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gwsinger
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The following two definitions are taken directly from Rudin's Principles of Mathematical Analysis.
(1) OPEN SUBSET DEFINITION: If [itex]G[/itex] is an open subset of some metric space [itex]X[/itex], then [itex]G \subset X[/itex] and for any [itex]p \in G[/itex] we can find some [itex]r_{p} > 0[/itex] such that the conditions [itex]d(p,q) < r_p[/itex], [itex]q \in X[/itex] implies [itex]q \in G[/itex].
(2) RELATIVE OPENNESS DEFINITION: Suppose [itex]E \subset Y \subset X[/itex]. We say that [itex]E[/itex] is open relative to [itex]Y[/itex] if for each [itex]p \in E[/itex] there is associated an [itex]r_p > 0[/itex] such that [itex]q \in E[/itex] whenever [itex]d(p,q) < r_p[/itex] and [itex]q \in Y[/itex].
1. Couldn't we just say that [itex]E[/itex] is open relative to [itex]Y[/itex] if and only if [itex]E[/itex] is an open subset of [itex]Y[/itex]? Rudin never flat out says this but I just wanted to make sure I wasn't missing something.
2. Suppose [itex]X[/itex] has an isolated point [itex]i[/itex] and it happens that [itex]i \in G[/itex]. While it's true that this would preclude [itex]X[/itex] from being an open set (since an open set must be comprised solely of internal points and [itex]i[/itex] is an isolated point), wouldn't it still be a possibility that [itex]G[/itex] could be an open subset of [itex]X[/itex]? After all, it's (trivially) true that for [itex]i[/itex] we could find some ball [itex]B[/itex] that satisfies the condition set forth in the definition (1) above. If this is true, wouldn't it follow that an open subset is not necessarily itself an open set?
(1) OPEN SUBSET DEFINITION: If [itex]G[/itex] is an open subset of some metric space [itex]X[/itex], then [itex]G \subset X[/itex] and for any [itex]p \in G[/itex] we can find some [itex]r_{p} > 0[/itex] such that the conditions [itex]d(p,q) < r_p[/itex], [itex]q \in X[/itex] implies [itex]q \in G[/itex].
(2) RELATIVE OPENNESS DEFINITION: Suppose [itex]E \subset Y \subset X[/itex]. We say that [itex]E[/itex] is open relative to [itex]Y[/itex] if for each [itex]p \in E[/itex] there is associated an [itex]r_p > 0[/itex] such that [itex]q \in E[/itex] whenever [itex]d(p,q) < r_p[/itex] and [itex]q \in Y[/itex].
1. Couldn't we just say that [itex]E[/itex] is open relative to [itex]Y[/itex] if and only if [itex]E[/itex] is an open subset of [itex]Y[/itex]? Rudin never flat out says this but I just wanted to make sure I wasn't missing something.
2. Suppose [itex]X[/itex] has an isolated point [itex]i[/itex] and it happens that [itex]i \in G[/itex]. While it's true that this would preclude [itex]X[/itex] from being an open set (since an open set must be comprised solely of internal points and [itex]i[/itex] is an isolated point), wouldn't it still be a possibility that [itex]G[/itex] could be an open subset of [itex]X[/itex]? After all, it's (trivially) true that for [itex]i[/itex] we could find some ball [itex]B[/itex] that satisfies the condition set forth in the definition (1) above. If this is true, wouldn't it follow that an open subset is not necessarily itself an open set?