arXiv subjects

José Ayala

Publications and source records attributed to José Ayala.

17 recordsLinked to original sources

arXiv2025

Compression Maps Between Polytopes

A shape of a combinatorial polytope is a convex embedding into Euclidean space. We provide necessary and sufficient conditions for a piecewise linear map between two shapes of the same polytope to be a compression (respectively a weak compression), meaning a distance decreasing (respectively a distance non-increasing) map between distinct pairs of points. We establish that there is a partial order on the space of shapes given by the relation of having a weak compression map between pairs of shapes. Finally, we construct a compression metric on the projective shape space of a polytope; the space of convex Euclidean realizations modulo rigid motions and homothety. For the projective space shape of a simplex, we show that the compression metric is complete. For general polytopes, we establish that the projective shape space has a natural completion given by the projective shape space of weakly convex realizations.

math.MG↗

arXiv2025

Causality and thermodynamics in anisotropic fluids

We propose a thermodynamic formalism, within the particle-frame, for the energy-momentum tensor of irreversible anisotropic imperfect fluids subject to causality. Building on the Israel-Stewart extension of Eckart's theory, we further generalize these formalisms to incorporate anisotropic effects while ensuring the preservation of causality. In this framework, the second law of thermodynamics includes an additional term accounting for the system's anisotropy, which we derive explicitly in closed form for both first- and second-order theories. Notably, when anisotropy is removed, our model recovers Eckart's theory at first order and Israel-Stewart's at second order.

gr-qc↗

arXiv2025

Thin Gordian Unlinks

A gordian unlink is a finite number of unknots that are not topologically linked, each with prescribed length and thickness, and that cannot be disentangled into the trivial link by an isotopy preserving length and thickness throughout. In this note, we provide the first examples of gordian unlinks. As a consequence, we identify the existence of isotopy classes of unknots that differ from those in classical knot theory. More generally, we present a one-parameter family of gordian unlinks with thickness ranging in $[1,2)$ and absolute curvature bounded by 1, concluding that thinner normal tubes lead to different rope geometries than those previously considered. Knots or links in the one-parameter model introduced here are called thin knots or links. When the thickness is equal to 2, we obtain the standard model for geometric knots, also called thick knots.

math.GT↗

arXiv2025

Profiles of Critical Flat Ribbon Knots

The main open problem in geometric knot theory is to provide a tabulation of knots based on an energy criterion, with the goal of presenting this tabulation in terms of global energy minimisers within isotopy classes, often referred to as ideal knots. Recently, the first examples of minimal length diagrams and their corresponding length values have been determined by Ayala, Kirszenblat, and Rubinstein. This article is motivated by the scarcity of examples despite several decades of intense research. Here, we compute the minimal ribbonlength for some well-known knot diagrams, including the Salomon knot and the Turk's head knot. We also determine the minimal ribbonlength for the granny knot and square knot using a direct method. We conclude by providing the ribbonlength for infinite classes of critical ribbon knots, along with conjectures aimed at relating ribbonlength to knot invariants in pursuit of a metric classification of knots.

math.GT↗

arXiv2025

Geometric Constraints in Link Isotopy

We prove the existence of families of distinct isotopy classes of physical unknots through the key concept of parametrised thickness. These unknots have prescribed length, tube thickness, a uniform bound on curvature, and cannot be disentangled into a thickened round circle by an isotopy that preserves these constraints throughout. In particular, we establish the existence of \emph{gordian unknots}: embedded tubes that are topologically trivial but geometrically locked, confirming a long-standing conjecture. These arise within the space $\mathcal{U}_1$ of thin unknots in $\mathbb{R}^3$, and persist across a stratified family $\{ \mathcal{U}_τ\}_{τ\in [0,2]}$, where $τ$ denotes the tube diameter, or thickness. The constraints on curvature and self-distance fragment the isotopy class of the unknot into infinitely many disconnected components, revealing a stratified structure governed by geometric thresholds. This unveils a rich hierarchy of geometric entanglement within topologically trivial configurations.

math.GT↗

arXiv2025

Gordian Unlinks

This paper gives the first examples of gordian unlinks. The components of these unlinks cannot be separated while maintaining constant length and thickness. We construct infinite families of 2-component gordian unlinks and also construct $n$-component gordian unlinks for each $n \geq 2$.

math.GT↗

arXiv2023

The Radiant Massive Magnetic Dipole

We present an exact, time-dependent solution for the Einstein field equations that models the coupling between an anisotropic fluid and a magnetic field in an axially symmetric space-time. By carefully selecting the metric components, we achieve a convenient separation of variables that enables us to solve Einstein's field equations and obtain a solution that evolves into the Gutsunaev-Manko massive magnetic dipole. The analysis of the thermodynamic quantities suggests that this solution may represent a pulse of radiation emitted by a massive object with magnetic properties as for example pulsars or neutron stars.

gr-qc↗

arXiv2021

On the Blaschke rolling disk theorem

The Blaschke rolling disk theorem is a classical inclusion principle in differential geometry. This states that a planar convex domain whose boundary is a curve of class $C^2$ with (signed) curvature not exceeding a positive constant $κ$ is such that for each point on its boundary there exists a disk of radius ${1}/κ$ tangent to the boundary included in the closure of the domain. We describe geometric conditions relying exclusively on curvature and independent of any kind of convexity that allows us to give necessary and sufficient conditions for the existence of rolling disks for planar domains that are not necessarily convex. We finish by presenting an algorithm leading to a decomposition of any planar domain into a finite number of maximal rolling regions.

math.DG↗

arXiv2021

A Gradient Descent Method for The Dubins Traveling Salesman Problem

We propose a combination of a bounding procedure and gradient descent method for solving the Dubins traveling salesman problem, that is, the problem of finding a shortest curvature-constrained tour through a finite number of points in the euclidean plane. The problem finds applications in path planning for robotic vehicles and unmanned aerial vehicles, where a minimum turning radius prevents the vehicle from taking sharp turns. In this paper, we focus on the case where any two points are separated by at least four times the minimum turning radius, which is most interesting from a practical standpoint. The bounding procedure efficiently determines the optimal order in which to visit the points. The gradient descent method, which is inspired by a mechanical model, determines the optimal trajectories of the tour through the points in a given order, and its computation time scales linearly with the number of points. In experiments on nine points, the bounding procedure typically explores no more than a few sequences before finding the optimal sequence, and the gradient descent method typically converges to within 1\% of optimal in a single iteration.

math.OC↗

arXiv2020

Census of bounded curvature paths

A bounded curvature path is a continuously differentiable piece-wise $C^2$ path with bounded absolute curvature connecting two points in the tangent bundle of a surface. These paths have been widely considered in computer science and engineering since the bound on curvature models the trajectory of the motion of robots under turning circle constraints. Analyzing global properties of spaces of bounded curvature paths is not a simple matter since the length variation between length minimizers of arbitrary close endpoints or directions is in many cases discontinuous. In this note, we develop a simple technology allowing us to partition the space of spaces of bounded curvature paths into one-parameter families. These families of spaces are classified in terms of the type of connected components their elements have (homotopy classes, isotopy classes, or isolated points) as we vary a parameter defined in the reals. Consequently, we answer a question raised by Dubins (Pac J Math 11(2), 1961).

math.MG↗

arXiv2020

Immersed flat ribbon knots

We study the minimum ribbonlength for immersed planar ribbon knots and links. Our approach is to embed the space of such knots and links into a larger more tractable space of disk diagrams. When length minimisers in disk diagram space are ribbon, then these solve the ribbonlength problem. We also provide examples when minimisers in the space of disk diagrams are not ribbon and state some conjectures. We compute the minimal ribbonlength of some small knot and link diagrams and certain infinite families of link diagrams. Finally we present a bound for the number of crossings for a diagram yielding the minimum ribbonlength of a knot or link amongst all diagrams.

math.GT↗

arXiv2017

Non-uniqueness of the homotopy class of bounded curvature paths

A bounded curvature path is a continuously differentiable piecewise $C^2$ path with a bounded absolute curvature that connects two points in the tangent bundle of a surface. In this work, we analyze the homotopy classes of bounded curvature paths for points in the tangent bundle of the Euclidean plane. We show the existence of connected components of bounded curvature paths that do not correspond to those under (regular) homotopies obtaining the first results in the theory outside optimality. An application to robotics is presented.

math.MG↗

arXiv2016

Topological dynamics on finite directed graphs

In this work we establish that finite directed graphs give rise to semiflows on the power set of their nodes. We analyze the topological dynamics for semiflows on finite directed graphs by characterizing Morse decompositions, recurrence behavior and attractor-repeller pairs under weaker assumptions. As is expected, the discrete metric plays an important role in our constructions and their consequences. The connections between the semiflow, graph theory and Markov chains are here explored. We lay the foundation for a dynamical systems approach to hybrid systems with Markov chain type perturbations.

math.DS↗

arXiv2015

Length minimising bounded curvature paths in homotopy classes

Choose two points in the tangent bundle of the Euclidean plane $(x,X),(y,Y)\in T{ \mathbb R}^2$. In this work we characterise the immersed length minimising paths with a prescribed bound on the curvature starting at $x$, tangent to $X$; finishing at $y$, tangent to $Y$, in each connected component of the space of paths with a prescribed bound on the curvature from $(x,X)$ to $(y,Y)$.

math.MG↗

arXiv2015

The classification of homotopy classes of bounded curvature paths

A bounded curvature path is a continuously differentiable piecewise $C^2$ path with bounded absolute curvature that connects two points in the tangent bundle of a surface. In this note we give necessary and sufficient conditions for two bounded curvature paths, defined in the Euclidean plane, to be in the same connected component while keeping the curvature bounded at every stage of the deformation. Following our previous results here we finish a program started by Lester Dubins in 1961.

math.MG↗

arXiv2015

On the topology of the spaces of curvature constrained plane curves

It is well known that plane curves with the same endpoints are homotopic. An analogous claim for plane curves with the same endpoints and bounded curvature still remains open. In this work we find necessary and sufficient conditions for two plane curves with bounded curvature to be deformed, one to another, by a continuous one-parameter family of curves also having bounded curvature. We conclude that the space of these curves has either one or two connected components, depending on the distance between the endpoints. The classification theorem here presented answers a question raised in 1961 by L. E. Dubins.

math.GT↗

arXiv2015

A geometric approach to shortest bounded curvature paths

Consider two elements in the tangent bundle of the Euclidean plane $(x,X),(y,Y)\in T{\mathbb R}^2$. In this work we address the problem of characterizing the paths of bounded curvature and minimal length starting at $x$, finishing at $y$ and having tangents at these points $X$ and $Y$ respectively. This problem was first investigated in the late 50's by Lester Dubins. In this note we present a constructive proof of Dubins' result giving special emphasis on the geometric nature of this problem.

math.MG↗