the remaining a and b vertices would make the VSS description of the cyclic polytope fully rigorous.
- [§5.2, Corollary 1.5.1] Please include the version or commit of the GitHub repository and a short description of the computational enumeration so that the claim that all listed values are attained can be reproduced independently.
Circularity Check
No circular reduction found: the main bounds are derived from external results (Upper Bound Theorem, Gale evenness, Steinitz, O'Neil width) and computational enumeration; the self-citation [6] is not load-bearing. The skeptical concern about Theorem 1.1's UBT step is a correctness gap, not a circularity.
full rationale
The paper's central claim, Theorem 1.1, is intended as an upper bound theorem for the number of vertices in hyperplane slices. The proof attempts to pass from the Upper Bound Theorem (external, McMullen) to a bound on slice vertices via Lemma 2.1. Even if that step is logically insufficient, it is not circular: the paper does not define the maximum slice vertex count in terms of the cyclic polytope, and it does not feed the target inequality back into its assumptions. The cyclic polytope values are computed directly from Gale's Evenness Condition, an independent classical result. The hypercube results rely on O'Neil's width result, also external, plus their own poset framework; the poset framework is not an ansatz imported from prior work but is introduced and proved in the paper. The paper cites its own prior work [6] for algorithmic context and for enumerating directions, but not to fix any constant or to prove any theorem used in the main derivation. The fragile step highlighted in the proof of Theorem 1.5 (the assertion that moving the hyperplane back cannot drop the vertex count by exactly one because central-level vertices have more than two incident edges in adjacent levels) is a geometric lemma stated without full detail; however, it does not reduce the conclusion to an input or to a fitted parameter. Likewise, the skeptical objection to the UBT step in Theorem 1.1 is a proof gap about why bounding slices of the cyclic polytope bounds all polytopes, not a circularity by definition. Overall, the derivation chain is not self-referential in a way that would make the predictions equivalent to their inputs.
Assumptions & free parameters
assumptions (8)
- standard math Upper Bound Theorem: Cd(n) maximizes each entry of the f-vector among d-polytopes with n vertices
- standard math Gale's Evenness Condition characterizes facets of cyclic polytopes
- standard math Balinski's theorem: the graph of a d-polytope is d-connected
- standard math Menger's theorem: k-connected graphs contain k disjoint paths between any two vertices
- standard math Steinitz's theorem: every 3-connected planar graph is realizable as a 3-polytope
- standard math O'Neil's 1971 width result: the maximum number of edges cut by a slice of Qd is ceil(d/2) times C(d, floor(d/2))
- standard math Any circuit and any cutset of a graph share an even number of edges
- domain assumption The linked Python program correctly enumerates all slice vertex counts for Q6 and Q7
Cite this review
Pith. "Pith review of On the Number of Vertices in a Hyperplane Section of a Polytope." pith.science (2026). https://pith.science/paper/ZBVUP7OL
@misc{pith2026241212419,
author = {Pith},
title = {Pith review of: On the Number of Vertices in a Hyperplane Section of a Polytope},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZBVUP7OL}},
note = {Machine review of arXiv:2412.12419}
}
abstract
We study the slices or sections of a convex polytope by affine hyperplanes. We present results on two key problems: First, we provide tight bounds on the maximum number of vertices attainable by a hyperplane slice of $d$-polytope (a sort of upper bound theorem) and discuss a new algorithm to find all sections. Second, we investigate the sequence of numbers of vertices produced by the different slices over all possible hyperplanes and analyze the gaps that arise in that sequence. We study these sequences for three-dimensional polytopes and for hypercubes. Our results were obtained with the help of large computational experiments, and we report on new data generated for hypercubes.
Figures
Figures from the paper (3 more)
Reference graph
Works this paper leans on
-
[16]
Combinatorics of sections of polyt opes and Coxeter groups in Lobachevsky spaces
Askold Khovanskii. Combinatorics of sections of polyt opes and Coxeter groups in Lobachevsky spaces. In The Coxeter legacy , pages 129–157. Amer. Math. Soc., Providence, RI, 2006
work page 2006
-
[1]
Volumes of sections of cubes and related prob lems
Keith Ball. Volumes of sections of cubes and related prob lems. In Geometric Aspects of Functional Analysis: Israel Seminar (GAF A) 1987–88 , pages 251–260. Springer, 2006
work page 1987
-
[2]
The minimum number of vertices of a simpl e polytope
David Barnette. The minimum number of vertices of a simpl e polytope. Israel Journal of Mathematics, 10:121–125, 1971
work page 1971
-
[3]
A proof of the lower bound conjecture for convex polytopes
David Barnette. A proof of the lower bound conjecture for convex polytopes. Pacific Journal of Mathematics , 46:349–354, 1973
work page 1973
-
[4]
David Barnette. On generating planar graphs. Discrete Mathematics, 7(3-4):199–208, 1974
work page 1974
-
[5]
John Adrian Bondy and Uppaluri Siva Ramachandra Murty. Graph theory. Springer Publishing Company, Incorporated, 2008. 20
work page 2008
-
[6]
The best ways to slice a polytope
Marie-Charlotte Brandenburg, Jes´ us De Loera, and Chia ra Meroni. The best ways to slice a polytope. Mathematics of Computation , 94(352):1003–1042, 2025
work page 2025
-
[7]
Same average in every di rection
Imre B´ ar´ any and Gabor Domokos. Same average in every di rection. Bulletin Math´ ematique de la Soci´ et´ e des Sciences Math´ ematiques de Roumanie, 67(2):125–138, 2024
work page 2024
Show all 28 references
-
[8]
Don Chakerian and Dave Logothetti
G. Don Chakerian and Dave Logothetti. Cube slices, picto rial triangles, and probability. Math. Mag., 64(4):219–241, 1991. doi:10.2307/2690829
1991 doi
-
[9]
Sections of hyper-cube in five dimensions
Hiroshi Fukuda, Nobuaki Muto, Kikuko Goto, and Gisaku Na kamura. Sections of hyper-cube in five dimensions. Forma, 12(1):15–33, 1997
1997
-
[10]
Poset topology, mo ves, and bruhat interval polytope lattices, 2024
Christian Gaetz and Patricia Hersh. Poset topology, mo ves, and bruhat interval polytope lattices, 2024. URL: https://arxiv.org/abs/2410.08076, arXiv:2410.08076
2024
-
[11]
Neighborly and cyclic polytopes
David Gale. Neighborly and cyclic polytopes. In Proc. Sympos. Pure Math , volume 7, pages 225–232, 1963
1963
-
[12]
Inequalities for sections and projections of convex bodies
Apostolos Giannopoulos, Alexander Koldobsky, and Art em Zvavitch. Inequalities for sections and projections of convex bodies. Harmonic analysis and convexity , 9:223–256, 2023
2023
-
[13]
Bernd Gonska and G¨ unter M. Ziegler. Inscribable stack ed polytopes. arXiv preprint arXiv:1111.5322, 2011
2011 arXiv
-
[14]
Springer, 2nd edition, 2003
Branko Gr¨ unbaum.Convex Polytopes, volume 221 of Graduate Texts in Mathematics. Springer, 2nd edition, 2003
2003
-
[15]
Posets arising as 1-skeleta of simple p olytopes, the nonrevisiting path conjecture, and poset topology
Patricia Hersh. Posets arising as 1-skeleta of simple p olytopes, the nonrevisiting path conjecture, and poset topology. Discret. Comput. Geom. , 72(4):1463–1492, 2024. URL: https://doi.org/10.1007/s00454-023-00588-9 , doi:10.1007/S00454-023-00588-9
2024 doi
-
[17]
Affirmative resolution of Bourgain’s slicing problem using Guan’s bound, 2024
Boaz Klartag and Joseph Lehec. Affirmative resolution of Bourgain’s slicing problem using Guan’s bound, 2024. URL: https://arxiv.org/abs/2412.15044, arXiv:2412.15044
2024 arXiv
-
[18]
The slicing problem by Bourgain
Bo´ az Klartag and Vitali Milman. The slicing problem by Bourgain. In Artur Avila, Michael Th. Rassias, and Yakov Sinai, editors, Analysis at Large: Dedicated to the Life and Work of Jean Bourgain , pages 203–231. Springer International Publishing, Cham, 2022. doi:10.1007/978-...
2022 doi
-
[19]
Harmonic Analysis and Convexity , volume 9
Alexander Koldobsky and Alexander Volberg. Harmonic Analysis and Convexity , volume 9. Walter de Gruyter GmbH & Co KG, 2023
2023
-
[20]
Oeuvres compl` etes de Laplace, volume 7
Pierre Simon Laplace. Oeuvres compl` etes de Laplace, volume 7. Gautier-Villars, 1835
-
[21]
Cutting the d-cube
Jim Lawrence. Cutting the d-cube. J. Res. Nat. Bur. Standards , 84(1):51–53 (1978), 1979. doi:10.6028/jres.084.004
1978 doi
-
[22]
The maximum numbers of faces of a convex polytope
Peter McMullen. The maximum numbers of faces of a convex polytope. Mathematika, 17(2):179–184, 1970
1970
-
[23]
Comonotone curves and polyhedra
Theodore S Motzkin. Comonotone curves and polyhedra. Bull. Amer. Math. Soc. , 63, 1957. 21
1957
-
[24]
Extremal sections and pro jections of certain convex bodies: a survey
Piotr Nayar and Tomasz Tkocz. Extremal sections and pro jections of certain convex bodies: a survey. 2022. arXiv:2210.00885
2022 arXiv
-
[25]
Hyperplane cuts of an n-cube
Patrick E O’Neil. Hyperplane cuts of an n-cube. Discrete Mathematics , 1(2):193–195, 1971
1971
-
[26]
Polygons as sections of higher-dimensional polytopes
Arnau Padrol and Julian Pfeifle. Polygons as sections of higher-dimensional polytopes. Elec- tron. J. Combin. , 22(1):Paper 1.24, 16, 2015. doi:10.37236/4315
2015 doi
-
[27]
Shallow sections of the hypercube
Lionel Pournin. Shallow sections of the hypercube. Israel Journal of Mathematics , 11 2022. doi:10.1007/s11856-022-2400-9
2022 doi
-
[28]
Lectures on polytopes , volume 152
G¨ unter M Ziegler. Lectures on polytopes , volume 152. Springer Science & Business Media, 2012. 22
2012
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.