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Semispaces of configurations, cell complexes of arrangements. (English) Zbl 0551.05002
From the authors’ introduction: ”In [J. Comb. Theory, Ser. A 29, 220-235 (1980; Zbl 0448.05016)] and [Geom. Dedicata 12, 63-74 (1982; Zbl 0494.51002)] we introduced the idea of associating to each numbered configuration of n points, as well as to each numbered arrangement of n lines, a circular sequence of permutations of $$\{$$ 1,...,n$$\}$$, which encodes in combinatorial terms the orientation properties of the configuration or arrangement; this device has since proven fruitful in solving a number of open problems on configurations and arrangements. The same combinatorial tool, which we call an allowable sequence of permutations, can be used to study equivalence relations on configurations and arrangements as well, and this is what we do in the present paper. Our results include: (i) a criterion, in terms of their associated sequences of permutations, for two numbered configurations of points to be semispace-equivalent, and a related criterion, also in terms of allowable sequences, for two numbered arrangements of lines (or pseudolines) to give rise to isomorphic cell complexes; (ii) a combinatorial characterization of the cell complexes determined by arrangements of pseudolines; (iii) the solution of a discrete version of the isotopy problem; and (iv) the result that if, for a configuration $$\{P_ 1,...,P_ n\}$$, one knows how many points lie to the left of each directed line $$P_ iP_ j$$, then one can reconstruct precisely which ones do. We end the paper with some open problems.”
Reviewer: J.Libicher

##### MSC:
 05A05 Permutations, words, matrices 51-02 Research exposition (monographs, survey articles) pertaining to geometry 05-02 Research exposition (monographs, survey articles) pertaining to combinatorics 51A99 Linear incidence geometry 05B25 Combinatorial aspects of finite geometries
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