# Annulus (mathematics)

In mathematics, an annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped object, especially a region bounded by two concentric circles. The adjectival form is annular (as in annular eclipse).

The open annulus is topologically equivalent to both the open cylinder S1 × (0,1) and the punctured plane.

The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r:

$A = \pi R^2 - \pi r^2 = \pi(R^2 - r^2)\,.$

The area of an annulus can be obtained from the length of the longest interval that can lie completely inside the annulus, 2*d in the accompanying diagram. This can be proven by the Pythagorean theorem; the length of the longest interval that can lie completely inside the annulus will be tangent to the smaller circle and form a right angle with its radius at that point. Therefore d and r are the sides of a right angled triangle with hypotenuse R and the area is given by:

$A = \pi\left(R^2 - r^2\right) = \pi d^2 \,.$

The area can also be obtained via calculus by dividing the annulus up into an infinite number of annuli of infinitesimal width and area 2πρ dρ and then integrating from ρ = r to ρ = R:

$A = \int_r^R 2\pi\rho\, d\rho = \pi\left(R^2 - r^2\right).$

The area of an annulus sector of angle θ, with θ measured in radians, is given by:

$A = \frac{\theta}{2} \left(R^2 - r^2\right)$

## Complex structure

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

$r < |z-a| < R.\,$

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

As a subset of the complex plane, an annulus can be considered as a Riemann surface. The complex structure of an annulus depends only on the ratio r/R. Each annulus ann(a; r, R) can be holomorphically mapped to a standard one centered at the origin and with outer radius 1 by the map

$z \mapsto \frac{z-a}{R}.$

The inner radius is then r/R < 1.

The Hadamard three-circle theorem is a statement about the maximum value a holomorphic function may take inside an annulus.