The Annulus reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Annulus

An annulus (from Latin anulus, "little ring") is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. The adjective form is annular (for example, an annular eclipse).

In complex analysis an annulus ann(a; r, R) in the complex plane is an open region defined by:

If r is 0, the region is known as the punctured disk of radius R around the point a.

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