# Contents

## Definition

###### Definition

(finer/coarser topologies)

Let $X$ be a set, and let $\tau_1, \tau_2 \in P(X)$ be two topologies on $X$, hence two choices of open subsets for $X$, making it a topological space. If

$\tau_1 \subset \tau_2$

hence if every open subset of $X$ with respect to $\tau_1$ is also regarded as open by $\tau_2$, then one says that

• the topology $\tau_2$ is finer than the topology $\tau_2$

• the topology $\tau_1$ is coarser than the topology $\tau_2$.

## Examples

###### Example

(discrete and co-discrete topology)

Let $S$ be any set. Then there are always the following two extreme possibilities of equipping $X$ with a topology $\tau \subset P(X)$, and hence making it a topological space:

1. $\tau \coloneq P(S)$ the set of all open subsets;

this is called the discrete topology on $S$, it is the finest topology (def. 1) on $X$,

we write $Disc(S)$ for the resulting topological space;

2. $\tau \coloneqq \{ \emptyset, S \}$ the set contaning only the empty subset of $S$ and all of $S$ itself;

this is called the codiscrete topology on $S$, it is the coarsest topology (def. 1) on $X$

we write $CoDisc(S)$ for the resulting topological space.

## References

Last revised on July 10, 2017 at 23:19:18. See the history of this page for a list of all contributions to it.