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

An often used model for the corresponding closed annulus is the subspace {(x,y)1x 2+y 24} 2\{(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 November 18, 2014 22:34:09 by Urs Schreiber (