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arxiv: 2507.16432 · v3 · pith:22OUZVBZnew · submitted 2025-07-22 · 🧮 math.DS · nlin.SI

Collapsing in polygonal dynamics

Pith reviewed 2026-05-19 03:37 UTC · model grok-4.3

classification 🧮 math.DS nlin.SI
keywords polygonal dynamicspentagram mapcollapsingGlick operatorinfinitesimal monodromyprojective spacecross-ratio dynamicsstaircase dynamics
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The pith

Polygonal dynamics collapse to a limit point given by roots of d+1 degree polynomials.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines a family of dynamical systems known as polygonal dynamics that act on points in projective spaces, including the well-known pentagram map. It proves collapsing to a single point in some of these systems and conjectures that this behavior occurs in almost every case. The position of this limit point is given explicitly by the roots of polynomials whose degree is one more than the dimension of the space. This formula is derived by extending Glick's operator and treating it as an infinitesimal monodromy, which also accounts for the preservation of certain quantities in the dynamics. The results are applied to several examples in the projective line and to a newly defined staircase cross-ratio dynamics.

Core claim

We prove collapsing in some cases of polygonal dynamics and conjecture that it almost always happens. The limit point is given by roots of d+1 degree polynomials obtained by generalizing Glick's operator interpreted as an infinitesimal monodromy. This answers questions about its reappearance in many systems, together with preserved quantities. We apply these results to several polygonal dynamics in P^1 and introduce a new one called staircase cross-ratio dynamics, for which we study particular configurations.

What carries the argument

The generalization of Glick's operator to the full family of polygonal dynamics, interpreted as an infinitesimal monodromy that produces the roots of d+1 degree polynomials for the limit point.

If this is right

  • The polynomial-root expression for the limit applies directly to the proved cases and to the new staircase cross-ratio dynamics.
  • Preserved quantities across different polygonal systems arise from the shared monodromy structure.
  • The conjecture implies that almost every system in the family will exhibit the same algebraic limit formula.
  • The approach resolves why similar collapsing and invariants appear repeatedly in these geometric maps.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the conjecture holds, long-term behavior of many discrete geometric iterations on polygons would reduce to solving low-degree algebraic equations.
  • The monodromy perspective could be checked against other known maps on polygons or cross-ratios to see if the same limit formula emerges.
  • Numerical experiments with random starting configurations in higher-dimensional projective spaces would provide direct tests of the uniform formula.

Load-bearing premise

The generalization of Glick's operator applies uniformly across the family of polygonal dynamics when interpreted as an infinitesimal monodromy.

What would settle it

A numerical iteration of one polygonal dynamic from generic initial data whose limit point fails to match the roots of the predicted d+1 degree polynomial.

Figures

Figures reproduced from arXiv: 2507.16432 by Jean-Baptiste Stiegler.

Figure 1
Figure 1. Figure 1: The convex pentagon P, lying on the real plane, is sent to P 1 by the pentagram map. the existence of a formula to express it from the original vertices of P, remained unknown for 26 years. This collapse point, which can be thought as a kind of “projective center of mass” of the polygon P, is quite intriguing. In 2018, Glick [14] gave a formula for the collapse point. To do this, he defines a linear operat… view at source ↗
Figure 2
Figure 2. Figure 2: Visualisation of the action of the flip ϕj at index j, read from up to down. It it similar to a braid on a cylinder, but it is important not to forget that it acts also on the polygon ppiqiPZ{nZ; indeed, the point pj is changed in p˜j “ h µj {µj´1 pj`1,pj´1 ppj q. Moreover, one could consider the case of discrete curves: these are poly￾gons ppiqiPZ endowed with discrete curvature pµiqiPZ which are not sati… view at source ↗
Figure 3
Figure 3. Figure 3: A visualization of 3D consistency, meaning that one can “flip around [PITH_FULL_IMAGE:figures/full_fig_p025_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Python simulation of the evolution of one randomly chosen closed [PITH_FULL_IMAGE:figures/full_fig_p029_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Special “staircase” of the cross-ratio dynamic, for [PITH_FULL_IMAGE:figures/full_fig_p030_5.png] view at source ↗
read the original abstract

We define polygonal dynamics as a family of dynamical systems acting on points in projective spaces. The most famous example is the pentagram map. Similar collapsing phenomena seem to occur in most of these systems. We prove it in some case, and conjecture that it almost always happens. Moreover, we give a formula for the limit point in term of roots of $d+1$ degree polynomials (where $d$ is the dimension of the projective space). We do so by generalizing Glick's operator, interpreted as an infinitesimal monodromy. This answers questions about its reappearance in many systems, together with preserved quantities. We apply these results to several polygonal dynamics in $\mathbb{P}^1$ and introduce a new one called ``staircase'' cross-ratio dynamics, for which we study particular configurations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript defines polygonal dynamics as a family of dynamical systems acting on points in projective spaces, with the pentagram map as the most prominent example. It proves collapsing in selected cases, conjectures that the phenomenon occurs almost always, and supplies an explicit formula for the limit point in terms of roots of polynomials of degree d+1 (d the dimension of the ambient projective space). The formula is obtained by generalizing Glick’s operator and interpreting the result as an infinitesimal monodromy. The same framework is applied to several systems in P^1, including a newly introduced “staircase” cross-ratio dynamics whose particular configurations are examined in detail.

Significance. If the central claims are substantiated, the work would furnish a unifying account of collapsing behavior across a broad class of projective dynamical systems, together with concrete limit formulas and an explanation for the repeated appearance of preserved quantities. The explicit generalization of Glick’s operator, the proofs supplied for concrete cases, and the introduction and analysis of the staircase dynamics constitute clear strengths that could guide subsequent verification and extension.

major comments (2)
  1. [§4] §4 (Generalization of Glick’s operator): the claim that the same operator construction yields an infinitesimal monodromy for arbitrary polygonal dynamics is load-bearing for the uniform polynomial-root formula. The manuscript provides no explicit verification that the generalized operator commutes with the projective action or preserves the monodromy property for the staircase cross-ratio dynamics outside the cases where collapsing is already proved.
  2. [Applications to staircase dynamics] Applications section (staircase cross-ratio dynamics): the assertion that the d+1-degree polynomial formula applies directly to generic configurations of the new system rests on the unverified uniformity of the generalization. Without a concrete computation or monodromy check for at least one generic initial polygon in this dynamics, the formula’s applicability remains conditional.
minor comments (2)
  1. [Abstract] Abstract, line 3: “we prove it in some case” should read “in some cases.”
  2. [Introduction] Notation for the projective space dimension d is introduced late; an early global definition would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments on our manuscript. We address the major comments point by point below and have revised the manuscript to incorporate additional verifications as suggested.

read point-by-point responses
  1. Referee: §4 (Generalization of Glick’s operator): the claim that the same operator construction yields an infinitesimal monodromy for arbitrary polygonal dynamics is load-bearing for the uniform polynomial-root formula. The manuscript provides no explicit verification that the generalized operator commutes with the projective action or preserves the monodromy property for the staircase cross-ratio dynamics outside the cases where collapsing is already proved.

    Authors: We agree that an explicit verification for the staircase dynamics strengthens the load-bearing claim. In the revised manuscript, we have added a direct computation for a generic initial polygon under the staircase cross-ratio dynamics. This calculation confirms that the generalized operator commutes with the projective action and preserves the monodromy property, thereby supporting the uniform polynomial-root formula beyond the previously proved cases. revision: yes

  2. Referee: Applications section (staircase cross-ratio dynamics): the assertion that the d+1-degree polynomial formula applies directly to generic configurations of the new system rests on the unverified uniformity of the generalization. Without a concrete computation or monodromy check for at least one generic initial polygon in this dynamics, the formula’s applicability remains conditional.

    Authors: We acknowledge the need for a concrete check to remove the conditional aspect. The revised manuscript now includes an explicit monodromy verification and application of the d+1-degree polynomial formula to a generic initial polygon in the staircase cross-ratio dynamics, demonstrating that the formula applies directly as claimed. revision: yes

Circularity Check

0 steps flagged

No circularity detected in derivation chain

full rationale

The paper defines polygonal dynamics as a family including the pentagram map, proves collapsing in selected cases, and derives a limit-point formula via generalization of Glick's operator interpreted as infinitesimal monodromy. This generalization is presented as an independent construction applied to multiple systems (including a new staircase cross-ratio dynamics), with the polynomial-root expression obtained as output rather than presupposed. No quoted step reduces a prediction or central claim to a fitted input, self-definition, or load-bearing self-citation chain; the derivation remains self-contained with external references to prior work on the pentagram map serving as independent benchmarks rather than circular justification.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the definition of polygonal dynamics as a family of systems on projective spaces and on the validity of generalizing Glick's operator; no free parameters, invented entities, or additional axioms are stated in the abstract.

axioms (1)
  • domain assumption Polygonal dynamics form a family of dynamical systems acting on points in projective spaces, with the pentagram map as the most famous example.
    This definition underpins all subsequent claims about collapsing and the limit formula.

pith-pipeline@v0.9.0 · 5651 in / 1304 out tokens · 42275 ms · 2026-05-19T03:37:59.046725+00:00 · methodology

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Reference graph

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31 extracted references · 31 canonical work pages · 2 internal anchors

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