Projective hierarchy

"Projective set" redirects here. For the card game, see Projective Set (game).

In the mathematical field of descriptive set theory, a subset $A$ of a Polish space $X$ is projective if it is ${𝜮}_n^1$ for some positive integer $n$. Here $A$ is

• ${𝜮}_1^1$ if $A$ is analytic
• ${𝜫}_n^1$ if the complement of $A$, $X\setminus A$, is ${𝜮}_n^1$
• ${𝜮}_{n+1}^1$ if there is a Polish space $Y$ and a ${𝜫}_n^1$ subset $C\subseteq X\times Y$ such that $A$ is the projection of $C$ onto $X$; that is, $A=\{x\in X\mid \mathrm{\exists }y\in Y:(x,y)\in C\}.$

The choice of the Polish space $Y$ in the third clause above is not very important; it could be replaced in the definition by a fixed uncountable Polish space, say Baire space or Cantor space or the real line.

Relationship to the analytical hierarchy

There is a close relationship between the relativized analytical hierarchy on subsets of Baire space (denoted by lightface letters $\Sigma$ and $\Pi$) and the projective hierarchy on subsets of Baire space (denoted by boldface letters $𝜮$ and $𝜫$). Not every ${𝜮}_n^1$ subset of Baire space is ${\Sigma }_n^1$. It is true, however, that if a subset X of Baire space is ${𝜮}_n^1$ then there is a set of natural numbers A such that X is ${\Sigma }_n^{1,A}$. A similar statement holds for ${𝜫}_n^1$ sets. Thus the sets classified by the projective hierarchy are exactly the sets classified by the relativized version of the analytical hierarchy. This relationship is important in effective descriptive set theory. Stated in terms of definability, a set of reals is projective iff it is definable in the language of second-order arithmetic from some real parameter.

A similar relationship between the projective hierarchy and the relativized analytical hierarchy holds for subsets of Cantor space and, more generally, subsets of any effective Polish space.

Table

Pointclasses
Lightface		Boldface
Σ0 0 = Π0 0 = Δ0 0 (sometimes the same as Δ0 1)		Σ0 0 = Π0 0 = Δ0 0 (if defined)
Δ0 1 = recursive		Δ0 1 = clopen
Σ0 1 = recursively enumerable	Π0 1 = co-recursively enumerable	Σ0 1 = G = open	Π0 1 = F = closed
Δ0 2		Δ0 2
Σ0 2	Π0 2	Σ0 2 = Fσ	Π0 2 = Gδ
Δ0 3		Δ0 3
Σ0 3	Π0 3	Σ0 3 = Gδσ	Π0 3 = Fσδ
⋮		⋮
Σ0 <ω = Π0 <ω = Δ0 <ω = Σ1 0 = Π1 0 = Δ1 0 = arithmetical		Σ0 <ω = Π0 <ω = Δ0 <ω = Σ1 0 = Π1 0 = Δ1 0 = boldface arithmetical
⋮		⋮
Δ0 α (α recursive)		Δ0 α (α countable)
Σ0 α	Π0 α	Σ0 α	Π0 α
⋮		⋮
Σ0 ωCK 1 = Π0 ωCK 1 = Δ0 ωCK 1 = Δ1 1 = hyperarithmetical		Σ0 ω1 = Π0 ω1 = Δ0 ω1 = Δ1 1 = B = Borel
Σ1 1 = lightface analytic	Π1 1 = lightface coanalytic	Σ1 1 = A = analytic	Π1 1 = CA = coanalytic
Δ1 2		Δ1 2
Σ1 2	Π1 2	Σ1 2 = PCA	Π1 2 = CPCA
Δ1 3		Δ1 3
Σ1 3	Π1 3	Σ1 3 = PCPCA	Π1 3 = CPCPCA
⋮		⋮
Σ1 <ω = Π1 <ω = Δ1 <ω = Σ2 0 = Π2 0 = Δ2 0 = analytical		Σ1 <ω = Π1 <ω = Δ1 <ω = Σ2 0 = Π2 0 = Δ2 0 = P = projective
⋮		⋮

See also

• Borel hierarchy