Unions and Intersections of Closed Sets

Let \(S \subset \mathbf{R}^n\). We call \(x \in \mathbf{R}^n\) a boundary point of \(S\) if for every \(\varepsilon > 0\) the ball \(B(x, \varepsilon)\) centered at \(x\) of radius \(\varepsilon\) contains at least one point \(y \in S\) and at least one point \(z \notin S\). The set of all boundary points of \(S\) is called the boundary of \(S\) denoted \(\partial S\). We say that \(S\) is closed if \(\partial S \subset S\).

Now let \({S_\alpha}\) be a collection of closed sets indexed by \(A\) (that is \(\alpha\) ranges over all possible values in \(A\)). We would like to show that the set
S = \bigcap_{\alpha \in A} S_\alpha
is closed. (Recall that \(x \in \bigcap_{\alpha \in A} S_\alpha\) if and only if \(x \in S_\alpha\) for all \(\alpha \in A\).) To this end, we must show that every \(x \in \partial S\) is also in \(S\). To see this is the case, let \(x \in \partial S\). Since \(x \in \partial S\), for every \(\varepsilon > 0\), there exist \(y \in S\) and \(z \notin S\) such that \(y, z \in B(x, \varepsilon)\). Since \(y \in S\), from the definition of \(S\), we must have \(y \in S_\alpha\) for all \(\alpha\). Therefore, \(x \in S_\alpha \cup \partial S_\alpha\) for each \(\alpha\), hence \(x \in S_\alpha\) for each \(\alpha\) as \(\partial S_\alpha \subset S_\alpha\). Thus we may conclude that
x \in \bigcap_{\alpha \in A} S_\alpha = S
which is what we needed to show.

Now we move on to consider unions of closed sets. First let \(S = \bigcup_{i = 1}^k S_i\) be a finite union of closed sets. Suppose \(x \in \partial S\). Again, for every \(\varepsilon > 0\) there exists \(y \in S\) and \(z \notin S\) with \(y, z \in B(x, \varepsilon)\). We would like to show that \(x \in S\), hence \(S\) is closed. Consider a sequence \(y_1, y_2, \ldots \in S\) with \(|x – y_i|

Will Rosenbaum

Saarbrücken, Germany

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