Valuation_(measure_theory) Knowpia

In measure theory, or at least in the approach to it via the domain theory, a valuation is a map from the class of open sets of a topological space to the set of positive real numbers including infinity, with certain properties. It is a concept closely related to that of a measure, and as such, it finds applications in measure theory, probability theory, and theoretical computer science.

Domain/Measure theory definition edit

Let $\scriptstyle (X,{\mathcal {T}})$ be a topological space: a valuation is any set function

v:{\mathcal {T}}\to \mathbb {R} ^{+}\cup \{+\infty \}

satisfying the following three properties

{\begin{array}{lll}v(\varnothing )=0&&\scriptstyle {\text{Strictness property}}\\v(U)\leq v(V)&{\mbox{if}}~U\subseteq V\quad U,V\in {\mathcal {T}}&\scriptstyle {\text{Monotonicity property}}\\v(U\cup V)+v(U\cap V)=v(U)+v(V)&\forall U,V\in {\mathcal {T}}&\scriptstyle {\text{Modularity property}}\,\end{array}}

The definition immediately shows the relationship between a valuation and a measure: the properties of the two mathematical object are often very similar if not identical, the only difference being that the domain of a measure is the Borel algebra of the given topological space, while the domain of a valuation is the class of open sets. Further details and references can be found in Alvarez-Manilla, Edalat & Saheb-Djahromi 2000 and Goubault-Larrecq 2005.

Continuous valuation edit

A valuation (as defined in domain theory/measure theory) is said to be continuous if for every directed family $\scriptstyle \{U_{i}\}_{i\in I}$ of open sets (i.e. an indexed family of open sets which is also directed in the sense that for each pair of indexes $i$ and $j$ belonging to the index set $I$ , there exists an index $k$ such that $\scriptstyle U_{i}\subseteq U_{k}$ and $\scriptstyle U_{j}\subseteq U_{k}$ ) the following equality holds:

v\left(\bigcup _{i\in I}U_{i}\right)=\sup _{i\in I}v(U_{i}).

This property is analogous to the τ-additivity of measures.

Simple valuation edit

A valuation (as defined in domain theory/measure theory) is said to be simple if it is a finite linear combination with non-negative coefficients of Dirac valuations, that is,

v(U)=\sum _{i=1}^{n}a_{i}\delta _{x_{i}}(U)\quad \forall U\in {\mathcal {T}}

where

a_{i}

is always greater than or at least equal to zero for all index

i

. Simple valuations are obviously continuous in the above sense. The supremum of a directed family of simple valuations (i.e. an indexed family of simple valuations which is also directed in the sense that for each pair of indexes

i

and

j

belonging to the index set

I

, there exists an index

k

such that

\scriptstyle v_{i}(U)\leq v_{k}(U)\!

and

\scriptstyle v_{j}(U)\leq v_{k}(U)\!

) is called quasi-simple valuation

{\bar {v}}(U)=\sup _{i\in I}v_{i}(U)\quad \forall U\in {\mathcal {T}}.\,

Examples edit

Dirac valuation edit

Let $\scriptstyle (X,{\mathcal {T}})$ be a topological space, and let $x$ be a point of $X$ : the map

\delta _{x}(U)={\begin{cases}0&{\mbox{if}}~x\notin U\\1&{\mbox{if}}~x\in U\end{cases}}\quad {\text{ for all }}U\in {\mathcal {T}}

is a valuation in the domain theory/measure theory, sense called Dirac valuation. This concept bears its origin from distribution theory as it is an obvious transposition to valuation theory of Dirac distribution: as seen above, Dirac valuations are the "bricks" simple valuations are made of.

Notes edit

^ Details can be found in several arXiv papers of prof. Semyon Alesker.

Works cited edit

Alvarez-Manilla, Maurizio; Edalat, Abbas; Saheb-Djahromi, Nasser (2000), "An extension result for continuous valuations", Journal of the London Mathematical Society, 61 (2): 629–640, CiteSeerX 10.1.1.23.9676, doi:10.1112/S0024610700008681.
Goubault-Larrecq, Jean (2005), "Extensions of valuations", Mathematical Structures in Computer Science, 15 (2): 271–297, doi:10.1017/S096012950400461X

External links edit

Alesker, Semyon, "various preprints on valuation s", arXiv preprint server, primary site at Cornell University. Several papers dealing with valuations on convex sets, valuations on manifolds and related topics.
The nLab page on valuations

[1] Details can be found in several arXiv papers of prof. Semyon Alesker.

[a]

Summary