# Annulus (mathematics)

In mathematics, an annulus (the Latin word for "little ring" is anulus, with plural anuli) is a ring-shaped object — 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. Informally, it has the shape of a hardware washer.

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:

${\displaystyle A=\pi R^{2}-\pi r^{2}=\pi (R^{2}-r^{2}).}$

The area of an annulus is determined by the length of the longest line segment within the annulus, which is the chord tangent to the inner circle, 2d in the accompanying diagram. This can be shown using the Pythagorean theorem, since this line is tangent to the smaller circle and perpendicular to its radius at that point, so d and r are sides of a right-angled triangle with hypotenuse R, and the area of the annulus is given by

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

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:

${\displaystyle A=\int _{r}^{R}2\pi \rho \,d\rho =\pi \left(R^{2}-r^{2}\right).}$

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

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

## Complex structure

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

${\displaystyle r<|z-a|

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

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

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.