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Section Summary
Important ideas that we discussed in this section include the following.
We can consider a set to be a well-defined collection of elements.
A subset of a set is any collection of elements from that set. That is, a subset \(S\) of a set \(X\) is a set with the property that if \(s \in S\text{,}\) then \(s \in X\text{.}\)
If \(X\) and \(Y\) are sets, then the union \(X \cup Y\) is the set
\begin{equation*}
X \cup Y = \{z \mid z \in X \text{ or } z \in Y\}\text{.}
\end{equation*}
The union of an arbitrary collection \(\{X_{\alpha}\}\) of sets for \(\alpha\) in some indexing set \(I\) is the set
\begin{equation*}
\bigcup_{\alpha \in I} X_{\alpha} = \{z \mid z \in X_{\beta} \text{ for some } \beta \in I\}\text{.}
\end{equation*}
If \(X\) and \(Y\) are sets, then the intersection \(X \cap Y\) is the set
\begin{equation*}
X \cap Y = \{z \mid z \in X \text{ and } z \in Y\}\text{.}
\end{equation*}
The intersection of an arbitrary collection \(\{X_{\alpha}\}\) of sets for \(\alpha\) in some indexing set \(I\) is the set
\begin{equation*}
\bigcap_{\alpha \in I} X_{\alpha} = \{z \mid z \in X_{\beta} \text{ for all } \beta \in I\}\text{.}
\end{equation*}
If \(X\) is a set and \(A\) is a subset of \(X\text{,}\) then the complement of \(A\) in \(X\) is the set
\begin{equation*}
A^c = \{x \in X \mid x \notin A\}\text{.}
\end{equation*}
If \(\{X_{i}\}\) is a collection of sets with \(i\) in some indexing set \(I\text{,}\) where \(I\) is finite or \(I\) is the set of positive integers, the Cartesian product \(\Pi_{i \in I} X_i\) of the sets \(X_{i}\) as the set of all ordered tuples of the form \((x_i)\) where \(i \in I\text{.}\)