Multi-valued mapping
point-to-set mapping
A mapping associating with each element x of a set X a subset \Gamma ( x) of a set Y . If for each x \in X the set \Gamma ( x) consists of one element, then the mapping \Gamma is called single-valued. A multi-valued mapping \Gamma can be treated as a single-valued mapping of X into 2 ^ {Y} , that is, into the set of all subsets of Y .
For two multi-valued mappings \Gamma _ {i} : X \rightarrow Y , i = 1 , 2 , their inclusion is naturally defined: \Gamma _ {1} \subset \Gamma _ {2} if \Gamma _ {1} ( x) \subset \Gamma _ {2} ( x) for all x \in X . For any family of multi-valued mappings \Gamma _ \alpha : X \rightarrow Y , \alpha \in A , the union and intersection are defined: \Gamma = \cup _ {\alpha \in A } \Gamma _ \alpha if \Gamma ( x) = \cup _ {\alpha \in A } \Gamma _ \alpha ( x) for all x \in X and \Gamma = \cap _ {\alpha \in A } \Gamma _ \alpha if \Gamma ( x) = \cap _ {\alpha \in A } \Gamma _ \alpha ( x) for all x \in X . For any family of multi-valued mappings \Gamma _ \alpha : X \rightarrow Y _ \alpha , \alpha \in A , the multi-valued mapping \Gamma = \prod _ {\alpha \in A } \Gamma _ \alpha : X \rightarrow \prod _ {\alpha \in A } Y _ \alpha is called the Cartesian product of the multi-valued mappings \Gamma _ \alpha if \Gamma ( x) = \prod _ {\alpha \in A } \Gamma _ \alpha ( x) . A section of a multi-valued mapping \Gamma is a single-valued mapping f : X \rightarrow Y such that f ( x) \in \Gamma ( x) for all x \in X . The graph of a multi-valued mapping \Gamma is the set G ( \Gamma ) = \{ {( x , y ) \in X \times Y } : {y \in \Gamma ( x) } \} .
A multi-valued mapping \Gamma of a topological space X into a topological space Y is called upper semi-continuous if for every open set U \subset Y the set \Gamma ^ {+} ( u) = \{ {x \in X } : {\Gamma ( x) \subset U, \Gamma ( x) \neq \emptyset } \} is open in X , or equivalently: For any x \in X and any neighbourhood U of \Gamma ( x) there is a neighbourhood O x of x such that \Gamma ( Ox) \subset U , where \Gamma ( Ox ) = \cup \{ {\Gamma ( y) } : {y \in Ox } \} . A multi-valued mapping from a topological space X to a topological space Y is called lower semi-continuous if for any open set U \subset Y the set \Gamma ^ {-} ( U) = \{ {x \in X } : {\Gamma ( x) \cap U \neq \emptyset } \} is open in X . If a multi-valued mapping satisfies both properties simultaneously, then it is called a continuous multi-valued mapping.
Let Y be a topological vector space. A multi-valued mapping \Gamma : X\rightarrow Y is called convex-compact valued if \Gamma ( x) is a convex compact set for all x \in X . For a finite set of multi-valued mappings \Gamma _ {i} : X \rightarrow Y , i \in I , the algebraic sum \Gamma = \sum _ {i \in I } \Gamma _ {i} is defined by \Gamma ( x) = \sum _ {i \in I } \Gamma _ {i} ( x) . The intersection of any (finite) family of upper semi-continuous (respectively, continuous) multi-valued mappings is upper semi-continuous (respectively, continuous). The Cartesian product of a finite family of upper semi-continuous multi-valued mappings is upper semi-continuous. The algebraic sum of a finite family of upper semi-continuous (convex-compact valued) mappings is upper semi-continuous (convex-compact valued). The intersection and Cartesian product of any family of convex-compact valued mappings is convex-compact valued.
Let X be a paracompact space and Y a locally convex metric linear space (cf. Locally convex space; Linear space; Metric space). Let \Gamma : X \rightarrow Y be a multi-valued mapping which is upper semi-continuous and is such that \Gamma ( x) is closed in Y for every x \in X . Then the multi-valued mapping \Gamma admits continuous sections. Let ( X , \mathfrak A ) and ( Y , \mathfrak B ) be spaces with given \sigma - algebras \mathfrak A and \mathfrak B ; a multi-valued mapping \Gamma : ( X , \mathfrak A ) \rightarrow ( Y , \mathfrak B ) is called measurable if the graph G ( \Gamma ) belongs to the smallest \sigma - algebra \mathfrak A \times \mathfrak B of X \times Y containing all sets of the form A \times B , where A \in \mathfrak A and B \in \mathfrak B . If \Gamma is a measurable multi-valued mapping from ( X , \mathfrak A ) to a complete separable metric space ( Y , \mathfrak B ) , where \mathfrak B is the Borel \sigma - algebra of Y , then \Gamma has measurable sections f .
References
[1] | K. Kuratowski, "Topology" , 1–2 , Acad. Press (1966–1968) (Translated from French) |
Comments
A multi-valued mapping is also called a set-valued or many-valued mapping. Sections are also called selections.
Theorems which prove that certain kinds of multi-valued mappings admit selections are called selection theorems. The measurable selection theorem stated in the last sentence of the main article above is known as von Neumann's measurable choice theorem. A number of selection theorems and some applications are discussed in [a4].
References
[a1] | E. Michael, "Continuous selections" Ann. of Math. , 63 (1956) pp. 361–382 |
[a2] | E.A. Michael, "A survey of continuous selections" W.M. Fleischmann (ed.) , Set valued mappings, selections and topological properties of ![]() |
[a3] | K. Przeslawski, D. Yost, "Continuity properties of selectors and Michael's Theorem" Mich. Math. J. , 36 (1989) pp. 113–134 |
[a4] | T. Parthasarathy, "Selection theorems and their applications" , Lect. notes in math. , 263 , Springer (1972) |
Multi-valued mapping. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Multi-valued_mapping&oldid=47923