2,859 research outputs found

    Triangle-Free Triangulations, Hyperplane Arrangements and Shifted Tableaux

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    Flips of diagonals in colored triangle-free triangulations of a convex polygon are interpreted as moves between two adjacent chambers in a certain graphic hyperplane arrangement. Properties of geodesics in the associated flip graph are deduced. In particular, it is shown that: (1) every diagonal is flipped exactly once in a geodesic between distinguished pairs of antipodes; (2) the number of geodesics between these antipodes is equal to twice the number of Young tableaux of a truncated shifted staircase shape.Comment: figure added, plus several minor change

    Flag weak order on wreath products

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    A generating set for the wreath product \ZZ_r \wr S_n which leads to a nicely behaved weak order is presented, and properties of the resulting order are studied.Comment: 20 pages, 2 figures; corrected and added proofs and explanation

    On Degrees in the Hasse Diagram of the Strong Bruhat Order

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    For a permutation π\pi in the symmetric group SnS_n let the {\it total degree} be its valency in the Hasse diagram of the strong Bruhat order on SnS_n, and let the {\it down degree} be the number of permutations which are covered by π\pi in the strong Bruhat order. The maxima of the total degree and the down degree and their values at a random permutation are computed. Proofs involve variants of a classical theorem of Tur\'an from extremal graph theory.Comment: 14 pages, minor corrections; to appear in S\'em. Lothar. Combi
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