# Set

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An aggregate, totality, collection of any objects whatever, called its elements, which have a common characteristic property. "A set is many, conceivable to us as one" (G. Cantor). This is not in a true sense a logical definition of the notion of a set, rather it is just an explanation (because defining the notion means finding a generic idea to which the given idea belongs as a species; but a set is, unfortunately, itself a broad notion in mathematics and logic). In this connection it is possible either to give a list of the elements of the set, an enumeration of it, or to give a rule for determining whether or not a given object belongs to the set considered, a description of it (moreover, the first is really acceptable only when the question concerns finite sets).

For a meaningful development of "naive" set theory such an explanation is quite sufficient, because for the mathematical theory it is only essential to define the relations between the elements of a set (or between the sets themselves), and not their nature. To describe sets which may be elements of another set, to avoid the so-called antinomies (cf. Antinomy) one introduces, for example, the terminology "class" . And then, speaking more formally, set theory deals with objects called classes (cf. Class), for which there is defined a relation of membership, and a set itself is defined as a class which is an element of some class.

Recently the unifying role of category theory (and, in particular, the notion of a universal set) has become more clear. The construction of a category is based on axiomatic set theory and allows one to consider, for example, such "large" collections as the category of all sets, groups, topological spaces, etc.

#### References

 [1] , Lectures on the sets of Georg Cantor , New ideas in mathematics , 6 , St. Petersburg (1914) (In Russian) [2] Yu.A. Shikhanovich, "Introduction to modern mathematics" , Moscow (1965) (In Russian) [3] N.I. Kondakov, "Logical reference-dictionary" , Moscow (1975) (In Russian) [4] N. Bourbaki, "Elements of mathematics. Theory of sets" , Addison-Wesley (1968) (Translated from French) [5] P.S. Novikov, "Elements of mathematical logic" , Oliver & Boyd and Acad. Press (1964) (Translated from Russian) [6] P.M. Cohn, "Universal algebra" , Reidel (1981) [7] J.R. Shoenfield, "Mathematical logic" , Addison-Wesley (1967)