n-sphere

In mathematics, an n-sphere is a topological space that is homeomorphic to a standard n-sphere, which is the set of points in (n + 1)-dimensional Euclidean space that are situated at a constant distance r from a fixed point, called the center. It is the generalization of an ordinary sphere in the ordinary three-dimensional space. The "radius" of a sphere is the constant distance of its points to the center. When the sphere has unit radius, it is usual to call it the unit n-sphere or simply the n-sphere for brevity. In terms of the standard norm, the n-sphere is defined as

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

2-sphere wireframe as an orthogonal projection

Just as a stereographic projection can project a sphere's surface to a plane, it can also project a 3-sphere into 3-space. This image shows three coordinate directions projected to 3-space: parallels (red), meridians (blue) and hypermeridians (green). Due to the conformal property of the stereographic projection, the curves intersect each other orthogonally (in the yellow points) as in 4D. All of the curves are circles: the curves that intersect ⟨0,0,0,1⟩ have an infinite radius (= straight line).

and an n-sphere of radius r can be defined as

${\displaystyle S^{n}(r)=\left\{x\in \mathbf {R} ^{n+1}:\left\|x\right\|=r\right\}.}$

The 0-sphere is a pair of points on the line, the 1-sphere is a circle in the plane, and the 2-sphere is an ordinary sphere within 3-dimensional space.

The dimension of n-sphere is n, and must not be confused with the dimension (n + 1) of the Euclidean space in which it is naturally embedded. An n-sphere is the surface or boundary of an (n + 1)-dimensional ball.

In particular:

• the pair of points at the ends of a (one-dimensional) line segment is a 0-sphere,
• a circle, which is the one-dimensional circumference of a (two-dimensional) disk, is a 1-sphere,
• the two-dimensional surface of a (three-dimensional) ball in three-dimensional space is a 2-sphere, often simply called a sphere,
• the three-dimensional boundary of a (four-dimensional) 4-ball in four-dimensional Euclidean is a 3-sphere, also known as a glome.
• the n – 1 dimensional boundary of a (n-dimensional) n-ball is an (n – 1)-sphere.

For n ≥ 2, the n-spheres that are differential manifolds can be characterized (up to a diffeomorphism) as the simply connected n-dimensional manifolds of constant, positive curvature. The n-spheres admit several other topological descriptions: for example, they can be constructed by gluing two n-dimensional Euclidean spaces together, by identifying the boundary of an n-cube with a point, or (inductively) by forming the suspension of an (n − 1)-sphere. The 1-sphere is the 1-manifold that is a circle, which is not simply connected. The 0-sphere is the 0-manifold consisting of two points, which is not even connected.

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 {c − r, 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.

Euclidean coordinates in (n + 1)-space

The set of points in (n + 1)-space, (x₁, x₂, ..., x_n+1), that define an n-sphere, ${\displaystyle S^{n}(r)}$, is represented by the equation:

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

where c = (c₁, c₂, ..., c_n+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

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

where ∗ 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

Main article: Ball (mathematics)

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 Rⁿ. This forms the basis for stereographic projection.[1]

Volume and surface area

See also: Volume of an n-ball

V_n(R) and S_n(R) are the n-dimensional volume of the n-ball and the surface area of the n-sphere embedded in dimension n + 1, respectively, of radius R.

The constants V_n and S_n (for R = 1, the unit ball and sphere) are related by the recurrences:

${\displaystyle {\begin{aligned}V_{0}&=1&V_{n+1}&={\frac {S_{n}}{n+1}}\\[6pt]S_{0}&=2&S_{n+1}&=2\pi V_{n}\end{aligned}}}$

The surfaces and volumes can also be given in closed form:

${\displaystyle {\begin{aligned}S_{n}(R)&={\frac {2\,\pi ^{\frac {n+1}{2}}}{\Gamma \left({\frac {n+1}{2}}\right)}}R^{n}\\[6pt]V_{n}(R)&={\frac {\pi ^{\frac {n}{2}}}{\Gamma \left({\frac {n}{2}}+1\right)}}R^{n}\end{aligned}}}$

where Γ is the gamma function. Derivations of these equations are given in this section.

Graphs of volumes (V) and surface areas (S) of n-balls of radius 1. In the SVG file, hover over a point to highlight it and its value.

In general, the volume of the n-ball in n-dimensional Euclidean space, and the surface area of the n-sphere in (n + 1)-dimensional Euclidean space, of radius R, are proportional to the nth power of the radius, R (with different constants of proportionality that vary with n). We write V_n(R) = V_nRⁿ for the volume of the n-ball and S_n(R) = S_nRⁿ for the surface area of the n-sphere, both of radius R, where V_n = V_n(1) and S_n = S_n(1) are the values for the unit-radius case.

In theory, one could compare the values of S_n(R) and S_m(R) for n ≠ m. However, this is not well-defined. For example, if n = 2 and m = 3 then the comparison is like comparing a number of square meters to a different number of cubic meters. The same applies to a comparison of V_n(R) and V_m(R) for n ≠ m.

Examples

The 0-ball consists of a single point. The 0-dimensional Hausdorff measure is the number of points in a set. So,

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

The unit 1-ball is the interval [−1,1] of length 2. So,

${\displaystyle V_{1}=2.}$

The 0-sphere consists of its two end-points, {−1,1}. So,

${\displaystyle S_{0}=2.}$

The unit 1-sphere is the unit circle in the Euclidean plane, and this has circumference (1-dimensional measure)

${\displaystyle S_{1}=2\pi .}$

The region enclosed by the unit 1-sphere is the 2-ball, or unit disc, and this has area (2-dimensional measure)

${\displaystyle V_{2}=\pi .}$

Analogously, in 3-dimensional Euclidean space, the surface area (2-dimensional measure) of the unit 2-sphere is given by

${\displaystyle S_{2}=4\pi .}$

and the volume enclosed is the volume (3-dimensional measure) of the unit 3-ball, given by

${\displaystyle V_{3}={\tfrac {4}{3}}\pi .}$

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

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

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

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

So,

${\displaystyle V_{n+1}={\frac {S_{n}}{n+1}}.}$

We can also represent the unit (n + 2)-sphere as a union of tori, each the product of a circle (1-sphere) with an n-sphere. Let r = cos θ and r² + R² = 1, so that R = sin θ and dR = cos θ dθ. Then,

${\displaystyle {\begin{aligned}S_{n+2}&=\int _{0}^{\frac {\pi }{2}}S_{1}r\cdot S_{n}R^{n}\,d\theta \\[6pt]&=\int _{0}^{\frac {\pi }{2}}S_{1}\cdot S_{n}R^{n}\cos \theta \,d\theta \\[6pt]&=\int _{0}^{1}S_{1}\cdot S_{n}R^{n}\,dR\\[6pt]&=S_{1}\int _{0}^{1}S_{n}R^{n}\,dR\\[6pt]&=2\pi V_{n+1}.\end{aligned}}}$

Since S₁ = 2π V₀, the equation

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

holds for all n.

This completes the derivation of the recurrences:

${\displaystyle {\begin{aligned}V_{0}&=1&V_{n+1}&={\frac {S_{n}}{n+1}}\\[6pt]S_{0}&=2&S_{n+1}&=2\pi V_{n}\end{aligned}}}$

Closed forms

Combining the recurrences, we see that

${\displaystyle V_{n+2}=2\pi {\frac {V_{n}}{n+2}}.}$

So it is simple to show by induction on k that,

${\displaystyle {\begin{aligned}V_{2k}&={\frac {\left(2\pi \right)^{k}}{(2k)!!}}={\frac {\pi ^{k}}{k!}}\\[6pt]V_{2k+1}&={\frac {2\left(2\pi \right)^{k}}{(2k+1)!!}}={\frac {2k!\left(4\pi \right)^{k}}{(2k+1)!}}\end{aligned}}}$

where !! denotes the double factorial, defined for odd natural numbers 2k + 1 by (2k + 1)!! = 1 × 3 × 5 × ... × (2k − 1) × (2k + 1) and similarly for even numbers (2k)!! = 2 × 4 × 6 × ... × (2k − 2) × (2k).

In general, the volume, in n-dimensional Euclidean space, of the unit n-ball, is given by

${\displaystyle V_{n}={\frac {\pi ^{\frac {n}{2}}}{\Gamma \left({\frac {n}{2}}+1\right)}}={\frac {\pi ^{\frac {n}{2}}}{\left({\frac {n}{2}}\right)!}}}$

where Γ is the gamma function, which satisfies Γ(1/2) = √π, Γ(1) = 1, and Γ(x + 1) = xΓ(x), and so Γ(x + 1) = x!, and where we conversely define x! = Γ(x + 1) for any x.

By multiplying V_n by Rⁿ, differentiating with respect to R, and then setting R = 1, we get the closed form

${\displaystyle S_{n-1}={\frac {n\pi ^{\frac {n}{2}}}{\Gamma \left({\frac {n}{2}}+1\right)}}={\frac {2\pi ^{\frac {n}{2}}}{\Gamma \left({\frac {n}{2}}\right)}}.}$

for the (n-1)-dimensional volume of the sphere S^n-1.

Other relations

The recurrences can be combined to give a "reverse-direction" recurrence relation for surface area, as depicted in the diagram:

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

n refers to the dimension of the ambient Euclidean space, which is also the intrinsic dimension of the solid whose volume is listed here, but which is 1 more than the intrinsic dimension of the sphere whose surface area is listed here. The curved red arrows show the relationship between formulas for different n. The formula coefficient at each arrow's tip equals the formula coefficient at that arrow's tail times the factor in the arrowhead (where the n in the arrowhead refers to the n value that the arrowhead points to). If the direction of the bottom arrows were reversed, their arrowheads would say to multiply by 2π/n − 2. Alternatively said, the surface area S_n+1 of the sphere in n + 2 dimensions is exactly 2πR times the volume V_n enclosed by the sphere in n dimensions.

Index-shifting n to n − 2 then yields the recurrence relations:

${\displaystyle {\begin{aligned}V_{n}&={\frac {2\pi }{n}}V_{n-2}\\[6pt]S_{n-1}&={\frac {2\pi }{n-2}}S_{n-3}\end{aligned}}}$

where S₀ = 2, V₁ = 2, S₁ = 2π and V₂ = π.

The recurrence relation for V_n can also be proved via integration with 2-dimensional polar coordinates:

${\displaystyle {\begin{aligned}V_{n}&=\int _{0}^{1}\int _{0}^{2\pi }V_{n-2}\left({\sqrt {1-r^{2}}}\right)^{n-2}\,r\,d\theta \,dr\\[6pt]&=\int _{0}^{1}\int _{0}^{2\pi }V_{n-2}\left(1-r^{2}\right)^{{\frac {n}{2}}-1}\,r\,d\theta \,dr\\[6pt]&=2\pi V_{n-2}\int _{0}^{1}\left(1-r^{2}\right)^{{\frac {n}{2}}-1}\,r\,dr\\[6pt]&=2\pi V_{n-2}\left[-{\frac {1}{n}}\left(1-r^{2}\right)^{\frac {n}{2}}\right]_{r=0}^{r=1}\\[6pt]&=2\pi V_{n-2}{\frac {1}{n}}={\frac {2\pi }{n}}V_{n-2}.\end{aligned}}}$

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, ... φ_n−1, where the angles φ₁, φ₂, ... φ_n−2 range over [0,π] radians (or over [0,180] degrees) and φ_n−1 ranges over [0,2π) radians (or over [0,360) degrees). If x_i are the Cartesian coordinates, then we may compute x₁, ... x_n from r, φ₁, ... φ_n−1 with: [2]

${\displaystyle {\begin{aligned}x_{1}&=r\cos(\varphi _{1})\\x_{2}&=r\sin(\varphi _{1})\cos(\varphi _{2})\\x_{3}&=r\sin(\varphi _{1})\sin(\varphi _{2})\cos(\varphi _{3})\\&\vdots \\x_{n-1}&=r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\cos(\varphi _{n-1})\\x_{n}&=r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\sin(\varphi _{n-1}).\end{aligned}}}$

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

${\displaystyle {\begin{aligned}r&={\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{2}}^{2}+{x_{1}}^{2}}}\\[6pt]\varphi _{1}&=\operatorname {arccot} {\frac {x_{1}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{2}}^{2}}}}&&=\arccos {\frac {x_{1}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{1}}^{2}}}}\\[6pt]\varphi _{2}&=\operatorname {arccot} {\frac {x_{2}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{3}}^{2}}}}&&=\arccos {\frac {x_{2}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+\cdots +{x_{2}}^{2}}}}\\[6pt]&\vdots &&\vdots \\[6pt]\varphi _{n-2}&=\operatorname {arccot} {\frac {x_{n-2}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}}}}&&=\arccos {\frac {x_{n-2}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}+{x_{n-2}}^{2}}}}\\[6pt]\varphi _{n-1}&=2\operatorname {arccot} {\frac {x_{n-1}+{\sqrt {x_{n}^{2}+x_{n-1}^{2}}}}{x_{n}}}&&={\begin{cases}\arccos {\frac {x_{n-1}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}}}}&x_{n}\geq 0\\[6pt]2\pi -\arccos {\frac {x_{n-1}}{\sqrt {{x_{n}}^{2}+{x_{n-1}}^{2}}}}&x_{n}<0\end{cases}}\,.\end{aligned}}}$

where if x_k ≠ 0 for some k but all of x_k+1, ... x_n are zero then φ_k = 0 when x_k > 0, and φ_k = π (180 degrees) when x_k < 0.

There are some special cases where the inverse transform is not unique; φ_k for any k will be ambiguous whenever all of x_k, x_k+1, ... x_n are zero; in this case φ_k may be chosen to be zero.

Spherical volume and area elements

Expressing the angular measures in radians, the volume element in n-dimensional Euclidean space will be found from the Jacobian of the transformation:

${\displaystyle {\begin{pmatrix}\cos(\varphi _{1})&-r\sin(\varphi _{1})&0&0&\cdots &0\\\sin(\varphi _{1})\cos(\varphi _{2})&r\cos(\varphi _{1})\cos(\varphi _{2})&-r\sin(\varphi _{1})\sin(\varphi _{2})&0&\cdots &0\\\vdots &\vdots &\vdots &\ddots &&\vdots \\&&&&&0\\\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\cos(\varphi _{n-1})&\cdots &\cdots &&&-r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\sin(\varphi _{n-1})\\\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\sin(\varphi _{n-1})&r\cos(\varphi _{1})\cdots \sin(\varphi _{n-1})&\cdots &&&r\sin(\varphi _{1})\cdots \sin(\varphi _{n-2})\cos(\varphi _{n-1})\end{pmatrix}}}$

${\displaystyle {\begin{aligned}d^{n}V&=\left|\det {\frac {\partial (x_{i})}{\partial \left(r,\varphi _{j}\right)}}\right|dr\,d\varphi _{1}\,d\varphi _{2}\cdots d\varphi _{n-1}\\[6pt]&=r^{n-1}\sin ^{n-2}(\varphi _{1})\sin ^{n-3}(\varphi _{2})\cdots \sin(\varphi _{n-2})\,dr\,d\varphi _{1}\,d\varphi _{2}\cdots d\varphi _{n-1}\end{aligned}}}$

and the above equation for the volume of the n-ball can be recovered by integrating:

${\displaystyle V_{n}=\int _{\varphi _{n-1}=0}^{2\pi }\int _{\varphi _{n-2}=0}^{\pi }\cdots \int _{\varphi _{1}=0}^{\pi }\int _{r=0}^{R}d^{n}V.}$

Similarly the surface area element of the (n − 1)-sphere, 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 e^isφ_j for the angle j = n − 1 in concordance with the spherical harmonics.

Stereographic projection

Main article: 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⁻¹ of radius 1 will map to the (n − 1)-dimensional hyperplane Rⁿ⁻¹ perpendicular to the x_n-axis as

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

Generating random points

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

A set of uniformly distributed points on the surface of a unit 2-sphere generated using Marsaglia's algorithm.

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,... x_n). 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,... x_n) in the unit n-cube by sampling each x_i 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.

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 u^​1⁄_nx 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,...,x_n+2) is a point selected uniformly from the unit (n + 1)-sphere, then (x₁,x₂,...,x_n) is uniformly distributed within the unit n-ball (i.e., by simply discarding two coordinates).[3]

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.

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. Has a natural Lie group structure; isomorphic to O(1). Parallelizable.

1-sphere

Also known as the circle. Has a nontrivial fundamental group. Abelian Lie group structure U(1); the circle group. Topologically equivalent to the real projective line, RP¹. Parallelizable. SO(2) = U(1).

2-sphere

Also known as the sphere. Complex structure; see Riemann sphere. Equivalent to the complex projective line, CP¹. SO(3)/SO(2).

3-sphere

Also known as the glome. Parallelizable, principal U(1)-bundle over the 2-sphere, Lie group structure Sp(1), where also

${\displaystyle \mathrm {Sp} (1)\cong \mathrm {SO} (4)/\mathrm {SO} (3)\cong \mathrm {SU} (2)\cong \mathrm {Spin} (3)}$.

4-sphere

Equivalent 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).

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.[4]

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

Equivalent 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:

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

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.[5] 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.

See also

• Affine sphere
• Conformal geometry
• Exotic sphere
• Homology sphere
• Homotopy groups of spheres
• Homotopy sphere
• Hyperbolic group
• Hypercube
• Inversive geometry
• Loop (topology)
• Manifold
• Möbius transformation
• Orthogonal group
• Spherical cap
• Volume of an n-ball
• Wigner semicircle distribution

Notes

1. James W. Vick (1994). Homology theory, p. 60. Springer
2. 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.
3. 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.
4. 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.
5. Meshulam, Roy (2001-01-01). "The Clique Complex and Hypergraph Matching". Combinatorica. 21 (1): 89–94. doi:10.1007/s004930170006. ISSN 1439-6912.

References

• Flanders, Harley (1989). Differential forms with applications to the physical sciences. New York: Dover Publications. ISBN 978-0-486-66169-8.
• Moura, Eduarda; Henderson, David G. (1996). Experiencing geometry: on plane and sphere. Prentice Hall. ISBN 978-0-13-373770-7 (Chapter 20: 3-spheres and hyperbolic 3-spaces).
• Weeks, Jeffrey R. (1985). The Shape of Space: how to visualize surfaces and three-dimensional manifolds. Marcel Dekker. ISBN 978-0-8247-7437-0 (Chapter 14: The Hypersphere).
• Marsaglia, G. (1972). "Choosing a Point from the Surface of a Sphere". Annals of Mathematical Statistics. 43 (2): 645–646. doi:10.1214/aoms/1177692644.
• Huber, Greg (1982). "Gamma function derivation of n-sphere volumes". Amer. Math. Monthly. 89 (5): 301–302. doi:10.2307/2321716. JSTOR 2321716. MR 1539933.
• Barnea, Nir (1999). "Hyperspherical functions with arbitrary permutational symmetry: Reverse construction". Phys. Rev. A. 59 (2): 1135–1146. Bibcode:1999PhRvA..59.1135B. doi:10.1103/PhysRevA.59.1135.

External links

• Weisstein, Eric W. "Hypersphere". MathWorld.