Suslin operation

In mathematics, the Suslin operation 𝓐 is an operation that constructs a set from a collection of sets indexed by finite sequences of positive integers. The Suslin operation was introduced by Alexandrov (1916) and Suslin (1917). In Russia it is sometimes called the A-operation after Alexandrov. It is usually denoted by the symbol 𝓐 (a calligraphic capital letter A).

Definitions

A Suslin scheme is a family P = {Ps: s ∈ Ο‰>Ο‰} of subsets of a set X indexed by finite sequences of non-negative integers. The Suslin operation applied to this scheme produces the set 𝓐P = UxβˆˆΟ‰Ο‰ ∩nβˆˆΟ‰ Pxβ†Ύn.

Alternatively, suppose we have Suslin scheme, in other words a function M from finite sequences of positive integers n1,...,nk to sets Mn1,...,nk. The result of the Suslin operation is the set

𝓐(M) = βˆͺ (Mn1 ∩ Mn1,n2 ∩ Mn1,n2, n3 ∩ ...)

where the union is taken over all infinite sequences n1,...,nk,...

If M is a family of subsets of a set X, then 𝓐(M) is the family of subsets of X obtained by applying the Suslin operation 𝓐 to all collections as above where all the sets Mn1,...,nk are in M. The Suslin operation on collections of subsets of X has the property that 𝓐(𝓐(M)) = 𝓐(M). The family 𝓐(M) is closed under taking countable unions or intersections, but is not in general closed under taking complements.

If M is the family of closed subsets of a topological space, then the elements of 𝓐(M) are called Suslin sets, or analytic sets if the space is a Polish space.

References

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