The concept of set is fundamental in mathematics. It is not our purpose to present here an axiomatic account of set theory. Instead we shall assume an intuitive understanding of the terms “set” and “belongs to.” Informally speaking, we say that a *set* is a collection of objects (or elements).

If *S* is a set and *x* is an element of the set *S*, we say *x belongs to S*, and we write An element of a set *S* is also called a member of *S*. If *x* does not belong to *S*, we write .

Let *A* and *B* be sets. We say that *A* and *B* are ...

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