Mathematics – Combinatorics
Scientific paper
2011-01-05
Mathematics
Combinatorics
57 pages, 39 figures
Scientific paper
Define an arrangement of double pseudolines as a finite family of separating simple closed curves embedded in a real two-dimensional projective plane with the property that any two intersect in exactly four transversal intersection points and induce a cell decomposition of their underlying projective plane. We show that the dual arrangement of any finite family of pairwise disjoint convex bodies of any real two-dimensional projective geometry is an arrangement of double pseudolines and that, conversely, any arrangement of double pseudolines is isomorphic to the dual arrangement of a finite family of pairwise disjoint convex bodies of a real two-dimensional projective geometry. Furthermore we provide a simple axiomatic characterization of the class of isomorphism classes of indexed arrangements of oriented double pseudolines and we establish a one-to-one and onto correspondence between this latter class and the class of chirotopes of finite indexed families of pairwise disjoint oriented convex bodies of real two-dimensional projective geometries. Here by the chirotope of a finite indexed family of pairwise disjoint oriented convex bodies we mean the map that assigns to each triple of indices the finite set of relative positions of the convex bodies of the triple of indices with respect to each of the lines of the geometry. Similar results concerning arrangements of double pseudolines in Mobius strips and finite families of pairwise disjoint convex bodies of real two-dimensional affine geometries are reported.
Habert Luc
Pocchiola Michel
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