Analytic set

In the mathematical field of descriptive set theory, a subset of a Polish space formula_1 is an analytic set if it is a continuous image of a Polish space. These sets were first defined by and his student .
Definition.
There are several equivalent definitions of analytic set. The following conditions on a subspace "A" of a Polish space "X" are equivalent:
An alternative characterization, in the specific, important, case that formula_1 is Baire space ωω, is that the analytic sets are precisely the projections of trees on formula_8. Similarly, the analytic subsets of Cantor space 2ω are precisely the projections of trees on formula_9.
Properties.
Analytic subsets of Polish spaces are closed under countable unions and intersections, continuous images, and inverse images. 
The complement of an analytic set need not be analytic. Suslin proved that if the complement of an analytic set is analytic then the set is Borel. (Conversely any Borel set is analytic and Borel sets are closed under complements.) Luzin proved more generally that any two disjoint analytic sets are separated by a Borel set: in other words there is a Borel set containing one and disjoint from the other. This is sometimes called the "Luzin separability principle" (though it was implicit in the proof of Suslin's theorem).
Analytic sets are always Lebesgue measurable (indeed, universally measurable) and have the property of Baire and the perfect set property.
Projective hierarchy.
Analytic sets are also called formula_10 (see projective hierarchy). Note that the bold font in this symbol is not the Wikipedia convention, but rather is used distinctively from its lightface counterpart formula_11 (see analytical hierarchy). The complements of analytic sets are called coanalytic sets, and the set of coanalytic sets is denoted by formula_12. 
The intersection formula_13 is the set of Borel sets.