REVIEW 4 major objections 4 minor 32 references
Decremental Greedy Polygons and Polyhedra Without Sharp Angles
T0 review · 4 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Every monotone bottleneck subset problem has a unique maximal optimal subset, and decremental greedy always finds it.
desk verdict The 2D max-min-angle algorithm and monotone bottleneck framework are solid and new, but the 3D solid-angle theorem rests on an unproved monotonicity claim and the appendix contains a false intermediate equality, so it deserves serious review but needs 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 load-bearing object is the monotone bottleneck subset problem: given a set $U$, a quality $q(x,S)$ for each $x \in S \subseteq U$ that is monotone non-increasing as $S$ shrinks, and a goal of maximizing $Q(S)=\min_{x\in S} q(x,S)$ over nonempty subsets. The proof mechanism is the observation that the union $M$ of all optimal subsets is itself optimal: for any $x$ in that union, $q(x,M) \ge q(x,X) \ge \beta$ because monotonicity says an element's quality can only be better in a larger set. Hence every element outside $M$ has quality below the optimum in any superset of $M$, so deleting elements outside $M$ is always safe. The decremental greedy algorithm—delete any element with quality at most the current best and record improvements—and the known-$\beta$ variant both terminate at $M$. In the geometric applications this is instantiated by dynamic convex hull data structures that report the sharpest-angle hull vertex; in the graph applications, by bottleneck cycles in directed, mixed, and polar graphs.
What would settle it
Take a random set of 6 to 8 points in $\mathbb{R}^3$, compute the convex hull solid angle at every hull vertex before and after deleting each other hull vertex, and compare the greedy output with a brute-force search over all subsets for the maximum minimum solid angle; any deletion that increases a survivor's solid angle, or any greedy-versus-brute-force mismatch, would falsify the monotonicity behind the $O(n^2)$ polyhedron algorithm.
Extended reading notes
Core claim
The paper's central claim is that when the quality of each element is monotone non-increasing under deletion of other elements, the bottleneck subset problem—find a nonempty set maximizing the minimum quality of its members—can be solved by deleting low-quality elements. Theorem 1 proves that the union of all optimal subsets is itself optimal, so optimal subsets have a unique maximal member, and that both the decremental greedy algorithm and the known-$\beta$ algorithm reach this maximal optimal subset regardless of tie-breaking. Applied to geometry, the quality of a point is its interior angle when it is a convex hull vertex and $2\pi$ otherwise, or in 3D its solid angle, and monotonicity makes it safe to delete the sharpest remaining hull vertex; the best hull seen during deletion is a max-min-angle polygon or a max-min-solid-angle polyhedron. The paper also formalizes bottleneck cycles in directed, mixed, and polar graphs, giving deterministic algorithms for max-min-angle closed polygonal curves in 3D and proving hardness for the simple-curve restriction.
Load-bearing premise
The entire framework rests on the monotonicity axiom: deleting points can only keep a remaining hull vertex's angle, or solid angle, the same or smaller, so a sharp corner never appears at a survivor; this is obvious for 2D interior angles, but for 3D solid angles it is asserted rather than proved.
Editorial extensions
If this is right
- A max-min-angle closed polygonal curve through $n$ planar points can be found in $O(n\log n)$, and an optimal curve can be assumed convex.
- A max-min-solid-angle non-self-intersecting polyhedral surface through $n$ 3D points can be found in $O(n^2)$, and an optimal surface can be assumed a convex polyhedron.
- Max-min-angle closed 3D curves allowing repeated vertices can be found in near-cubic time: $O(n^3\log^* n)$ with repeated segments, and $O(n^3(\log n\log\log n)^2)$ without repeated segments.
- Requiring the 3D curve to be simple, or unknotted, makes the max-min-angle problem NP-complete.
- Graph degeneracy, computed by repeatedly deleting a minimum-degree vertex, is a special case of the same framework.
Reading between the lines
- The same greedy template could be applied to other monotone geometric quality measures, such as minimum edge length or minimum circumradius, wherever deleting points cannot improve the quality of a survivor.
- The 3D solid-angle monotonicity is the most exposed assumption: a proof of it would close the only gap between the abstract theorem and the polyhedron algorithm, while a small counterexample would invalidate the $O(n^2)$ result.
- The antimatroid characterization suggests that the correctness of the algorithm depends only on the monotone availability of elements, not on the geometric origin of the quality measure, so the framework may extend to other bottleneck objectives beyond angles and degrees.
- The NP-hardness boundary for 3D curves indicates that similar greedy methods are unlikely to exist for curve classes carrying global non-self-intersection constraints.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces a formal framework of monotone bottleneck subset problems, in which a quality measure q(x,S) is monotone nondecreasing as the ambient set S grows. It proves that every such problem has a unique maximal bottleneck subset, and that a decremental greedy algorithm that repeatedly removes an element whose quality is no better than the best quality seen so far always finds this subset. The framework is applied to three geometric settings: (1) computing a max-min-angle polygon in a planar point set in O(n log n) time, (2) computing a max-min-solid-angle convex polyhedron in a 3D point set in O(n^2) time, and (3) computing max-min-angle closed polygonal curves in 3D via reductions to bottleneck cycles in directed, mixed, and polar graphs. The paper also shows NP-hardness for the non-self-intersecting variant of the 3D curve problem, and connects the removal sequences to antimatroids. Appendix A is devoted to the antimatroid property of the decremental greedy algorithm, Appendix B to the NP-hardness proof, and Appendix C to the graph bottleneck-cycle algorithms.
Significance. If the claims are fully substantiated, the paper provides an elegant and useful unification of several known algorithms, including graph degeneracy, with new applications. The 2D max-min-angle polygon result is a clean improvement over dynamic programming approaches, and the bottleneck-cycle algorithms for directed, mixed, and polar graphs are likely to be of independent interest. The paper is generally clearly written, and Theorem 1 is a genuinely useful structural insight. However, the 3D polyhedron result rests on an unproved monotonicity assertion, the dynamic hull data structure is presented only as a sketch, and Appendix A contains a false intermediate claim. These issues prevent the manuscript from being acceptable in its present form, though they appear to be repairable within the scope of the paper.
major comments (4)
- [Section 3.2, Lemma 4] The monotonicity of the solid-angle quality function is asserted but not proved. The correctness of the decremental greedy algorithm in Theorem 5 relies on q(x,S) ≤ q(x,T) for x∈S⊂T; please provide a proof, e.g., via the fact that conv(S)⊆conv(T) implies the tangent cone at a surviving hull vertex of the smaller hull is contained in the corresponding tangent cone of the larger hull, and handle explicitly the cases where x is interior to one of the two hulls. The current one-sentence justification in Lemma 4 ('This can only remove vertices and improve the solid angle at the remaining vertices') is insufficient for the application of Theorem 1.
- [Appendix A, Lemma 8] The statement 'Note that Q(X+) = better(Q)' is false in general. For monotone quality functions one can construct instances in which the union of all subsets with quality at least better(Q) has a strictly higher quality than better(Q); only the inequality Q(X+) ≥ better(Q) is guaranteed. Since the subsequent argument only needs Q(S+) > Q(T), the proof can be repaired by replacing the equality with this inequality, but the false claim must be corrected.
- [Section 3.2, Theorem 5] The O(n^2) time bound relies on a dynamic hull data structure that is only sketched. Please specify how two child convex hulls are merged in linear time in 3D (or cite a reference for linear-time merging of convex polytopes), and explain how the global hull is queried to find the vertex with minimum solid angle in O(n) time after each deletion. The present description leaves the claimed per-deletion O(n) update time insufficiently supported.
- [Appendix B] The NP-hardness reduction for maxmin-angle simple polygons in 3D is presented as a high-level sketch. Several quantitative assertions (the choice of ε, the Ω(1/n^2) spacing, the perturbation argument using O(1/n^3) movements, and the claimed angle gap) need precise statements and proofs to make the reduction rigorous. As it stands, the proof cannot be checked in detail from the text; please expand it or clearly mark the result as conditional on the described construction.
minor comments (4)
- [Appendix C.4, Theorem 16] The phrase 'in time O(m(log m log log m)^2 per edge removal' appears to be a typo; if it were per removal the total would be quadratic. Please state the intended total time or per-removal bound.
- [Appendix B] The threshold θ is inconsistently given as '2π − Θ(ε)' and later as 'π − Θ(1/n)'; the latter is the correct form since polygonal angles are at most π.
- [Conclusions] In the final paragraph, 'given pints' should be 'given points'.
- [Appendix A, Lemma 8] The monotonicity statement of t contains a missing parenthesis: it should read Q(t(S)) ≥ Q(t(S')).
Circularity Check
No circularity: the derivation chain is self-contained, with the unproved 3D solid-angle monotonicity a correctness gap rather than a circular step.
full rationale
The paper's central framework is developed from first principles: Theorem 1 is proved directly from the definition of monotone quality, and the geometric algorithms apply that theorem to independently defined angle and solid-angle measures. The 2D angle monotonicity is argued geometrically, and the 3D solid-angle monotonicity is asserted in Lemma 4 without a proof; this is an unsupported correctness-critical claim, but it is not circular, because it is not derived from the target theorem or from any fitted parameter. The polygonal-curve results reduce to independently defined bottleneck-cycle problems in directed and polar graphs, with algorithms proved in Appendix C. References to the author's own prior work are contextual (e.g., curve reconstruction and minimum-area k-gons) and are not load-bearing for the new results. No prediction is equivalent to its input by construction, and no self-citation chain forces the conclusions. The absence of a proof for a monotonicity assertion is a correctness risk, not a circularity, so the appropriate circularity score is 0.
Assumptions & free parameters
assumptions (5)
- domain assumption Monotonicity of quality: for x ∈ S ⊂ T, q(x,S) ≤ q(x,T)
- domain assumption Interior angle at a convex hull vertex is non-increasing when other vertices are deleted
- domain assumption Solid angle at a vertex of a convex polyhedron is non-increasing when other vertices are deleted
- standard math Dynamic convex hull data structures with O(log n) update (2D) and O(n) per deletion (3D) exist
- ad hoc to paper In the NP-hardness reduction, points on a unit sphere can be sampled with spacing ε to achieve angle thresholds and gaps
Cite this review
Pith. "Pith review of Decremental Greedy Polygons and Polyhedra Without Sharp Angles." pith.science (2026). https://pith.science/paper/R6YADY3W
@misc{pith2026250704538,
author = {Pith},
title = {Pith review of: Decremental Greedy Polygons and Polyhedra Without Sharp Angles},
year = {2026},
howpublished = {\url{https://pith.science/paper/R6YADY3W}},
note = {Machine review of arXiv:2507.04538}
}
abstract
We show that the max-min-angle polygon in a planar point set can be found in time $O(n\log n)$ and a max-min-solid-angle convex polyhedron in a three-dimensional point set can be found in time $O(n^2)$. We also study the maxmin-angle polygonal curve in 3d, which we show to be $\mathsf{NP}$-hard to find if repetitions are forbidden but can be found in near-cubic time if repeated vertices or line segments are allowed, by reducing the problem to finding a bottleneck cycle in a graph. We formalize a class of problems on which a decremental greedy algorithm can be guaranteed to find an optimal solution, generalizing our max-min-angle and bottleneck cycle algorithms, together with a known algorithm for graph degeneracy.
Figures
Figures from the paper (3 more)
Reference graph
Works this paper leans on
- [1]
- [2]
-
[3]
D. Attali. r -regular shape reconstruction from unorganized points . Comput. Geom. 10(4):239 -- 247, 1998, http://dx.doi.org/10.1016/S0925-7721(98)00013-3 doi: 10.1016/S0925-7721(98)00013-3
-
[4]
D. Avis and D. Rappaport. Computing the largest empty convex subset of a set of points . Proc. 1st Symp. on Computational Geometry (SOCG '85) , pp. 161 -- 167. ACM, 1985, http://dx.doi.org/10.1145/323233.323255 doi: 10.1145/323233.323255
-
[5]
G. S. Brodal and R. Jacob. Dynamic planar convex hull . Proc. 43rd Symp. Foundations of Computer Science (FOCS 2002) , pp. 617 -- 626. IEEE Computer Society, 2002, http://dx.doi.org/10.1109/SFCS.2002.1181985 doi: 10.1109/SFCS.2002.1181985
arXiv 2002
-
[6]
K. Buchin and W. Mulzer. Delaunay triangulations in O ( sort (n) ) time and more . J. ACM 58(2):1 -- 27, 2011, http://dx.doi.org/10.1145/1944345.1944347 doi: 10.1145/1944345.1944347
arXiv 2011
- [7]
-
[8]
T. M. Chan and K. Tsakalidis. Optimal deterministic algorithms for 2-d and 3-d shallow cuttings . Discrete Comput. Geom. 56(4):866 -- 881, 2016, http://dx.doi.org/10.1007/s00454-016-9784-4 doi: 10.1007/s00454-016-9784-4
Show all 32 references
-
[9]
Chv \'a tal and G
V. Chv \'a tal and G. Klincsek. Finding largest convex subsets . Congressus Numerantium 29:453 -- 460, 1980, https://www.ams.org/mathscinet-getitem?mr=608447 MR608447
1980
-
[10]
Duan and S
R. Duan and S. Pettie. Fast algorithms for (max, min)-matrix multiplication and bottleneck shortest paths . Proc. 20th ACM-SIAM Symp. on Discrete Algorithms (SODA '09) , pp. 384 -- 391, 2009, https://portal.acm.org/citation.cfm?id=1496813
2009
-
[11]
Eppstein, M
D. Eppstein, M. Overmars, G. Rote, and G. J. Woeginger. Finding minimum area k -gons . Discrete Comput. Geom. 7(1):45 -- 58, 1992, http://dx.doi.org/10.1007/BF02187823 doi: 10.1007/BF02187823 , https://www.ams.org/mathscinet-getitem?mr=1134451 MR1134451
1992 doi
-
[12]
Fern \'a ndez, R
E. Fern \'a ndez, R. Garfinkel, and R. Arbiol. Mosaicking of aerial photographic maps via seams defined by bottleneck shortest paths . Operations Research 46(3):293 -- 304, 1998, http://dx.doi.org/10.1287/opre.46.3.293 doi: 10.1287/opre.46.3.293
1998 doi
-
[13]
Formann, T
M. Formann, T. Hagerup, J. Haralambides, M. Kaufmann, F. T. Leighton, A. Symvonis, E. Welzl, and G. Woeginger. Drawing graphs in the plane with high resolution . SIAM J. Comput. 22(5):1035 -- 1052, 1993, http://dx.doi.org/10.1137/0222063 doi: 10.1137/0222063 , https://www.ams....
1993 doi
-
[14]
H. N. Gabow and R. E. Tarjan. Algorithms for two bottleneck optimization problems . J. Algorithms 9(3):411 -- 417, 1988, http://dx.doi.org/10.1016/0196-6774(88)90031-4 doi: 10.1016/0196-6774(88)90031-4 , https://www.ams.org/mathscinet-getitem?mr=955149 MR955149
1988 doi
-
[15]
Hershberger and S
J. Hershberger and S. Suri. Applications of a semi-dynamic convex hull algorithm . BIT 32(2):249 -- 267, June 1992, http://dx.doi.org/10.1007/bf01994880 doi: 10.1007/bf01994880
1992 doi
-
[16]
B. Joe. Delaunay versus max-min solid angle triangulations for three-dimensional mesh generation . Int. J. Numer. Methods Eng. 31(5):987 -- 997, 1991, http://dx.doi.org/10.1002/nme.1620310511 doi: 10.1002/nme.1620310511
1991 doi
-
[17]
Korte, L
B. Korte, L. Lov \'a sz, and R. Schrader. Chapter III: Abstract Convexity -- Antimatroids . Greedoids , pp. 19 -- 43. Springer-Verlag, 1991, http://dx.doi.org/10.1007/978-3-642-58191-5_3 doi: 10.1007/978-3-642-58191-5_3
1991 doi
-
[18]
Malitz and A
S. Malitz and A. Papakostas. On the angular resolution of planar graphs . SIAM J. Discrete Math. 7(2):172 -- 183, 1994, http://dx.doi.org/10.1137/S0895480193242931 doi: 10.1137/S0895480193242931 , https://www.ams.org/mathscinet-getitem?mr=1271989 MR1271989
1994 doi
-
[19]
D. W. Matula and L. L. Beck. Smallest-last ordering and clustering and graph coloring algorithms . J. ACM 30(3):417 -- 427, 1983, http://dx.doi.org/10.1145/2402.322385 doi: 10.1145/2402.322385 , https://www.ams.org/mathscinet-getitem?mr=0709826 MR0709826
1983
-
[20]
S. A. Mitchell. Approximating the maxmin-angle covering triangulation . Comput. Geom. 7(1 -- 2):93 -- 111, 1997, http://dx.doi.org/10.1016/0925-7721(95)00046-1 doi: 10.1016/0925-7721(95)00046-1
1997 doi
-
[21]
Ohrhallinger, J
S. Ohrhallinger, J. Peethambaran, A. D. Parakkat, T. K. Dey, and R. Muthuganapathy. 2D points curve reconstruction survey and benchmark . Computer Graphics Forum 40(2):611 -- 632, 2021, http://dx.doi.org/10.1111/cgf.142659 doi: 10.1111/cgf.142659
2021 doi
-
[22]
M. H. Overmars. The Design of Dynamic Data Structures . Lecture Notes in Computer Science 156. Springer, 1983, http://dx.doi.org/10.1007/BFB0014927 doi: 10.1007/BFB0014927
1983 doi
-
[23]
M. Pollack. The maximum capacity through a network . Operations Research] 8(5):733 -- 736, 1960, http://dx.doi.org/10.1287/opre.8.5.733 doi: 10.1287/opre.8.5.733
1960 doi
-
[24]
M. Schulze. A new monotonic, clone-independent, reversal symmetric, and Condorcet-consistent single-winner election method . Social Choice and Welfare 36(2):267 -- 303, 2011, http://dx.doi.org/10.1007/s00355-010-0475-4 doi: 10.1007/s00355-010-0475-4
2011 doi
-
[25]
R. Sibson. Locally equiangular triangulations . The Computer J. 21(3):243 -- 245, 1978, http://dx.doi.org/10.1093/comjnl/21.3.243 doi: 10.1093/comjnl/21.3.243
1978 doi
-
[26]
Ullah, K
E. Ullah, K. Lee, and S. Hassoun. An algorithm for identifying dominant-edge metabolic pathways . IEEE/ACM International Conference on Computer-Aided Design (ICCAD 2009) , pp. 144 -- 150, 2009, https://ieeexplore.ieee.org/document/5361299
2009
-
[27]
P. M. Camerini. The min-max spanning tree problem and some extensions . Information Processing Letters 7(1):10 -- 14, 1978, http://dx.doi.org/10.1016/0020-0190(78)90030-3 doi: 10.1016/0020-0190(78)90030-3
1978 doi
-
[28]
M. Cook. Still life theory . New Constructions in Cellular Automata , pp. 93 -- 118. Oxford University Press, Santa Fe Institute Studies in the Sciences of Complexity, 2003
2003
-
[29]
A. V. Goldberg and A. V. Karzanov. Path problems in skew-symmetric graphs . Combinatorica 16(3):353 -- 382, 1996, http://dx.doi.org/10.1007/bf01261321 doi: 10.1007/bf01261321
1996 doi
-
[30]
J. Holm, E. Rotenberg, and M. Thorup. Dynamic bridge-finding in O( ^2 n) amortized time . Proc. 29th ACM -- SIAM Symp. Discrete Algorithms (SODA 2018) , pp. 35 -- 52. SIAM, 2018, http://dx.doi.org/10.1137/1.9781611975031.3 doi: 10.1137/1.9781611975031.3
2018 doi
-
[31]
J. F. Lynch. The equivalence of theorem proving and the interconnection problem . ACM SIGDA Newsletter 5(3):31 -- 36, 1975, http://dx.doi.org/10.1145/1061425.1061430 doi: 10.1145/1061425.1061430
1975
-
[32]
B. Zelinka. Polar graphs and railway traffic . Aplikace Matematiky 19:169 -- 176, 1974
1974
Reviewed August 6, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.