Cospanning characterizations of violator and co-violator spaces
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Given a finite set E and an operator sigma:2^E–>2^E, two subsets X,Y of the ground set E are cospanning if sigma(X)=sigma(Y) (Korte, Lovasz, Schrader; 1991). We investigate cospanning relations on violator spaces. A notion of a violator space was introduced in (Gartner, Matousek, Rust, Skovrovn; 2008) as a combinatorial framework that encompasses linear programming and other geometric optimization problems. Violator spaces are defined by violator operators. We introduce co-violator spaces based on contracting operators known also as choice functions. Let alpha,beta:2^E–>2^E be a violator operator and a co-violator operator, respectively. Cospanning characterizations of violator spaces allow us to obtain some new properties of violator operators, co-violator operators, and their interconnections. In particular, we show that uniquely generated violator spaces enjoy so-called Krein-Milman properties, i.e., alpha(beta(X))=alpha(X) and beta(alpha(X))=beta(X) for every subset X of the ground set E.
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