57 research outputs found

    Realizations of the associahedron and cyclohedron

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    We describe many different realizations with integer coordinates for the associahedron (i.e. the Stasheff polytope) and for the cyclohedron (i.e. the Bott-Taubes polytope) and compare them to the permutahedron of type A_n and B_n respectively. The coordinates are obtained by an algorithm which uses an oriented Coxeter graph of type A_n or B_n respectively as only input and which specialises to a procedure presented by J.-L. Loday for a certain orientation of A_n. The described realizations have cambrian fans of type A and B as normal fans. This settles a conjecture of N. Reading for cambrian fans of these types.Comment: v2: 18 pages, 7 figures; updated version has revised introduction and updated Section 4; v3: 21 pages, 2 new figures, added statement (b) in Proposition 1.4. and 1.7 plus extended proof; added references [1], [29], [30]; minor changes with respect to presentatio

    On inversion sets and the weak order in Coxeter groups

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    In this article, we investigate the existence of joins in the weak order of an infinite Coxeter group W. We give a geometric characterization of the existence of a join for a subset X in W in terms of the inversion sets of its elements and their position relative to the imaginary cone. Finally, we discuss inversion sets of infinite reduced words and the notions of biconvex and biclosed sets of positive roots.Comment: 22 pages; 10 figures; v2 some references were added; v2: final version, to appear in European Journal of Combinatoric

    Automata, reduced words, and Garside shadows in Coxeter groups

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    In this article, we introduce and investigate a class of finite deterministic automata that all recognize the language of reduced words of a finitely generated Coxeter system (W,S). The definition of these automata is straightforward as it only requires the notion of weak order on (W,S) and the related notion of Garside shadows in (W,S), an analog of the notion of a Garside family. Then we discuss the relations between this class of automata and the canonical automaton built from Brink and Howlett's small roots. We end this article by providing partial positive answers to two conjectures: (1) the automata associated to the smallest Garside shadow is minimal; (2) the canonical automaton is minimal if and only if the support of all small roots is spherical, i.e., the corresponding root system is finite.Comment: 21 pages, 7 figures; v2: 23 pages, 8 figures, Remark 3.15 added, accepted in Journal of Algebra, computational sectio

    Polytopal realizations of finite type g\mathbf{g}-vector fans

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    This paper shows the polytopality of any finite type g\mathbf{g}-vector fan, acyclic or not. In fact, for any finite Dynkin type Γ\Gamma, we construct a universal associahedron Assoun(Γ)\mathsf{Asso}_{\mathrm{un}}(\Gamma) with the property that any g\mathbf{g}-vector fan of type Γ\Gamma is the normal fan of a suitable projection of Assoun(Γ)\mathsf{Asso}_{\mathrm{un}}(\Gamma).Comment: 27 pages, 9 figures; Version 2: Minor changes in the introductio
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