Definition:Open Set/Metric Space

Definition
Let $M = \left({A, d}\right)$ be a metric space.

Let $U \subseteq A$.

Then $U$ is an open set in $M$ it is a neighborhood of each of its points.

That is:
 * $\forall y \in U: \exists \epsilon \in \R_{>0}: B_\epsilon \left({y}\right) \subseteq U$

where $B_\epsilon \left({y}\right)$ is the open $\epsilon$-ball of $y$.

That is, for every point $y$ in $U$, we can find an $\epsilon \in \R_{>0}$, dependent on that $y$, such that the open $\epsilon$-ball of $y$ lies entirely inside $U$.

Another way of saying the same thing is that one can not get out of $U$ by moving an arbitrarily small distance from any point in $U$.

It is important to note that, in general, the values of $\epsilon$ depend on $y$.

That is, it is not required that:
 * $\exists \epsilon \in \R_{>0}: \forall y \in U: B_\epsilon \left({y}\right) \subseteq U$

Also known as
An open set in $M$ can also be referred to as:
 * open in $M$
 * a $d$-open set
 * $d$-open.

An open set in $M$ is sometimes seen written as open subset of $A$.

Also see

 * Open Ball is Open Set
 * Open Set may not be Open Ball


 * Definition:Topology Induced by Metric: the set of all open sets in a given metric space