# nLab annulus

### Context

#### Topology

topology

algebraic topology

## Examples

An (open) annulus is a topological space that is homeomorphic to the disk with an interior point removed: $D^2 \setminus \{0\}$.

An often used model for the corresponding closed annulus is the subspace $\{(x,y)\mid 1\leq x^2 + y^2 \leq 4\} \subset \mathbb{R}^2$, of the plane consisting of the point lying between a unit circle and a circle of radius 2, both centred on the origin.

Revised on October 27, 2010 12:42:13 by Tim Porter (95.147.236.0)