# n-sphere

(Redirected from Hypersphere)

In mathematics, an n-sphere or hypersphere is an n-dimensional generalization of the 1-dimensional circle and 2-dimensional sphere to any non-negative integer n. The n-sphere is the setting for n-dimensional spherical geometry.

Considered extrinsically, as a hypersurface embedded in (n + 1)-dimensional Euclidean space, an n-sphere is the locus of points at equal distance (the radius) from a given center point. Its interior, consisting of all points closer to the center than the radius, is an (n + 1)-dimensional ball. In particular:

Given a Cartesian coordinate system, the unit n-sphere of radius 1 can be defined as:

${\displaystyle S^{n}=\left\{x\in \mathbb {R} ^{n+1}:\left\|x\right\|=1\right\}.}$

Considered intrinsically, when n ≥ 1, the n-sphere is a Riemannian manifold of positive constant curvature, and is orientable. The geodesics of the n-sphere are called great circles.

The stereographic projection maps the n-sphere onto n-space with a single adjoined point at infinity; under the metric thereby defined, ${\displaystyle \mathbb {R} ^{n}\cup \{\infty \}}$ is a model for the n-sphere.

In the more general setting of topology, any topological space that is homeomorphic to the unit n-sphere is called an n-sphere. Under inverse stereographic projection, the n-sphere is the one-point compactification of n-space. The n-spheres admit several other topological descriptions: for example, they can be constructed by gluing two n-dimensional spaces together, by identifying the boundary of an n-cube with a point, or (inductively) by forming the suspension of an (n − 1)-sphere. When n ≥ 2 it is simply connected; the 1-sphere (circle) is not simply connected; the 0-sphere is not even connected, consisting of two discrete points.

## Description

For any natural number n, an n-sphere of radius r is defined as the set of points in (n + 1)-dimensional Euclidean space that are at distance r from some fixed point c, where r may be any positive real number and where c may be any point in (n + 1)-dimensional space. In particular:

• a 0-sphere is a pair of points {cr, c + r}, and is the boundary of a line segment (1-ball).
• a 1-sphere is a circle of radius r centered at c, and is the boundary of a disk (2-ball).
• a 2-sphere is an ordinary 2-dimensional sphere in 3-dimensional Euclidean space, and is the boundary of an ordinary ball (3-ball).
• a 3-sphere is a 3-dimensional sphere in 4-dimensional Euclidean space.

### Cartesian coordinates

The set of points in (n + 1)-space, (x1, x2, ..., xn+1), that define an n-sphere, Sn(r), is represented by the equation:

${\displaystyle r^{2}=\sum _{i=1}^{n+1}(x_{i}-c_{i})^{2},}$

where c = (c1, c2, ..., cn+1) is a center point, and r is the radius.

The above n-sphere exists in (n + 1)-dimensional Euclidean space and is an example of an n-manifold. The volume form ω of an n-sphere of radius r is given by

${\displaystyle \omega ={\frac {1}{r}}\sum _{j=1}^{n+1}(-1)^{j-1}x_{j}\,dx_{1}\wedge \cdots \wedge dx_{j-1}\wedge dx_{j+1}\wedge \cdots \wedge dx_{n+1}={\star }dr}$

where ${\displaystyle {\star }}$ is the Hodge star operator; see Flanders (1989, §6.1) for a discussion and proof of this formula in the case r = 1. As a result,

${\displaystyle dr\wedge \omega =dx_{1}\wedge \cdots \wedge dx_{n+1}.}$

### n-ball

The space enclosed by an n-sphere is called an (n + 1)-ball. An (n + 1)-ball is closed if it includes the n-sphere, and it is open if it does not include the n-sphere.

Specifically:

• A 1-ball, a line segment, is the interior of a 0-sphere.
• A 2-ball, a disk, is the interior of a circle (1-sphere).
• A 3-ball, an ordinary ball, is the interior of a sphere (2-sphere).
• A 4-ball is the interior of a 3-sphere, etc.

### Topological description

Topologically, an n-sphere can be constructed as a one-point compactification of n-dimensional Euclidean space. Briefly, the n-sphere can be described as Sn = ℝn ∪ {∞}, which is n-dimensional Euclidean space plus a single point representing infinity in all directions. In particular, if a single point is removed from an n-sphere, it becomes homeomorphic to n. This forms the basis for stereographic projection.[1]

## Volume and area

Let Sn−1 be the surface area of the unit (n − 1)-sphere of radius 1 embedded in n-dimensional Euclidean space, and let Vn be the volume of its interior, the unit n-ball. The surface area of an arbitrary (n − 1)-sphere is proportional to the (n − 1)st power of the radius, and the volume of an arbitrary n-ball is proportional to the nth power of the radius.

The 0-ball is sometimes defined as a single point. The 0-dimensional Hausdorff measure is the number of points in a set. So

${\displaystyle V_{0}=1.}$

A unit 1-ball is a line segment whose points have a single coordinate in the interval [−1, 1] of length 2, and the 0-sphere consists of its two end-points, with coordinate {−1, 1}.

${\displaystyle S_{0}=2,\quad V_{1}=2.}$

A unit 1-sphere is the unit circle in the Euclidean plane, and its interior is the unit disk (2-ball).

${\displaystyle S_{1}=2\pi ,\quad V_{2}=\pi .}$

The interior of a 2-sphere in three-dimensional space is the unit 3-ball.

${\displaystyle S_{2}=4\pi ,\quad V_{3}={\tfrac {4}{3}}\pi .}$

In general, Sn−1 and Vn are given in closed form by the expressions

${\displaystyle S_{n-1}={\frac {2\pi ^{n/2}}{\Gamma {\bigl (}{\frac {n}{2}}{\bigr )}}},\quad V_{n}={\frac {\pi ^{n/2}}{\Gamma {\bigl (}{\frac {n}{2}}+1{\bigr )}}}}$

where Γ is the gamma function.

As n tends to infinity, the volume of the unit n-ball (ratio between the volume of an n-ball of radius 1 and an n-cube of side length 1) tends to zero.[2]

### Recurrences

The surface area, or properly the n-dimensional volume, of the n-sphere at the boundary of the (n + 1)-ball of radius R is related to the volume of the ball by the differential equation

${\displaystyle S_{n}R^{n}={\frac {dV_{n+1}R^{n+1}}{dR}}={(n+1)V_{n+1}R^{n}}.}$

Equivalently, representing the unit n-ball as a union of concentric (n − 1)-sphere shells,

${\displaystyle V_{n+1}=\int _{0}^{1}S_{n}r^{n}\,dr={\frac {1}{n+1}}S_{n}.}$

We can also represent the unit (n + 2)-sphere as a union of products of a circle (1-sphere) with an n-sphere. Then ${\displaystyle S_{n+2}=2\pi V_{n+1}.}$ Since S1 = 2π V0, the equation

${\displaystyle S_{n+1}=2\pi V_{n}}$

holds for all n. Along with the base cases ${\displaystyle S_{0}=2,}$ ${\displaystyle V_{1}=2}$ from above, these recurrences can be used to compute the surface area of any sphere or volume of any ball.

## Spherical coordinates

We may define a coordinate system in an n-dimensional Euclidean space which is analogous to the spherical coordinate system defined for 3-dimensional Euclidean space, in which the coordinates consist of a radial coordinate r, and n − 1 angular coordinates φ1, φ2, ..., φn−1, where the angles φ1, φ2, ..., φn−2 range over [0, π] radians (or over [0, 180] degrees) and φn−1 ranges over [0, 2π) radians (or over [0, 360) degrees). If xi are the Cartesian coordinates, then we may compute x1, ..., xn from r, φ1, ..., φn−1 with:[3]

{\displaystyle {\begin{aligned}x_{1}&=r\cos(\varphi _{1}),\\[5mu]x_{2}&=r\sin(\varphi _{1})\cos(\varphi _{2}),\\[5mu]x_{3}&=r\sin(\varphi _{1})\sin(\varphi _{2})\cos(\varphi _{3}),\\&\qquad \vdots \\x_{n-1}&=r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\cos(\varphi _{n-1}),\\[5mu]x_{n}&=r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\sin(\varphi _{n-1}).\end{aligned}}}

Except in the special cases described below, the inverse transformation is unique:

{\displaystyle {\begin{aligned}r&={\textstyle {\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{2}}^{2}+{x_{1}}^{2}}}},\\[5mu]\varphi _{1}&=\operatorname {atan2} \left({\textstyle {\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{2}}^{2}}}},x_{1}\right),\\[5mu]\varphi _{2}&=\operatorname {atan2} \left({\textstyle {\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{3}}^{2}}}},x_{2}\right),\\&\qquad \vdots \\\varphi _{n-2}&=\operatorname {atan2} \left({\textstyle {\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}}}},x_{n-2}\right),\\[5mu]\varphi _{n-1}&=\operatorname {atan2} \left(x_{n},x_{n-1}\right).\end{aligned}}}

where atan2 is the two-argument arctangent function.

There are some special cases where the inverse transform is not unique; φk for any k will be ambiguous whenever all of xk, xk+1, ... xn are zero; in this case φk may be chosen to be zero. (For example, for the 2-sphere, when the polar angle is 0 or π then the point is one of the poles, zenith or nadir, and the choice of azimuthal angle is arbitrary.)

### Spherical volume and area elements

To express the volume element of n-dimensional Euclidean space in terms of spherical coordinates, let ${\displaystyle s_{k}=\sin \varphi _{k}}$ and ${\displaystyle c_{k}=\cos \varphi _{k}}$ for concision, then observe that the Jacobian matrix of the transformation is:

${\displaystyle J_{n}={\begin{pmatrix}c_{1}&-rs_{1}&0&0&\cdots &0\\s_{1}c_{2}&rc_{1}c_{2}&-rs_{1}s_{2}&0&\cdots &0\\\vdots &\vdots &\vdots &&\ddots &\vdots \\&&&&&0\\s_{1}\cdots s_{n-2}c_{n-1}&\cdots &\cdots &&&-rs_{1}\cdots s_{n-2}s_{n-1}\\s_{1}\cdots s_{n-2}s_{n-1}&rc_{1}\cdots s_{n-1}&\cdots &&&{\phantom {-}}rs_{1}\cdots s_{n-2}c_{n-1}\end{pmatrix}}.}$

The determinant of this matrix can be calculated by induction. When n = 2, a straightforward computation shows that the determinant is r. For larger n, observe that Jn can be constructed from Jn−1 as follows. Except in column n, rows n − 1 and n of Jn are the same as row n − 1 of Jn−1, but multiplied by an extra factor of cos φn−1 in row n − 1 and an extra factor of sin φn−1 in row n. In column n, rows n − 1 and n of Jn are the same as column n − 1 of row n − 1 of Jn−1, but multiplied by extra factors of sin φn−1 in row n − 1 and cos φn−1 in row n, respectively. The determinant of Jn can be calculated by Laplace expansion in the final column. By the recursive description of Jn, the submatrix formed by deleting the entry at (n − 1, n) and its row and column almost equals Jn−1, except that its last row is multiplied by sin φn−1. Similarly, the submatrix formed by deleting the entry at (n, n) and its row and column almost equals Jn−1, except that its last row is multiplied by cos φn−1. Therefore the determinant of Jn is

{\displaystyle {\begin{aligned}|J_{n}|&=(-1)^{(n-1)+n}(-rs_{1}\dotsm s_{n-2}s_{n-1})(s_{n-1}|J_{n-1}|)\\&\qquad {}+(-1)^{n+n}(rs_{1}\dotsm s_{n-2}c_{n-1})(c_{n-1}|J_{n-1}|)\\&=(rs_{1}\dotsm s_{n-2}|J_{n-1}|(s_{n-1}^{2}+c_{n-1}^{2})\\&=(rs_{1}\dotsm s_{n-2})|J_{n-1}|.\end{aligned}}}

Induction then gives a closed-form expression for the volume element in spherical coordinates

{\displaystyle {\begin{aligned}d^{n}V&=\left|\det {\frac {\partial (x_{i})}{\partial \left(r,\varphi _{j}\right)}}\right|dr\,d\varphi _{1}\,d\varphi _{2}\cdots d\varphi _{n-1}\\&=r^{n-1}\sin ^{n-2}(\varphi _{1})\sin ^{n-3}(\varphi _{2})\cdots \sin(\varphi _{n-2})\,dr\,d\varphi _{1}\,d\varphi _{2}\cdots d\varphi _{n-1}.\end{aligned}}}

The formula for the volume of the n-ball can be derived from this by integration.

Similarly the surface area element of the (n − 1)-sphere of radius R, which generalizes the area element of the 2-sphere, is given by

${\displaystyle d_{S^{n-1}}V=R^{n-1}\sin ^{n-2}(\varphi _{1})\sin ^{n-3}(\varphi _{2})\cdots \sin(\varphi _{n-2})\,d\varphi _{1}\,d\varphi _{2}\cdots d\varphi _{n-1}.}$

The natural choice of an orthogonal basis over the angular coordinates is a product of ultraspherical polynomials,

{\displaystyle {\begin{aligned}&{}\quad \int _{0}^{\pi }\sin ^{n-j-1}\left(\varphi _{j}\right)C_{s}^{\left({\frac {n-j-1}{2}}\right)}\cos \left(\varphi _{j}\right)C_{s'}^{\left({\frac {n-j-1}{2}}\right)}\cos \left(\varphi _{j}\right)\,d\varphi _{j}\\[6pt]&={\frac {2^{3-n+j}\pi \Gamma (s+n-j-1)}{s!(2s+n-j-1)\Gamma ^{2}\left({\frac {n-j-1}{2}}\right)}}\delta _{s,s'}\end{aligned}}}

for j = 1, 2, ..., n − 2, and the eisφj for the angle j = n − 1 in concordance with the spherical harmonics.

### Polyspherical coordinates

The standard spherical coordinate system arises from writing n as the product ℝ × ℝn−1. These two factors may be related using polar coordinates. For each point x of n, the standard Cartesian coordinates

${\displaystyle \mathbf {x} =(x_{1},\dots ,x_{n})=(y_{1},z_{1},\dots ,z_{n-1})=(y_{1},\mathbf {z} )}$

can be transformed into a mixed polar–Cartesian coordinate system:

${\displaystyle \mathbf {x} =(r\sin \theta ,(r\cos \theta ){\hat {\mathbf {z} }}).}$

This says that points in n may be expressed by taking the ray starting at the origin and passing through ${\displaystyle {\hat {\mathbf {z} }}=\mathbf {z} /\lVert \mathbf {z} \rVert \in S^{n-2}}$, rotating it towards ${\displaystyle (1,0,\dots ,0)}$ by ${\displaystyle \theta =\arcsin y_{1}/r}$, and traveling a distance ${\displaystyle r=\lVert \mathbf {x} \rVert }$ along the ray. Repeating this decomposition eventually leads to the standard spherical coordinate system.

Polyspherical coordinate systems arise from a generalization of this construction.[4] The space n is split as the product of two Euclidean spaces of smaller dimension, but neither space is required to be a line. Specifically, suppose that p and q are positive integers such that n = p + q. Then n = ℝp × ℝq. Using this decomposition, a point x ∈ ℝn may be written as

${\displaystyle \mathbf {x} =(x_{1},\dots ,x_{n})=(y_{1},\dots ,y_{p},z_{1},\dots ,z_{q})=(\mathbf {y} ,\mathbf {z} ).}$

This can be transformed into a mixed polar–Cartesian coordinate system by writing:

${\displaystyle \mathbf {x} =((r\sin \theta ){\hat {\mathbf {y} }},(r\cos \theta ){\hat {\mathbf {z} }}).}$

Here ${\displaystyle {\hat {\mathbf {y} }}}$ and ${\displaystyle {\hat {\mathbf {z} }}}$ are the unit vectors associated to y and z. This expresses x in terms of ${\displaystyle {\hat {\mathbf {y} }}\in S^{p-1}}$, ${\displaystyle {\hat {\mathbf {z} }}\in S^{q-1}}$, r ≥ 0, and an angle θ. It can be shown that the domain of θ is [0, 2π) if p = q = 1, [0, π] if exactly one of p and q is 1, and [0, π/2] if neither p nor q are 1. The inverse transformation is

{\displaystyle {\begin{aligned}r&=\lVert \mathbf {x} \rVert ,\\\theta &=\arcsin {\frac {\lVert \mathbf {y} \rVert }{\lVert \mathbf {x} \rVert }}=\arccos {\frac {\lVert \mathbf {z} \rVert }{\lVert \mathbf {x} \rVert }}=\arctan {\frac {\lVert \mathbf {y} \rVert }{\lVert \mathbf {z} \rVert }}.\end{aligned}}}

These splittings may be repeated as long as one of the factors involved has dimension two or greater. A polyspherical coordinate system is the result of repeating these splittings until there are no Cartesian coordinates left. Splittings after the first do not require a radial coordinate because the domains of ${\displaystyle {\hat {\mathbf {y} }}}$ and ${\displaystyle {\hat {\mathbf {z} }}}$ are spheres, so the coordinates of a polyspherical coordinate system are a non-negative radius and n − 1 angles. The possible polyspherical coordinate systems correspond to binary trees with n leaves. Each non-leaf node in the tree corresponds to a splitting and determines an angular coordinate. For instance, the root of the tree represents n, and its immediate children represent the first splitting into p and q. Leaf nodes correspond to Cartesian coordinates for Sn−1. The formulas for converting from polyspherical coordinates to Cartesian coordinates may be determined by finding the paths from the root to the leaf nodes. These formulas are products with one factor for each branch taken by the path. For a node whose corresponding angular coordinate is θi, taking the left branch introduces a factor of sin θi and taking the right branch introduces a factor of cos θi. The inverse transformation, from polyspherical coordinates to Cartesian coordinates, is determined by grouping nodes. Every pair of nodes having a common parent can be converted from a mixed polar–Cartesian coordinate system to a Cartesian coordinate system using the above formulas for a splitting.

Polyspherical coordinates also have an interpretation in terms of the special orthogonal group. A splitting n = ℝp × ℝq determines a subgroup

${\displaystyle \operatorname {SO} _{p}(\mathbb {R} )\times \operatorname {SO} _{q}(\mathbb {R} )\subseteq \operatorname {SO} _{n}(\mathbb {R} ).}$

This is the subgroup that leaves each of the two factors ${\displaystyle S^{p-1}\times S^{q-1}\subseteq S^{n-1}}$ fixed. Choosing a set of coset representatives for the quotient is the same as choosing representative angles for this step of the polyspherical coordinate decomposition.

In polyspherical coordinates, the volume measure on n and the area measure on Sn−1 are products. There is one factor for each angle, and the volume measure on n also has a factor for the radial coordinate. The area measure has the form:

${\displaystyle dA_{n-1}=\prod _{i=1}^{n-1}F_{i}(\theta _{i})\,d\theta _{i},}$

where the factors Fi are determined by the tree. Similarly, the volume measure is

${\displaystyle dV_{n}=r^{n-1}\,dr\,\prod _{i=1}^{n-1}F_{i}(\theta _{i})\,d\theta _{i}.}$

Suppose we have a node of the tree that corresponds to the decomposition n1+n2 = ℝn1 × ℝn2 and that has angular coordinate θ. The corresponding factor F depends on the values of n1 and n2. When the area measure is normalized so that the area of the sphere is 1, these factors are as follows. If n1 = n2 = 1, then

${\displaystyle F(\theta )={\frac {d\theta }{2\pi }}.}$

If n1 > 1 and n2 = 1, and if B denotes the beta function, then

${\displaystyle F(\theta )={\frac {\sin ^{n_{1}-1}\theta }{\mathrm {B} ({\frac {n_{1}}{2}},{\frac {1}{2}})}}\,d\theta .}$

If n1 = 1 and n2 > 1, then

${\displaystyle F(\theta )={\frac {\cos ^{n_{2}-1}\theta }{\mathrm {B} ({\frac {1}{2}},{\frac {n_{2}}{2}})}}\,d\theta .}$

Finally, if both n1 and n2 are greater than one, then

${\displaystyle F(\theta )={\frac {(\sin ^{n_{1}-1}\theta )(\cos ^{n_{2}-1}\theta )}{{\frac {1}{2}}\mathrm {B} ({\frac {n_{1}}{2}},{\frac {n_{2}}{2}})}}\,d\theta .}$

## Stereographic projection

Just as a two-dimensional sphere embedded in three dimensions can be mapped onto a two-dimensional plane by a stereographic projection, an n-sphere can be mapped onto an n-dimensional hyperplane by the n-dimensional version of the stereographic projection. For example, the point [x,y,z] on a two-dimensional sphere of radius 1 maps to the point [x/1 − z, y/1 − z] on the xy-plane. In other words,

${\displaystyle [x,y,z]\mapsto \left[{\frac {x}{1-z}},{\frac {y}{1-z}}\right].}$

Likewise, the stereographic projection of an n-sphere Sn of radius 1 will map to the (n − 1)-dimensional hyperplane n−1 perpendicular to the xn-axis as

${\displaystyle [x_{1},x_{2},\ldots ,x_{n}]\mapsto \left[{\frac {x_{1}}{1-x_{n}}},{\frac {x_{2}}{1-x_{n}}},\ldots ,{\frac {x_{n-1}}{1-x_{n}}}\right].}$

## Probability distributions

### Uniformly at random on the (n − 1)-sphere

To generate uniformly distributed random points on the unit (n − 1)-sphere (that is, the surface of the unit n-ball), Marsaglia (1972) gives the following algorithm.

Generate an n-dimensional vector of normal deviates (it suffices to use N(0, 1), although in fact the choice of the variance is arbitrary), x = (x1, x2, ..., xn). Now calculate the "radius" of this point:

${\displaystyle r={\sqrt {x_{1}^{2}+x_{2}^{2}+\cdots +x_{n}^{2}}}.}$

The vector 1/rx is uniformly distributed over the surface of the unit n-ball.

An alternative given by Marsaglia is to uniformly randomly select a point x = (x1, x2, ..., xn) in the unit n-cube by sampling each xi independently from the uniform distribution over (–1, 1), computing r as above, and rejecting the point and resampling if r ≥ 1 (i.e., if the point is not in the n-ball), and when a point in the ball is obtained scaling it up to the spherical surface by the factor 1/r; then again 1/rx is uniformly distributed over the surface of the unit n-ball. This method becomes very inefficient for higher dimensions, as a vanishingly small fraction of the unit cube is contained in the sphere. In ten dimensions, less than 2% of the cube is filled by the sphere, so that typically more than 50 attempts will be needed. In seventy dimensions, less than ${\displaystyle 10^{-24}}$ of the cube is filled, meaning typically a trillion quadrillion trials will be needed, far more than a computer could ever carry out.

### Uniformly at random within the n-ball

With a point selected uniformly at random from the surface of the unit (n − 1)-sphere (e.g., by using Marsaglia's algorithm), one needs only a radius to obtain a point uniformly at random from within the unit n-ball. If u is a number generated uniformly at random from the interval [0, 1] and x is a point selected uniformly at random from the unit (n − 1)-sphere, then u1/n x is uniformly distributed within the unit n-ball.

Alternatively, points may be sampled uniformly from within the unit n-ball by a reduction from the unit (n + 1)-sphere. In particular, if (x1, x2, ..., xn+2) is a point selected uniformly from the unit (n + 1)-sphere, then (x1, x2, ..., xn) is uniformly distributed within the unit n-ball (i.e., by simply discarding two coordinates).[5]

If n is sufficiently large, most of the volume of the n-ball will be contained in the region very close to its surface, so a point selected from that volume will also probably be close to the surface. This is one of the phenomena leading to the so-called curse of dimensionality that arises in some numerical and other applications.

### Distribution of the first coordinate

Let ${\displaystyle y=x_{1}^{2}}$ be the square of the first coordinate of a point sampled uniformly at random from the (n-1)-sphere, then its probability density function, for ${\displaystyle y\in [0,1]}$, is

${\displaystyle \rho (y)={\frac {\Gamma {\bigl (}{\frac {n}{2}}{\bigr )}}{{\sqrt {\pi }}\;\Gamma {\bigl (}{\frac {n-1}{2}}{\bigr )}}}(1-y)^{(n-3)/2}y^{-1/2}.}$

Let ${\displaystyle z=y/N}$ be the appropriately scaled version, then at the ${\displaystyle N\to \infty }$ limit, the probability density function of ${\displaystyle z}$ converges to ${\displaystyle (2\pi ze^{z})^{-1/2}}$. This is sometimes called the Porter–Thomas distribution.[6]

## Specific spheres

0-sphere
The pair of points R} with the discrete topology for some R > 0. The only sphere that is not path-connected. Parallelizable.
1-sphere
Commonly called a circle. Has a nontrivial fundamental group. Abelian Lie group structure U(1); the circle group. Homeomorphic to the real projective line.
2-sphere
Commonly simply called a sphere. For its complex structure, see Riemann sphere. Homeomorphic to the complex projective line
3-sphere
Parallelizable, principal U(1)-bundle over the 2-sphere, Lie group structure Sp(1).
4-sphere
Homeomorphic to the quaternionic projective line, HP1. SO(5) / SO(4).
5-sphere
Principal U(1)-bundle over CP2. SO(6) / SO(5) = SU(3) / SU(2). It is undecidable whether a given n-dimensional manifold is homeomorphic to Sn for n ≥ 5.[7]
6-sphere
Possesses an almost complex structure coming from the set of pure unit octonions. SO(7) / SO(6) = G2 / SU(3). The question of whether it has a complex structure is known as the Hopf problem, after Heinz Hopf.[8]
7-sphere
Topological quasigroup structure as the set of unit octonions. Principal Sp(1)-bundle over S4. Parallelizable. SO(8) / SO(7) = SU(4) / SU(3) = Sp(2) / Sp(1) = Spin(7) / G2 = Spin(6) / SU(3). The 7-sphere is of particular interest since it was in this dimension that the first exotic spheres were discovered.
8-sphere
Homeomorphic to the octonionic projective line OP1.
23-sphere
A highly dense sphere-packing is possible in 24-dimensional space, which is related to the unique qualities of the Leech lattice.

## Octahedral sphere

The octahedral n-sphere is defined similarly to the n-sphere but using the 1-norm

${\displaystyle S^{n}=\left\{x\in \mathbb {R} ^{n+1}:\left\|x\right\|_{1}=1\right\}}$

In general, it takes the shape of a cross-polytope.

The octahedral 1-sphere is a square (without its interior). The octahedral 2-sphere is a regular octahedron; hence the name. The octahedral n-sphere is the topological join of n + 1 pairs of isolated points.[9] Intuitively, the topological join of two pairs is generated by drawing a segment between each point in one pair and each point in the other pair; this yields a square. To join this with a third pair, draw a segment between each point on the square and each point in the third pair; this gives a octahedron.

## Notes

1. ^ James W. Vick (1994). Homology theory, p. 60. Springer
2. ^ Smith, David J.; Vamanamurthy, Mavina K. (1989). "How Small Is a Unit Ball?". Mathematics Magazine. 62 (2): 101–107. doi:10.1080/0025570X.1989.11977419. JSTOR 2690391.
3. ^ Blumenson, L. E. (1960). "A Derivation of n-Dimensional Spherical Coordinates". The American Mathematical Monthly. 67 (1): 63–66. doi:10.2307/2308932. JSTOR 2308932.
4. ^ N. Ja. Vilenkin and A. U. Klimyk, Representation of Lie groups and special functions, Vol. 2: Class I representations, special functions, and integral transforms, translated from the Russian by V. A. Groza and A. A. Groza, Math. Appl., vol. 74, Kluwer Acad. Publ., Dordrecht, 1992, ISBN 0-7923-1492-1, pp. 223–226.
5. ^ Voelker, Aaron R.; Gosmann, Jan; Stewart, Terrence C. (2017). Efficiently sampling vectors and coordinates from the n-sphere and n-ball (Report). Centre for Theoretical Neuroscience. doi:10.13140/RG.2.2.15829.01767/1.
6. ^ Livan, Giacomo; Novaes, Marcel; Vivo, Pierpaolo (2018), Livan, Giacomo; Novaes, Marcel; Vivo, Pierpaolo (eds.), "One Pager on Eigenvectors", Introduction to Random Matrices: Theory and Practice, SpringerBriefs in Mathematical Physics, Cham: Springer International Publishing, pp. 65–66, doi:10.1007/978-3-319-70885-0_9, ISBN 978-3-319-70885-0, retrieved 2023-05-19
7. ^ Stillwell, John (1993), Classical Topology and Combinatorial Group Theory, Graduate Texts in Mathematics, vol. 72, Springer, p. 247, ISBN 9780387979700.
8. ^ Agricola, Ilka; Bazzoni, Giovanni; Goertsches, Oliver; Konstantis, Panagiotis; Rollenske, Sönke (2018). "On the history of the Hopf problem". Differential Geometry and Its Applications. 57: 1–9. arXiv:1708.01068. doi:10.1016/j.difgeo.2017.10.014. S2CID 119297359.
9. ^ Meshulam, Roy (2001-01-01). "The Clique Complex and Hypergraph Matching". Combinatorica. 21 (1): 89–94. doi:10.1007/s004930170006. ISSN 1439-6912. S2CID 207006642.