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Dear reader of this post,

I have a question concerning the equivalence of two definitions of open subsets (in metric spaces). To avoid confusion, I will state the two definitions and then ask my questions.

The definition I found is: Given a metric space $X$ and let $S \subset Y \subset X$. $S$ is an open subset of $Y$ if $\forall p \in S, \exists r > 0 \ | \ d(p,q)<r, q \in X \rightarrow q \in S$. In contrast, I simply defined $S$ to be an open subset of $Y$ if it is open and a subset of $Y$. The two definition however seem (at least to me) to be quite different because for the set $S$ to be open one only considers whether $S$ consists solely of interior points or not. The first definition however checks the existence of a neighbourhood of $p$ such that all points which are element of the largest set $X$ are also part of the smallest set $S$.

I have the following questions in particular:

  • Is it really true that the definition of a open subset is not a combination of two definitions of an open set and a subset?
  • The part "$q \in X$" of the first definition appears a bit strange to me. Is it correct that we do not considers points in Y but instead of the larger set $X$? What is then an open subset if one considers two sets instead of three?

I am looking forward for your replies.

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up vote 1 down vote accepted

A simple example is $X=\mathbb R^2$ and $Y=\mathbb R\times\{0\}$. Then the standard definition makes the open sets of $Y$ the same as the open sets of $\mathbb R$, while your definition would have no open subsets of $Y$.

If $Y$ is open in $X$, then the two definitions coincide.

It turns out that $S$ is open in $Y$ (under the usual definition) if and only if $S=T\cap Y$ for some open subset $T\subseteq X$.

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Thomas, thank you for your reply! The last definition that you stated "It turns out [...]" is equivalent to the definition of S being open relative to Y, or not? – fabian Aug 1 '13 at 14:26
@fabian Yes. That was the definition of "open" we were given. – Thomas Andrews Aug 1 '13 at 14:36
You mean open is equal to open relative to? – fabian Aug 1 '13 at 14:40
@fabian: When we talk about being open in a subset $Y$ of a given space, we do indeed mean open relative to $Y$. When we just say "open," we mean open in the given space. – Cameron Buie Aug 1 '13 at 15:13

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