Editing Descriptive set theory

{{short description|Subfield of mathematical logic}}
In [[mathematical logic]], '''descriptive set theory''' ('''DST''') is the study of certain classes of "[[well-behaved]]" [[set (mathematics)|subset]]s of the [[real line]] and other [[Polish space]]s. As well as being one of the primary areas of research in [[set theory]], it has applications to other areas of mathematics such as [[functional analysis]], [[ergodic theory]], the study of [[operator algebras]] and [[Group action (mathematics)|group actions]], and [[mathematical logic]].

== Polish spaces ==

Descriptive set theory begins with the study of Polish spaces and their [[Borel set]]s.

A '''[[Polish space]]''' is a [[second-countable]] [[topological space]] that is [[metrizable]] with a [[complete metric]].  Heuristically, it is a complete [[separable metric space]] whose metric has been "forgotten". Examples include the [[real line]] <math>\mathbb{R}</math>, the [[Baire space (set theory)|Baire space]] <math>\mathcal{N}</math>, the [[Cantor space]] <math>\mathcal{C}</math>, and the [[Hilbert cube]] <math>I^{\mathbb{N}}</math>.

=== Universality properties ===

The class of Polish spaces has several universality properties, which show that there is no loss of generality in considering Polish spaces of certain restricted forms.

* Every Polish space is [[homeomorphic]] to a [[Gδ space|''G''<sub>&delta;</sub> subspace]] of the [[Hilbert cube]], and every ''G''<sub>&delta;</sub> subspace of the Hilbert cube is Polish.
* Every Polish space is obtained as a continuous image of Baire space; in fact every Polish space is the image of a continuous bijection defined on a closed subset of Baire space. Similarly, every compact Polish space is a continuous image of Cantor space.

Because of these universality properties, and because the Baire space <math>\mathcal{N}</math> has the convenient property that it is [[homeomorphic]] to <math>\mathcal{N}^\omega</math>, many results in descriptive set theory are proved in the context of Baire space alone.

== Borel sets ==

The class of '''[[Borel set]]s''' of a topological space ''X'' consists of all sets in the smallest [[sigma-algebra|&sigma;-algebra]] containing the open sets of ''X''. This means that the Borel sets of ''X'' are the smallest collection of sets such that:
* Every open subset of ''X'' is a Borel set.
* If ''A'' is a Borel set, so is <math>X \setminus A</math>. That is, the class of Borel sets are closed under complementation. 
* If ''A''<sub>''n''</sub> is a Borel set for each natural number ''n'', then the union <math>\bigcup A_n</math> is a Borel set. That is, the Borel sets are closed under countable unions.

A fundamental result shows that any two uncountable Polish spaces ''X'' and ''Y'' are [[Borel isomorphism|Borel isomorphic]]: there is a bijection from ''X'' to ''Y'' such that the preimage of any Borel set is Borel, and the image of any Borel set is Borel. This gives additional justification to the practice of restricting attention to Baire space and Cantor space, since these and any other Polish spaces are all isomorphic at the level of Borel sets.

=== Borel hierarchy ===

Each Borel set of a Polish space is classified in the '''[[Borel hierarchy]]''' based on how many times the operations of countable union and complementation must be used to obtain the set, beginning from open sets. The classification is in terms of [[countable set|countable]] [[ordinal number]]s. For each nonzero countable ordinal ''&alpha;'' there are classes <math>\mathbf{\Sigma}^0_\alpha</math>, <math>\mathbf{\Pi}^0_\alpha</math>, and <math>\mathbf{\Delta}^0_\alpha</math>. 
* Every open set is declared to be <math>\mathbf{\Sigma}^0_1</math>.
* A set is declared to be <math>\mathbf{\Pi}^0_\alpha</math> if and only if its complement is <math>\mathbf{\Sigma}^0_\alpha</math>.
* A set ''A'' is declared to be <math>\mathbf{\Sigma}^0_\delta</math>, ''&delta;'' > 1, if there is a sequence &lang; ''A''<sub>''i''</sub> &rang; of sets, each of which is <math>\mathbf{\Pi}^0_{\lambda(i)}</math> for some ''&lambda;''(''i'') < ''&delta;'', such that <math>A = \bigcup A_i</math>.
* A set is <math>\mathbf{\Delta}^0_\alpha</math> if and only if it is both <math>\mathbf{\Sigma}^0_\alpha</math> and <math>\mathbf{\Pi}^0_\alpha</math>.

A theorem shows that any set that is <math>\mathbf{\Sigma}^0_\alpha</math> or <math>\mathbf{\Pi}^0_\alpha</math> is <math>\mathbf{\Delta}^0_{\alpha + 1}</math>, and any <math>\mathbf{\Delta}^0_\beta</math> set is both <math>\mathbf{\Sigma}^0_\alpha</math> and <math>\mathbf{\Pi}^0_\alpha</math> for all ''&alpha;'' > ''&beta;''.  Thus the hierarchy has the following structure, where arrows indicate inclusion.

{{center|
<!-- replace this with a diagram -->
<math>
\begin{matrix}
& & \mathbf{\Sigma}^0_1 & & & & \mathbf{\Sigma}^0_2 & & \cdots \\
& \nearrow & & \searrow & & \nearrow \\
\mathbf{\Delta}^0_1 & & & & \mathbf{\Delta}^0_2 & & & & \cdots \\
& \searrow & & \nearrow & & \searrow \\
& & \mathbf{\Pi}^0_1 & & & & \mathbf{\Pi}^0_2 & & \cdots 
\end{matrix}\begin{matrix}
& &  \mathbf{\Sigma}^0_\alpha & & & \cdots \\
& \nearrow & & \searrow \\
\quad \mathbf{\Delta}^0_\alpha &  & & & \mathbf{\Delta}^0_{\alpha + 1} & \cdots \\
& \searrow & & \nearrow \\
& & \mathbf{\Pi}^0_\alpha & & & \cdots 
\end{matrix}
</math>
}}

=== Regularity properties of Borel sets ===

Classical descriptive set theory includes the study of regularity properties of Borel sets. For example, all Borel sets of a Polish space have the [[property of Baire]] and the [[perfect set property]].  Modern descriptive set theory includes the study of the ways in which these results generalize, or fail to generalize, to other classes of subsets of Polish spaces.

== Analytic and coanalytic sets ==

Just beyond the Borel sets in complexity are the [[analytic set]]s and [[coanalytic set]]s. A subset of a Polish space ''X'' is '''analytic''' if it is the continuous image of a Borel subset of some other Polish space. Although any continuous preimage of a Borel set is Borel, not all analytic sets are Borel sets. A set is '''coanalytic''' if its complement is analytic.

== Projective sets and Wadge degrees ==

Many questions in descriptive set theory ultimately depend upon [[set theory|set-theoretic]] considerations and the properties of [[ordinal number|ordinal]] and [[cardinal number]]s. This phenomenon is particularly apparent in the '''projective sets'''.   These are defined via the [[projective hierarchy]] on a Polish space ''X'':
* A set is declared to be <math>\mathbf{\Sigma}^1_1</math> if it is analytic.
* A set is <math>\mathbf{\Pi}^1_1</math> if it is coanalytic.
* A set ''A'' is <math>\mathbf{\Sigma}^1_{n+1}</math> if there is a <math>\mathbf{\Pi}^1_n</math> subset ''B'' of <math>X \times X</math> such that ''A'' is the projection of ''B'' to the first coordinate.
* A set ''A'' is <math>\mathbf{\Pi}^1_{n+1}</math> if there is a <math>\mathbf{\Sigma}^1_n</math> subset ''B'' of <math>X \times X</math> such that ''A'' is the projection of ''B'' to the first coordinate. 
* A set is <math>\mathbf{\Delta}^1_{n}</math> if it is both <math>\mathbf{\Pi}^1_n</math> and <math>\mathbf{\Sigma}^1_n</math> .

As with the Borel hierarchy, for each ''n'', any <math>\mathbf{\Delta}^1_n</math> set is both <math>\mathbf{\Sigma}^1_{n+1}</math> and <math>\mathbf{\Pi}^1_{n+1}</math>.

The properties of the projective sets are not completely determined by ZFC. Under the assumption [[axiom of constructibility|''V = L'']], not all projective sets have the perfect set property or the property of Baire.  However, under the assumption of [[projective determinacy]], all projective sets have both the perfect set property and the property of Baire. This is related to the fact that ZFC proves [[Borel determinacy]], but not projective determinacy.

There are also generic extensions of <math>L</math> for any natural number <math>n>2</math> in which <math>\mathcal P(\omega)\cap L</math> consists of all the lightface <math>\Delta^1_n</math> subsets of <math>\omega</math>.<ref>V. Kanovei, V. Lyubetsky, "[https://www.mdpi.com/2227-7390/8/9/1477 On the <math>\Delta^1_n</math> problem of Harvey Friedman]. In ''Mathematical Logic and its Applications'' (2020), DOI [https://doi.org/10.3390/math8091477 10.3380/math8091477].</ref>

More generally, the entire collection of sets of elements of a Polish space ''X'' can be grouped into equivalence classes, known as [[Wadge degree]]s, that generalize the projective hierarchy. These degrees are ordered in the [[Wadge hierarchy]]. The [[axiom of determinacy]] implies that the Wadge hierarchy on any Polish space is well-founded and of length [[Θ (set theory)|Θ]], with structure extending the projective hierarchy.

== Borel equivalence relations ==

A contemporary area of research in descriptive set theory studies '''[[Borel equivalence relation]]s'''.   A '''Borel equivalence relation''' on a Polish space ''X'' is a Borel subset of <math>X \times X</math> that is an [[equivalence relation]] on ''X''.

== Effective descriptive set theory ==

The area of [[effective descriptive set theory]] combines the methods of descriptive set theory with those of [[generalized recursion theory]] (especially [[hyperarithmetical theory]]). In particular, it focuses on [[lightface]] analogues of hierarchies of classical descriptive set theory. Thus the [[hyperarithmetic hierarchy]] is studied instead of the Borel hierarchy, and the [[analytical hierarchy]] instead of the projective hierarchy.  This research is related to weaker versions of set theory such as [[Kripke–Platek set theory]] and [[second-order arithmetic]].

==Table==

{{pointclasses}}

==See also==
*[[Pointclass]]
*[[Prewellordering]]
*[[Scale property]]

==References==
* {{cite book | authorlink=Alexander Kechris | author=Kechris, Alexander S. | title=Classical Descriptive Set Theory | url=https://archive.org/details/classicaldescrip0000kech | url-access=registration | publisher=Springer-Verlag | year=1994 | isbn=0-387-94374-9}}
* {{cite book | authorlink=Yiannis N. Moschovakis | author=Moschovakis, Yiannis N. | title=Descriptive Set Theory|url=https://www.math.ucla.edu/~ynm/books.htm | publisher=North Holland | year=1980|page=2 |isbn=0-444-70199-0}}

===Citations===
{{Reflist}}

== External links ==
* [http://www.math.uic.edu/~marker/math512/dst.pdf Descriptive set theory], David Marker, 2002. Lecture notes.

[[Category:Descriptive set theory| ]]