Geometrical and analytical characteristic properties of piecewise affine mappings (1111.1389v1)

Abstract: Let X and Y be finite dimensional normed spaces, F(X,Y) a collection of all mappings from X into Y. A mapping $P\in F(X,Y)$ is said to be piecewise affine if there exists a finite family of convex polyhedral subsets covering X and such that the restriction of $P$ on each subset of this family is an affine mapping. In the paper we prove a number of characterizations of piecewise affine mappings. In particular we show that a mapping $P: X \to Y$ is piecewise affine if and only if for any partial order $\preceq$ defined on Y by a polyhedral convex cone both the $\preceq$-epigraph and the $\preceq$-hypograph of P can be represented as the union of finitely many convex polyhedral subsets of $X \times Y$. When the space Y is ordered by a minihedral cone or equivalently when Y is a vector lattice the collection F(X,Y) endowed with standard pointwise algebraic operations and the pointwise ordering is a vector lattice too. In the paper we show that the collection of piecewise affine mappings coincides with the smallest vector sublattice of F(X,Y) containing all affine mappings. Moreover we prove that each convex (with respect to an ordering of Y by a minihedral cone) piecewise affine mapping is the least upper bound of finitely many affine mappings. The collection of all convex piecewise affine mappings is a convex cone in F(X,Y) the linear envelope of which coincides with the vector subspace of all piecewise affine mappings.