Infinity-Borel set
In set theory, a subset of a Polish space formula_1 is ∞-Borel if it can be obtained by starting with the open subsets of formula_1, and transfinitely iterating the operations of complementation and wellordered union. Note that the set of ∞-Borel sets may not actually be closed under wellordered union; see below. Formal definition. More formally: we define by simultaneous transfinite recursion the notion of ∞-Borel code, and of the interpretation of such codes. Since formula_1 is Polish, it has a countable base. Let formula_4 enumerate that base (that is, formula_5 is the formula_6 basic open set ). Now: Now a set is ∞-Borel if it is the interpretation of some ∞-Borel code. The axiom of choice implies that "every" set can be wellordered, and therefore that every subset of every Polish space is ∞-Borel. Therefore, the notion is interesting only in contexts where AC does not hold (or is not known to hold). Unfortunately, without the axiom of choice, it is not clear that the ∞-Borel sets "are" closed under wellordered union. This is because, given a wellordered union of ∞-Borel sets, each of the individual sets may have "many" ∞-Borel codes, and there may be no  way to choose one code for each of the sets, with which to form the code for the union. The assumption that every set of reals is ∞-Borel is part of AD+, an extension of the axiom of determinacy studied by Woodin. Incorrect definition. It is very tempting to read the informal description at the top of this article as claiming that the ∞-Borel sets are the smallest class of subsets of formula_1 containing all the open sets and closed under complementation and wellordered union. That is, one might wish to dispense with the ∞-Borel codes altogether and try a  definition like this: This set is manifestly closed under well-ordered unions, but without AC it cannot be proved equal to the ∞-Borel sets (as defined in the previous section). Specifically, it is instead the closure of the ∞-Borel sets under "all" well-ordered unions, even those for which a choice of codes cannot be made. Alternative characterization. For subsets of Baire space or Cantor space, there is a more concise (if less transparent) alternative definition, which turns out to be equivalent. A subset "A" of Baire space is ∞-Borel just in case there is a set of  ordinals "S" and a first-order formula "φ" of the language of set theory such that, for every "x" in Baire space, where "L"["S","x"] is Gödel's constructible universe relativized to "S" and "x". When using this definition, the ∞-Borel code is made up of the set "S" and the formula "φ", taken together.