Unique geodesics for Thompson’s metric
Annales de l'Institut Fourier, Volume 65 (2015) no. 1, pp. 315-348.

In this paper a geometric characterization of the unique geodesics in Thompson’s metric spaces is presented. This characterization is used to prove a variety of other geometric results. Firstly, it will be shown that there exists a unique Thompson’s metric geodesic connecting x and y in the cone of positive self-adjoint elements in a unital C * -algebra if, and only if, the spectrum of x -1/2 yx -1/2 is contained in {1/β,β} for some β1. A similar result will be established for symmetric cones. Secondly, it will be shown that if C is the interior of a finite-dimensional closed cone C, then the Thompson’s metric space (C ,d C ) can be quasi-isometrically embedded into a finite-dimensional normed space if, and only if, C is a polyhedral cone. Moreover, (C ,d C ) is isometric to a finite-dimensional normed space if, and only if, C is a simplicial cone. It will also be shown that if C is the interior of a strictly convex cone C with 3dimC<, then every Thompson’s metric isometry is projectively linear.

Nous présentons une caractérisation géométrique des géodésiques uniques des espaces métriques de Thompson. Nous utilisons cette caractérisation pour démontrer plusieurs autres résultats géométriques. D’abord, nous démontrons qu’il existe une géodésique unique de la métrique de Thompson entre x and y dans le cône d’éléments positifs autoadjoints dans une C * -algèbre unitale si et seulement s’il existe β1 tel que le spectre de x -1/2 yx -1/2 soit contenu dans {1/β,β}. Un résultat similaire est établi pour des cônes symétriques. Ensuite, nous démontrons que si C est l’intérieur d’un cône fermé C de dimension finie, il existe un plongement quasi-isométrique de l’espace métrique de Thompson (C ,d C ) dans un espace normé de dimension finie si et seulement si C est un cône polyédrale. De plus, (C ,d C ) est isométrique à un espace normé de dimension finie si et seulement si C est un cône simplicial. Par ailleurs, il est établi que pour C l’intérieur d’un cône C strictement convexe avec 3dimC<, chaque isométrie de la métrique de Thompson est projectivement linéaire.

DOI: 10.5802/aif.2932
Classification: 53C22, 51Fxx, 53C60
Keywords: Geodesics, Thompson’s (part) metric, Hilbert’s (projective) metric, cones, isometries
Mot clés : géodésiques, métrique de Thompson, métrique d’Hilbert, cônes, isométries
Lemmens, Bas 1; Roelands, Mark 2

1 School of Mathematics, Statistics & Actuarial Science, Cornwallis Building, University of Kent, Canterbury, Kent CT2 7NF, UK.
2 School of Mathematics, Statistics & Actuarial Science, Cornwallis Building, University of Kent, Canterbury, Kent CT2 7NF, UK
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Lemmens, Bas; Roelands, Mark. Unique geodesics for Thompson’s metric. Annales de l'Institut Fourier, Volume 65 (2015) no. 1, pp. 315-348. doi : 10.5802/aif.2932. https://aif.centre-mersenne.org/articles/10.5802/aif.2932/

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