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).
A Suslin scheme is a family of subsets of a set indexed by finite sequences of non-negative integers. The Suslin operation applied to this scheme produces the set
Alternatively, suppose we have a Suslin scheme, in other words a function from finite sequences of positive integers to sets . The result of the Suslin operation is the set
where the union is taken over all infinite sequences
If is a family of subsets of a set , then is the family of subsets of obtained by applying the Suslin operation to all collections as above where all the sets are in . The Suslin operation on collections of subsets of has the property that . The family is closed under taking countable unions or intersections, but is not in general closed under taking complements.
If is the family of closed subsets of a topological space, then the elements of are called Suslin sets, or analytic sets if the space is a Polish space.
For each finite sequence , let be the infinite sequences that extend . This is a clopen subset of . If is a Polish space and is a continuous function, let . Then is a Suslin scheme consisting of closed subsets of and .