In mathematics, an annulus (plural: annuli) is a ring-shaped object that is formed by the region between two concentric circles. It can be described in the following way: 1. **Definition**: An annulus is the set of points in a plane that are situated between two circles, typically denoted by an inner circle with radius \( r_1 \) and an outer circle with radius \( r_2 \), where \( r_2 > r_1 \).
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