# nLab annulus

## Idea

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.

Last revised on November 18, 2014 at 22:34:09. See the history of this page for a list of all contributions to it.