# Definition:Coarser Topology

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## Definition

Let $S$ be a set.

Let $\tau_1$ and $\tau_2$ be topologies on $S$.

Let $\tau_1 \subseteq \tau_2$.

Then $\tau_1$ is said to be **coarser** than $\tau_2$.

This can be expressed as:

- $\tau_1 \le \tau_2 := \tau_1 \subseteq \tau_2$

### Strictly Coarser

Let $\tau_1 \subsetneq \tau_2$.

Then $\tau_1$ is said to be **strictly coarser** than $\tau_2$.

This can be expressed as:

- $\tau_1 < \tau_2 := \tau_1 \subsetneq \tau_2$

## Also known as

The terms **weaker** or **smaller** are often encountered, meaning the same thing as **coarser**.

Unfortunately, the term **stronger** is also sometimes encountered, meaning exactly the same thing.

To remove any ambiguity as to which one is meant, it is recommended that **coarser** be used exclusively.

## Also see

- Definition:Finer Topology, the opposite of
**coarser topology**. - Definition:Discrete Topology
- Definition:Indiscrete Topology
- Discrete Topology is Finest Topology
- Indiscrete Topology is Coarsest Topology

## Sources

- 1964: Steven A. Gaal:
*Point Set Topology*... (previous) ... (next): Chapter $\text {I}$: Topological Spaces: $1$. Open Sets and Closed Sets - 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*(2nd ed.) ... (previous) ... (next): Part $\text I$: Basic Definitions: Section $1$: General Introduction