nLab
annulus

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

An often used model for the corresponding closed annulus is the subspace {(x,y)∣1≤x 2+y 2≤4}⊂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)
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