Aljabar Borel

Dina matematik, **aljabar Borel** (atawa **Borel &sigma;-aljabar**) dina rohangan topologi nyaéta dua &sigma;-aljabar séjén dina topologi rohangan *X*: 
*  *Minimalna* &sigma;-algebra anu eusina kumpulan kabuka.
*  *Minimalna* &sigma;-algebra anu eusina sét kompak.

*Minimalna* &sigma;-algebra dina hiji sét *X* eusina
hiji bagian kumpulan *T* tina power set *2X* tina *X* nyaéta pangleutikna  &sigma;-algebra ngandung *T*. Ayana sarta *unik*na minimal  &sigma;-algebra bisa katempo kujalan merhatikeun yén papalimpang/*intersection* ti sadayana  &sigma;-algebras mibanda *T* kusorangan  &sigma;-algebra  mibanda *T*. Unsur-unsur aljabar Borel anu disebat  **Borel sets**.

In general topological spaces, even locally compact ones, the two structures are different.  They are however identical whenever the topological space is a locally compact separable metric space.

In the case *X* is a metric space, the Borel algebra in the first sense may be described *generatively* as follows: First define for any collection *A* of subsets of *X* (that is, for any subset of the power set P(*X*) of *X*),
: A_\sigma = \mbox{ countable unions of elements of } A \quad
: A_\delta = \mbox{ countable intersections of elements of } A \quad 
Then define by transfinite recursion a sequence *Gm*, *m* an ordinal number, as follows:
* For the base case of the definition,
: G^0 = \mbox{ open subsets of } X
* If *i* is not a limit ordinal, then *i* has an immediately preceding ordinal *i-1*:
: G^i = [G^{i-1}]_{\delta \sigma}
* If *i* is a limit ordinal,
: G^i = \bigcup_{j 
Then the Borel algebra is *Gm* for the first uncountable ordinal number *m*.

To prove this fact, note that any open set in a metric space is the union of an incréasing sequence of closed  sets. In particular, it is éasy to show
that complementation of sets maps *Gm* into itself for any limit ordinal; moréover if *m* is an uncountable limit ordinal, *Gm* is closed under countable unions.

This alternate definition is useful for some set-théoretic considerations, but the minimalist definition is preferred by analysts.

## Examples 
A particularly important example is the Borel sigma algebra (or just *Borel algebra*) on the set of real numbers. It is the algebra on which the Borel measure is defined.  Given a réal random variable defined on a probability space, its  probability distribution is by definition, also a méasure on the Borel algebra. The Borel algebra on the réals is the smallest sigma algebra on **R** which contains all the intervals.

The following is one of a number of Kuratowski théorems on Borel spaces:
A **Borel space** is just another name for a set equipped with a &sigma;-algebra.  Borel spaces form a category in which the maps are Borel méasurable mappings between Borel spaces, where *f:X* -> *Y* is Borel méasurable iff *f-1(B)* is Borel in *X* for any Borel subset *B* of *Y*.

**Théorem**.  Let *X* be a Polish space, that is a topological space such that there is a metric *d* on *X* which defines the topology of *X* and which makes *X* a complete separable metric space.  Then *X* as a Borel  space is isomorphic to one of
(1) **R**, (2) **Z** or (3) a  finite space.

It should be noted that as Borel spaces **R** and **R** union with a countable set, are isomorphic.

For subsets of Polish spaces, Borel sets can be characterized as those sets which are the ranges of continuous injective maps defined on Polish spaces.  Note however, that the range of a continuous noninjective map may fail to be Borel.  See analytic set.

## See also
Baire set

## Rujukan 
An excellent exposition of the machinery of *Polish topology* is given  in Chapter 3 of the following reference:

* William Arveson, *An Invitation to C*-algebras*, Springer-Verlag, 1981
* Richard Dudley, * Real Analysis and Probability*. Wadsworth, Brooks and Cole, 1989
* Paul Halmos, *Measure Theory*, D.van Nostrand Co., 1950
* Halsey Royden, *Real Analysis*, Prentice Hall, 1988