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In mathematics, a **ball** is the volume space bounded by a sphere; it is also called a **solid sphere**.^{ [1] } It may be a **closed ball** (including the boundary points that constitute the sphere) or an **open ball** (excluding them).

- In Euclidean space
- Volume
- In general metric spaces
- In normed vector spaces
- p-norm
- General convex norm
- In topological spaces
- Regions
- See also
- References

These concepts are defined not only in three-dimensional Euclidean space but also for lower and higher dimensions, and for metric spaces in general. A *ball* or **hyperball** in n dimensions is called an **n-ball** and is bounded by an **( n − 1)-sphere**. Thus, for example, a ball in the Euclidean plane is the same thing as a disk, the area bounded by a circle. In Euclidean 3-space, a ball is taken to be the volume bounded by a 2-dimensional sphere. In a one-dimensional space, a ball is a line segment.

In other contexts, such as in Euclidean geometry and informal use, *sphere* is sometimes used to mean *ball*.

In Euclidean n-space, an (open) n-ball of radius r and center x is the set of all points of distance less than r from x. A closed n-ball of radius r is the set of all points of distance less than or equal to r away from x.

In Euclidean n-space, every ball is bounded by a hypersphere. The ball is a bounded interval when *n* = 1, is a ** disk ** bounded by a circle when *n* = 2, and is bounded by a sphere when *n* = 3.

The n-dimensional volume of a Euclidean ball of radius R in n-dimensional Euclidean space is:^{ [2] }

where Γ is Leonhard Euler's gamma function (which can be thought of as an extension of the factorial function to fractional arguments). Using explicit formulas for particular values of the gamma function at the integers and half integers gives formulas for the volume of a Euclidean ball that do not require an evaluation of the gamma function. These are:

In the formula for odd-dimensional volumes, the double factorial (2*k* + 1)!! is defined for odd integers 2*k* + 1 as (2*k* + 1)!! = 1 · 3 · 5 · … · (2*k* − 1) · (2*k* + 1).

Let (*M*, *d*) be a metric space, namely a set M with a metric (distance function) d. The open (metric) **ball of radius***r* > 0 centered at a point p in M, usually denoted by *B _{r}*(

The closed (metric) ball, which may be denoted by *B _{r}*[

Note in particular that a ball (open or closed) always includes p itself, since the definition requires *r* > 0.

The closure of the open ball *B _{r}*(

A ** unit ball ** (open or closed) is a ball of radius 1.

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

The open balls of a metric space can serve as a base, giving this space a topology, the open sets of which are all possible unions of open balls. This topology on a metric space is called the **topology induced by** the metric d.

Any normed vector space V with norm is also a metric space with the metric In such spaces, an arbitrary ball of points around a point with a distance of less than may be viewed as a scaled (by ) and translated (by ) copy of a *unit ball* Such "centered" balls with are denoted with

The Euclidean balls discussed earlier are an example of balls in a normed vector space.

In a Cartesian space ℝ^{n} with the p-norm L_{p}, that is

an open ball around the origin with radius is given by the set

For *n* = 2, in a 2-dimensional plane , "balls" according to the *L*_{1}-norm (often called the * taxicab * or *Manhattan* metric) are bounded by squares with their *diagonals* parallel to the coordinate axes; those according to the *L*_{∞}-norm, also called the Chebyshev metric, have squares with their *sides* parallel to the coordinate axes as their boundaries. The *L*_{2}-norm, known as the Euclidean metric, generates the well known discs within circles, and for other values of p, the corresponding balls are areas bounded by Lamé curves (hypoellipses or hyperellipses).

For *n* = 3, the *L*_{1}- balls are within octahedra with axes-aligned *body diagonals*, the *L*_{∞}-balls are within cubes with axes-aligned *edges*, and the boundaries of balls for L_{p} with *p* > 2 are superellipsoids. Obviously, *p* = 2 generates the inner of usual spheres.

More generally, given any centrally symmetric, bounded, open, and convex subset X of ℝ^{n}, one can define a norm on ℝ^{n} where the balls are all translated and uniformly scaled copies of X. Note this theorem does not hold if "open" subset is replaced by "closed" subset, because the origin point qualifies but does not define a norm on ℝ^{n}.

One may talk about balls in any topological space X, not necessarily induced by a metric. An (open or closed) n-dimensional **topological ball** of X is any subset of X which is homeomorphic to an (open or closed) Euclidean n-ball. Topological n-balls are important in combinatorial topology, as the building blocks of cell complexes.

Any open topological n-ball is homeomorphic to the Cartesian space ℝ^{n} and to the open unit n-cube (hypercube) (0, 1)^{n} ⊆ ℝ^{n}. Any closed topological n-ball is homeomorphic to the closed n-cube [0, 1]^{n}.

An n-ball is homeomorphic to an m-ball if and only if *n* = *m*. The homeomorphisms between an open n-ball B and ℝ^{n} can be classified in two classes, that can be identified with the two possible topological orientations of B.

A topological n-ball need not be smooth; if it is smooth, it need not be diffeomorphic to a Euclidean n-ball.

A number of special regions can be defined for a ball:

- Ball – ordinary meaning
- Disk (mathematics)
- Formal ball, an extension to negative radii
- Neighbourhood (mathematics)
- 3-sphere
- n-sphere, or hypersphere
- Alexander horned sphere
- Manifold
- Volume of an n-ball
- Octahedron – a 3-ball in the
*l*_{1}metric.

In mathematics, more specifically in functional analysis, a **Banach space** is a complete normed vector space. Thus, a Banach space is a vector space with a metric that allows the computation of vector length and distance between vectors and is complete in the sense that a Cauchy sequence of vectors always converges to a well defined limit that is within the space.

In mathematics, **Hausdorff dimension** is a measure of *roughness*, or more specifically, fractal dimension, that was first introduced in 1918 by mathematician Felix Hausdorff. For instance, the Hausdorff dimension of a single point is zero, of a line segment is 1, of a square is 2, and of a cube is 3. That is, for sets of points that define a smooth shape or a shape that has a small number of corners—the shapes of traditional geometry and science—the Hausdorff dimension is an integer agreeing with the usual sense of dimension, also known as the topological dimension. However, formulas have also been developed that allow calculation of the dimension of other less simple objects, where, solely on the basis of their properties of scaling and self-similarity, one is led to the conclusion that particular objects—including fractals—have non-integer Hausdorff dimensions. Because of the significant technical advances made by Abram Samoilovitch Besicovitch allowing computation of dimensions for highly irregular or "rough" sets, this dimension is also commonly referred to as the *Hausdorff–Besicovitch dimension.*

In mathematics, a **metric space** is a set together with a metric on the set. The metric is a function that defines a concept of *distance* between any two members of the set, which are usually called points. The metric satisfies a few simple properties. Informally:

In the part of mathematics referred to as topology, a **surface** is a two-dimensional manifold. Some surfaces arise as the boundaries of three-dimensional solids; for example, the sphere is the boundary of the solid ball. Other surfaces arise as graphs of functions of two variables; see the figure at right. However, surfaces can also be defined abstractly, without reference to any ambient space. For example, the Klein bottle is a surface that cannot be embedded in three-dimensional Euclidean space.

In mathematics, particularly in topology, an **open set** is an abstract concept generalizing the idea of an open interval in the real line. The simplest example is in metric spaces, where open sets can be defined as those sets which contain a ball around each of their points ; however, an open set, in general, can be very abstract: any collection of sets can be called open, as long as the union of an arbitrary number of open sets in the collection is open, the intersection of a finite number of open sets is open, and the space itself is open. These conditions are very loose, and they allow enormous flexibility in the choice of open sets. In the two extremes, every set can be open, or no set can be open but the space itself and the empty set.

In mathematics, an ** n-sphere** is a topological space that is homeomorphic to a

In mathematics, the ** L^{p} spaces** are function spaces defined using a natural generalization of the

In topology, a **discrete space** is a particularly simple example of a topological space or similar structure, one in which the points form a *discontinuous sequence*, meaning they are *isolated* from each other in a certain sense. The discrete topology is the finest topology that can be given on a set, i.e., it defines all subsets as open sets. In particular, each singleton is an open set in the discrete topology.

In real analysis the **Heine–Borel theorem**, named after Eduard Heine and Émile Borel, states:

In mathematics, a **hyperbolic space** is a homogeneous space that has a constant negative curvature, where in this case the curvature is the sectional curvature. It is hyperbolic geometry in more than 2 dimensions, and is distinguished from Euclidean spaces with zero curvature that define the Euclidean geometry, and elliptic geometry that have a constant positive curvature.

This is a glossary of some terms used in Riemannian geometry and metric geometry — it doesn't cover the terminology of differential topology.

In mathematics, **low-dimensional topology** is the branch of topology that studies manifolds, or more generally topological spaces, of four or fewer dimensions. Representative topics are the structure theory of 3-manifolds and 4-manifolds, knot theory, and braid groups. This can be regarded as a part of geometric topology. It may also be used to refer to the study of topological spaces of dimension 1, though this is more typically considered part of continuum theory.

In mathematics, a **norm** is a function from a real or complex vector space to the nonnegative real numbers that behaves in certain ways like the distance from the origin: it commutes with scaling, obeys a form of the triangle inequality, and is zero only at the origin. In particular, the Euclidean distance of a vector from the origin is a norm, called the Euclidean norm, or 2-norm, which may also be defined as the square root of the inner product of a vector with itself.

In mathematics, a **real coordinate space** of dimension n, written **R**^{n} or ℝ^{n}, is a coordinate space over the real numbers. This means that it is the set of the n-tuples of real numbers. With component-wise addition and scalar multiplication, it is a real vector space.

In topology and related branches of mathematics, **total-boundedness** is a generalization of compactness for circumstances in which a set is not necessarily closed. A totally bounded set can be covered by finitely many subsets of every fixed "size"

In linear algebra and related areas of mathematics a **balanced set**, **circled set** or **disk** in a vector space is a set S such that *aS* ⊆ *S* for all scalars a satisfying |*a*| ≤ 1.

In mathematics, a **manifold** is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or *n-manifold* for short, is a topological space with the property that each point has a neighborhood that is homeomorphic to the Euclidean space of dimension n.

In topology, a branch of mathematics, a **topological manifold** is a topological space which locally resembles real *n*-dimensional Euclidean space. Topological manifolds are an important class of topological spaces, with applications throughout mathematics. All manifolds are topological manifolds by definition. Other types of manifolds are formed by adding structure to a topological manifold. Every manifold has an "underlying" topological manifold, obtained by simply "forgetting" the added structure.

In the study of metric spaces in mathematics, there are various notions of two metrics on the same underlying space being "the same", or **equivalent**.

In mathematics, a **unit sphere** is the set of points of distance 1 from a fixed central point, where a generalized concept of distance may be used; a closed **unit ball** is the set of points of distance less than or equal to 1 from a fixed central point. Usually a specific point has been distinguished as the origin of the space under study and it is understood that a unit sphere or unit ball is centered at that point. Therefore, one speaks of "the" unit ball or "the" unit sphere.

- ↑
- ↑ Equation 5.19.4,
*NIST Digital Library of Mathematical Functions.*http://dlmf.nist.gov/,%5B%5D Release 1.0.6 of 2013-05-06.

- Smith, D. J.; Vamanamurthy, M. K. (1989). "How small is a unit ball?".
*Mathematics Magazine*.**62**(2): 101–107. doi:10.1080/0025570x.1989.11977419. JSTOR 2690391. - Dowker, J. S. (1996). "Robin Conditions on the Euclidean ball".
*Classical and Quantum Gravity*.**13**(4): 585–610. arXiv: hep-th/9506042 . Bibcode:1996CQGra..13..585D. doi:10.1088/0264-9381/13/4/003. - Gruber, Peter M. (1982). "Isometries of the space of convex bodies contained in a Euclidean ball".
*Israel Journal of Mathematics*.**42**(4): 277–283. doi:10.1007/BF02761407.

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