# Ball (mathematics)

(Redirected from Open ball)

In mathematics, a ball is the inside of a sphere; both concepts apply not only in the three-dimensional space but also for lower and higher dimensions, and for metric spaces in general.

## Metric spaces

Let M be a metric space. The (open) ball of radius r > 0 centred at a point p in M is defined as

$\displaystyle B_r(p) = \{ x \in M \mid d(x,p) < r \},$

where d is the distance function or metric. If the less-than symbol (<) is replaced by a less-than-or-equal-to (≤), the above definition becomes that of a closed ball:

$\displaystyle {\bar B}_r(p) = \{ x \in M \mid d(x,p) \le r \}$ .

Note in particular that a ball (open or closed) always includes p itself, since r > 0. A (open or closed) unit ball is a ball of radius 1.

A subset of a metric space is bounded if it is contained in a ball. A set is totally bounded if given any radius, it is covered by finitely many balls of that radius.

Open balls with respect to a metric d form a basis for the topology induced by d (by definition). This means, among other things, that all open sets in a metric space can be written as a union of open balls.

## Euclidean balls

In n-dimensional Euclidean space with the ordinary (Euclidean) metric, if the space is the line, the ball is an interval, and if the space is the plane, the ball is the disc inside a circle. A closed unit ball is denoted by Dn; its outside is the n-1-sphere Sn-1, e.g. the 3-sphere S3 is the outside of D4 in 4D. These and the corresponding objects in even higher dimensions are called hyperball and hypersphere. See the latter for "volumes" and "areas".

With other metrics the shape of a ball can be different; examples:

• in 2D:
• with the 1-norm (i.e. in taxicab geometry) a ball is a square with the diagonals parallel to the coordinate axes
• with the Chebyshev distance a ball is a square with the sides parallel to the coordinate axes
• in 3D:
• with the 1-norm a ball is a regular octahedron with the body diagonals parallel to the coordinate axes
• with the Chebyshev distance a ball is a cube with the edges parallel to the coordinate axes

## Topological balls

One may talk about balls in any topological space, not necessarily induced by a metric. An (open or closed) ball in such a space is a set which is homeomorphic to an (open or closed) Euclidean ball described above. A ball is known by its dimension: an n-dimensional ball is called an n-ball and denoted $\displaystyle B^n$ or $\displaystyle D^n$ . For distinct n and m, an n-ball is not homeomorphic to an m-ball. A ball need not be smooth; if it is smooth, it need not be diffeomorphic to the Euclidean ball.