REVIEW 2 major objections 3 minor 39 references
Shortest filling geodesics on hyperbolic surfaces
T0 review · 2 major / 3 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read For every closed orientable hyperbolic surface of genus g, the shortest total length of a filling multi-geodesic is exactly half the perimeter of the regular right-angled hyperbolic (8g−4)-gon, and a single filling geodesic attains it.
desk verdict Likely-true main theorem, but the proof has a load-bearing gap in the isoperimetric inequality and a concrete miscount in the construction; worth refereeing after 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 argument rests on two mechanisms: a reduction (Theorem 2.1) that converts any filling multi-geodesic into a filling geodesic graph of no greater length whose complementary polygons each have at least five sides, with the side-count identity sum(m_i − 4) = 8g − 8; and an isoperimetric inequality (Theorem 3.1) that bounds the total perimeter of such a polygon family below by P_{8g−4}. The inequality is proved by studying perimeter functions of regular hyperbolic polygons and relies on a generalization of Sanki-Vadnere's even-polygon isoperimetric result.
What would settle it
Construct a genus-g hyperbolic surface and a filling multi-geodesic of total length strictly less than 1/2 P_{8g−4}, or exhibit a filling multi-geodesic of length exactly 1/2 P_{8g−4} whose complement is not a regular right-angled (8g−4)-gon.
Extended reading notes
Core claim
The central discovery is that the filling length infimum over the moduli space of genus-g hyperbolic surfaces equals 1/2 P_{8g−4}, where P_k is the perimeter of the regular right-angled hyperbolic k-gon. The proof proceeds in two steps: first, any filling multi-geodesic is shortened to a filling geodesic graph whose complement has only polygons with at least five sides and satisfies sum(m_i − 4) = 8g − 8; second, an isoperimetric inequality shows the total perimeter of such a collection is at least P_{8g−4}, with equality only for a single regular right-angled (8g−4)-gon. An explicit construction exhibits a single filling geodesic of this length, so the bound is sharp.
Load-bearing premise
The lower bound rests on the reduction in Theorem 2.1, which asserts that every filling multi-geodesic can be replaced by a filling geodesic graph of no greater length whose complementary polygons all have at least five sides; if that reduction produced triangles or increased length, the isoperimetric inequality of Theorem 3.1 could not be applied and the bound would not follow.
Editorial extensions
If this is right
- The bound is sharp for every genus g ≥ 2: a single filling geodesic whose complement is a regular right-angled (8g−4)-gon has length exactly 1/2 P_{8g−4}.
- Equality is rigid: a filling multi-geodesic of length 1/2 P_{8g−4} must be a single geodesic whose complement is the regular right-angled (8g−4)-gon.
- For surfaces whose systole set fills, the kissing number satisfies sys(X) · kiss(X) ≥ 1/2 P_{8g−4}, yielding kiss(X) ≥ (3.525) g / log g for large g, improving the previous constant.
- The reduction and isoperimetric inequality apply to arbitrary filling multi-geodesics, including those with multiple intersection points and self-intersections, settling a conjecture previously known only for filling pairs.
Reading between the lines
- The triangle-free condition in the reduction is essential, since the isoperimetric inequality fails for triangles; this suggests that any relaxation of the filling-graph hypothesis would need an explicit correction term that accounts for triangular regions.
- The same two-step strategy (reduce the graph, then apply a polygon isoperimetric inequality) could be adapted to other settings, such as filling curves on punctured surfaces or on non-orientable surfaces, where the relevant regular polygon side count would change.
- The equality case forces the surface to be a regular polygonal gluing, hinting that global minimizers of filling length in moduli space are highly symmetric; it would be natural to ask whether these are the only critical points of the filling-length function.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims to determine the minimal length of a filling multi-geodesic on a closed orientable hyperbolic surface of genus g, minimized over moduli space. Theorem 1.1 states this minimum equals half the perimeter of a regular right-angled (8g−4)-gon, and that it is realized by a single filling geodesic. The proof has two main ingredients: Theorem 2.1, which replaces a filling multi-geodesic by a filling geodesic graph of no larger length whose complementary components are polygons with at least five sides and satisfy a side-count identity, and Theorem 3.1, an isoperimetric inequality comparing the perimeter sum of such polygons with the perimeter of a regular polygon of the same area. Section 4 constructs a hyperbolic surface and a single filling geodesic whose complement is the desired regular polygon.
Significance. If correct, the result settles a natural question about filling geodesics and extends the Aougab–Huang conjecture from filling pairs to arbitrary filling multi-geodesics. The construction of a single filling geodesic attaining the bound is explicit, and the corollaries on the kissing number of surfaces with filling systole are of independent interest. The paper also provides a clean statement of the isoperimetric inequality for polygons with at least four sides, which is a useful contribution in itself. However, the proof of the isoperimetric theorem is currently incomplete, so the main result is not yet established as written.
major comments (2)
- [Section 2.3, proof of Theorem 2.1] The theorem is stated for arbitrary hyperbolic mi-gons, but the proof from Proposition 3.8 onward only treats regular polygons. Proposition 3.8 assumes D1 and D2 are regular hyperbolic polygons, Lemma 3.10 assumes each Di is regular, and the induction in the proof of Theorem 3.1 compares the regular replacements eD_j. There is no argument showing that, among hyperbolic polygons with a fixed number of sides and fixed area, the regular polygon minimizes perimeter. Reference [10] (Bezdek) is listed but never cited in the proof. Consequently the key step in the proof of Theorem 1.1, namely ℓ(G) = 1/2 Σ Perim(D_i) ≥ 1/2 P_{8g−4}, does not follow for the arbitrary polygons produced by Theorem 2.1. This is the load-bearing connection between the graph reduction and the numerical lower bound, so the proof of Theorem 3.1 is incomplete as written.
- [Section 2.3, proof of Theorem 2.1] The iterative construction ends with the assertion: 'Since Γ is a finite filling graph, the repeating process will eventually stop.' This is not justified in the general case. The replacement step replaces an arc β by a shortest proper geodesic arc β′ that may not lie in Γ, so the intermediate graph Θ_i is no longer a subgraph of Γ. Finiteness of Γ therefore does not, by itself, bound the number of iterations. A rigorous termination argument—for instance, a monotone complexity measure for the pair (Θ_i, G_i) or a bound on the number of essential cutting curves needed to fill the surface—is required. The termination of the process is essential to the existence of the graph G, so this gap also affects the lower bound.
minor comments (3)
- [References] Reference [10] (Bezdek) is never cited in the body of the paper. Either it should be used to supply the missing regularization step in Theorem 3.1, or it should be removed.
- [Section 4] The claim that the curve α in Figure 12 is filling and that its complement is an (8g−4)-gon is stated without proof. Since the construction of the extremal surface depends on this assertion, a brief justification or a more detailed reading of the figure would be helpful.
- [Throughout] There are several typographical errors, including 'SHOR TEST' in the title, 'olygons' in Section 2.2, 'essencity' in Section 2.3, and some malformed summation indices such as 'kX k=1'. The manuscript would benefit from a careful proofreading pass.
Circularity Check
No circularity found: the main lower bound is derived from an independent isoperimetric inequality and a self-contained graph-reduction lemma; the noted proof gap in Theorem 3.1 is a completeness issue, not circularity.
full rationale
The derivation chain is not circular. Theorem 2.1 is a self-contained combinatorial-geometric reduction: it builds a filling convex geodesic graph G from Gamma with length(G) <= length(Gamma) and side counts m_i >= 5 satisfying sum(m_i - 4) = 8g - 8, using only Gauss-Bonnet and geodesic convexity arguments (Section 2). The lower bound then rests on Theorem 3.1, whose proof uses Proposition 3.5 from Sanki-Vadnere [29] - prior independent published work not by these authors - and calculus on the regular hyperbolic polygon perimeter function P_n(a). No equation in the paper defines the target (1/2)P_{8g-4} in terms of the quantities being bounded, and no fitted parameter is renamed as a prediction. The paper's self-citations ([32], [38]) appear only in background remarks on non-simple geodesics and are not load-bearing. The legitimate concern in this manuscript is a completeness gap in the proof of Theorem 3.1: the statement allows arbitrary hyperbolic m_i-gons, but from Proposition 3.8 onward the proof silently treats the D_i as regular (e.g., 'We consider a series of hyperbolic regular polygons eD1, eD2, ..., eDk' and then applies Proposition 3.8 to D1,D2), and the cited isoperimetric result [10] is never invoked to justify this regularization. That is a missing argument or correctness issue, not a circular reduction: Theorem 3.1 is not assumed in its own proof, and the main theorem would follow if the regularization lemma were supplied.
Assumptions & free parameters
assumptions (5)
- standard math Gauss-Bonnet theorem for hyperbolic surfaces and polygons
- standard math Hyperbolic polygon isoperimetric inequality: among convex hyperbolic m-gons of fixed area, the regular m-gon has minimal perimeter (Bezdek [10])
- domain assumption Sanki-Vadnere's Proposition 3.5 (convexity and monotonicity properties of perimeter functions)
- standard math Existence of a minimizer via Mumford compactness and the collar lemma
- ad hoc to paper Topological assertion that the curve in Figure 12 is filling with complement an (8g-4)-gon
Cite this review
Pith. "Pith review of Shortest filling geodesics on hyperbolic surfaces." pith.science (2026). https://pith.science/paper/NKKJTKL5
@misc{pith2026250612465,
author = {Pith},
title = {Pith review of: Shortest filling geodesics on hyperbolic surfaces},
year = {2026},
howpublished = {\url{https://pith.science/paper/NKKJTKL5}},
note = {Machine review of arXiv:2506.12465}
}
abstract
In this paper, we obtain the minimal length of a filling (multi-)geodesic on a genus $g$ hyperbolic surface in the moduli space of hyperbolic surfaces and show that it is realized by the geodesic whose complement is a right-angled regular $(8g-4)$-gon. A single geodesic realizing this minimum is provided.
Figures
Figures from the paper (10 more)
Reference graph
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