TAILIEUCHUNG - Báo cáo toán học: "An alternative definition of the notion valuation in the theory of near polygons"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: An alternative definition of the notion valuation in the theory of near polygons. | An alternative definition of the notion valuation in the theory of near polygons Bart De Bruyn Department of Pure Mathematics and Computer Algebra Ghent University Gent Belgium bdb@ Submitted Sep 13 2008 Accepted Jan 20 2009 Published Jan 30 2009 Mathematics Subject Classihcations 05B25 51E12 Abstract Valuations of dense near polygons were introduced in 9 . A valuation of a dense near polygon S P L I is a map f from the point-set P of S to the set N of nonnegative integers satisfying very nice properties with respect to the set of convex subspaces of S. In the present paper we give an alternative dehnition of the notion valuation and prove that both dehnitions are equivalent. In the case of dual polar spaces and many other known dense near polygons this alternative dehnition can be signihcantly simplihed. 1 Introduction Basic definitions A near polygon is a partial linear space S P L I I c P X L with the property that for every point p 2P and every line L 2 L there exists a unique point on L nearest to p. Here distances d - are measured in the collinearity graph r of S. If d is the diameter of r then the near polygon is called a near 2d-gon. A near 0-gon is a point and a near 2-gon is a line. Near quadrangles are usually called generalized quadrangles Payne and Thas 11 . If X1 and X2 are two nonempty sets of points of a near polygon S then d X1 X2 denotes the minimal distance between a point of X1 and a point of X2. If X1 x1 we will also write d x1 X2 instead of d x1 X2 . For every nonempty set X of points of S and every i 2 N v X denotes the set of all points y of S for which d y X i. If X is a singleton x then we will also write Fj x instead of Fj x . Postdoctoral Fellow of the Research Foundation - Flanders THE ELECTRONIC JOURNAL OF COMBINATORICS 16 2009 R16 1 A nonempty set X of points of a near polygon S is called a subspace if every line meeting X in at least two points has all its points in X. A subspace X is called convex if every point on

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