A set is a collection of distinct elements. Order does not matter, and repeated entries are counted once. The universal set \(\xi\) specifies all objects under consideration.
| Notation | Meaning |
|---|---|
| \(x\in A\); \(x\notin A\) | x is an element of A; x is not an element of A. |
| \(A\subseteq B\) | Every element of A is in B; equality is allowed. |
| \(A\subset B\) | A is a proper subset of B: every element of A is in B, and A is not B. |
| \(\varnothing\); \(n(A)\) | The empty set; the number of elements of A. |
In this chapter, \(\subset\) denotes a proper subset. Compare an element with a set using \(\in\); compare two sets using \(\subseteq\) or \(\subset\). If \(A=\{2,4\}\), then \(2\in A\) and \(\{2\}\subset A\), but \(\{2\}\notin A\).
Set-builder notation gives a rule and a domain. For example, \(\{x\in\mathbb Z:-2\le x<2\}=\{-2,-1,0,1\}\). The set \(\{0\}\) has one element; it is not \(\varnothing\).
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