Notre Dame Journal of Formal Logic

Coxeter Groups and Abstract Elementary Classes: The Right-Angled Case

Tapani Hyttinen and Gianluca Paolini

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We study classes of right-angled Coxeter groups with respect to the strong submodel relation of a parabolic subgroup. We show that the class of all right-angled Coxeter groups is not smooth and establish some general combinatorial criteria for such classes to be abstract elementary classes (AECs), for them to be finitary, and for them to be tame. We further prove two combinatorial conditions ensuring the strong rigidity of a right-angled Coxeter group of arbitrary rank. The combination of these results translates into a machinery to build concrete examples of AECs satisfying given model-theoretic properties. We exhibit the power of our method by constructing three concrete examples of finitary classes. We show that the first and third classes are nonhomogeneous and that the last two are tame, uncountably categorical, and axiomatizable by a single Lω1,ω-sentence. We also observe that the isomorphism relation of any countable complete first-order theory is κ-Borel reducible (in the sense of generalized descriptive set theory) to the isomorphism relation of the theory of right-angled Coxeter groups whose Coxeter graph is an infinite random graph.

Article information

Notre Dame J. Formal Logic, Volume 60, Number 4 (2019), 707-731.

Received: 7 February 2017
Accepted: 19 August 2018
First available in Project Euclid: 14 September 2019

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Digital Object Identifier

Primary: 03C48: Abstract elementary classes and related topics [See also 03C45]
Secondary: 05E15: Combinatorial aspects of groups and algebras [See also 14Nxx, 22E45, 33C80]

classification theory abstract elementary classes Coxeter groups


Hyttinen, Tapani; Paolini, Gianluca. Coxeter Groups and Abstract Elementary Classes: The Right-Angled Case. Notre Dame J. Formal Logic 60 (2019), no. 4, 707--731. doi:10.1215/00294527-2019-0027.

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