# Annulus (mathematics)

In mathematics, an **annulus** (the Latin word for "little ring" is *anulus* / *annulus*, with plural *anuli* / *annuli*) 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 *S*^{1} × (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*:

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, 2*d* in the accompanying diagram. That 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

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

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

## Complex structure

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

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

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.

## See also

- Annulus theorem (or conjecture)
- Visual calculus#Description, for an alternative approach to the area of the annulus
- Spherical shell
- Torus
- List of geometric shapes

## References

- "The Edge of the Universe: Celebrating Ten Years of Math Horizons". Retrieved 9 May 2017.

## External links

- Annulus definition and properties With interactive animation
- Area of an annulus, formula With interactive animation