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Lattices, injective metrics and the $K(\pi,1)$ conjecture

Thomas Haettel

Algebraic & Geometric Topology 24 (2024) 4007–4060
Abstract

Starting with a lattice with an action of or , we build a Helly graph or an injective metric space. We deduce that the orthoscheme complex of any bounded graded lattice is injective. We also prove a Cartan–Hadamard result for locally injective metric spaces. We apply this to show that any Garside group or any FC-type Artin group acts on an injective metric space and on a Helly graph. We also deduce that the natural piecewise metric on any Euclidean building of type Ãn extended, B~n, C~n or D~n is injective, and its thickening is a Helly graph.

Concerning Artin groups of Euclidean types Ãn and C~n, we show that the natural piecewise metric on the Deligne complex is injective, the thickening is a Helly graph, and it admits a convex bicombing. This gives a metric proof of the K(π,1) conjecture, as well as several other consequences usually known when the Deligne complex has a CAT(0) metric.

Keywords
injective metrics, Helly graphs, lattices, Artin groups, Euclidean buildings, Cartan–Hadamard
Mathematical Subject Classification
Primary: 20E42, 05B35, 52A35, 06A12
References
Publication
Received: 15 February 2023
Revised: 12 September 2023
Accepted: 23 October 2023
Published: 9 December 2024
Authors
Thomas Haettel
Institut Montpelliérain Alexander Grothendieck
Université de Montpellier, CNRS
Montpellier
France
IRL 3457, CRM-CNRS
Université de Montréal
Montréal
Canada

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