Editing N-sphere

{{Short description|Generalized sphere of dimension n (mathematics)}}
{{DISPLAYTITLE:''n''-sphere}}
[[File:Sphere wireframe.svg|thumb|2-sphere wireframe as an [[orthogonal projection]]]]
[[File:Hypersphere coord.PNG|right|thumb|Just as a [[stereographic projection]] can project a sphere's surface to a plane, it can also project a {{math|3}}-sphere into {{math|3}}-space. This image shows three coordinate directions projected to {{math|3}}-space:
parallels (red), [[Meridian (geography)|meridians]] (blue), and hypermeridians (green).
Due to the [[conformal map|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 {{angbr|0,0,0,1}} have an infinite radius (= straight line).]]

In [[mathematics]], an '''{{mvar|n}}-sphere''' or '''hypersphere''' is an {{tmath|n}}-[[dimension|dimensional]] generalization of the {{tmath|1}}-dimensional [[circle]] and {{tmath|2}}-dimensional [[sphere]] to any non-negative [[integer]] {{tmath|n}}.

The circle is considered [[1-dimensional]] and the sphere [[2-dimensional]] because a point within them has one and two degrees of freedom respectively. However, the typical [[embedding]] of the 1-dimensional circle is in 2-dimensional space, the 2-dimensional sphere is usually depicted embedded in [[3-dimensional space]], and a general {{tmath|n}}-sphere is embedded in an {{tmath|n+1}}-dimensional space. The term ''hyper''sphere is commonly used to distinguish spheres of dimension {{tmath|n \ge 3}} which are thus embedded in a space of dimension {{tmath|n+1 \ge 4}}, which means that they cannot be easily visualized. The {{tmath|n}}-sphere is the setting for {{tmath|n}}-dimensional [[spherical geometry]].

Considered extrinsically, as a [[hypersurface]] embedded in {{tmath|(n+1)}}-dimensional [[Euclidean space]], an {{tmath|n}}-sphere is the [[Locus (mathematics)|locus]] of [[point (geometry)|point]]s at equal [[Euclidean distance|distance]] (the ''[[radius]]'') from a given ''[[Center (geometry)|center]]'' point. Its [[interior (topology)|interior]], consisting of all points closer to the center than the radius, is an {{tmath|(n+1)}}-dimensional [[ball (mathematics)|ball]]. In particular:

* The {{tmath|0}}-sphere is the pair of points at the ends of a [[line segment]] ({{tmath|1}}-ball).
* The {{tmath|1}}-sphere is a [[circle]], the [[circumference]] of a [[disk (mathematics)|disk]] ({{tmath|2}}-ball) in the two-dimensional [[Euclidean plane|plane]].
* The {{tmath|2}}-sphere, often simply called a sphere, is the [[boundary (topology)|boundary]] of a {{tmath|3}}-ball in [[three-dimensional space]].
* The [[3-sphere|{{math|3}}-sphere]] is the boundary of a {{tmath|4}}-ball in [[four-dimensional space]].
* The {{tmath|(n-1)}}-sphere is the boundary of an {{tmath|n}}-ball.

Given a [[Cartesian coordinate system]], the ''[[unit n-sphere|unit {{tmath|n}}-sphere]]'' of radius {{tmath|1}} can be defined as:

:<math> S^n = \left\{ x \in \R^{n+1} : \left\| x \right\| = 1 \right\}.</math>

Considered intrinsically, when {{tmath|n \geq 1}}, the {{tmath|n}}-sphere is a [[Riemannian manifold]] of positive [[constant curvature]], and is [[Orientable manifold|orientable]]. The [[geodesics]] of the {{tmath|n}}-sphere are called [[great circle]]s.

The [[stereographic projection]] maps the {{tmath|n}}-sphere onto {{tmath|n}}-space with a single adjoined [[point at infinity]]; under the [[metric tensor|metric]] thereby defined, <math>\R^n \cup \{\infty\}</math> is a model for the {{tmath|n}}-sphere.

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

==Description==

For any [[natural number]] {{tmath|n}}, an {{tmath|n}}-sphere of radius {{tmath|r}} is defined as the set of points in {{tmath|(n+1)}}-dimensional [[Euclidean space]] that are at distance {{tmath|r}} from some fixed point {{tmath|\mathbf{c} }}, where {{tmath|r}} may be any [[Positive number|positive]] [[real number]] and where {{tmath|\mathbf{c} }} may be any point in {{tmath|(n+1)}}-dimensional space. In particular:
* a 0-sphere is a pair of points {{tmath| \{ c - r, c + r \} }}, and is the boundary of a line segment ({{tmath|1}}-ball).
* a [[1-sphere|{{math|1}}-sphere]] is a [[circle]] of radius {{tmath|r}} centered at {{tmath|\mathbf{c} }}, and is the boundary of a disk ({{tmath|2}}-ball).
* a [[2-sphere|{{math|2}}-sphere]] is an ordinary {{tmath|2}}-dimensional [[sphere]] in {{tmath|3}}-dimensional Euclidean space, and is the boundary of an ordinary ball ({{tmath|3}}-ball).
* a [[3-sphere|{{math|3}}-sphere]] is a {{tmath|3}}-dimensional sphere in {{tmath|4}}-dimensional Euclidean space.

=== Cartesian coordinates ===

The set of points in {{tmath|(n+1)}}-space, {{tmath|(x_1, x_2, \ldots, x_{n+1})}}, that define an {{tmath|n}}-sphere, {{tmath|S^n(r)}}, is represented by the equation:

:<math>r^2=\sum_{i=1}^{n+1} (x_i - c_i)^2 ,</math>

where {{tmath|1= \mathbf{c} = (c_1, c_2, \ldots, c_{n+1})}} is a center point, and {{tmath|r}} is the radius.

The above {{tmath|n}}-sphere exists in {{tmath|(n+1)}}-dimensional Euclidean space and is an example of an {{tmath|n}}-[[manifold]].  The [[volume form]] {{tmath|\omega}} of an {{tmath|n}}-sphere of radius {{tmath|r}} is given by

:<math>\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</math>

where <math>{\star}</math> is the [[Hodge star operator]]; see {{harvtxt|Flanders|1989|loc=§6.1}} for a discussion and proof of this formula in the case {{tmath|r {{=}} 1}}.  As a result,
:<math>dr \wedge \omega = dx_1 \wedge \cdots \wedge dx_{n+1}.</math>

=== ''n''-ball ===
{{main|Ball (mathematics)}}

The space enclosed by an {{tmath|n}}-sphere is called an {{tmath|(n+1)}}-[[Ball (mathematics)|ball]]. An {{tmath|(n+1)}}-ball is [[Closed set|closed]] if it includes the {{tmath|n}}-sphere, and it is [[Open set|open]] if it does not include the {{tmath|n}}-sphere.

Specifically:
* A {{tmath|1}}-''ball'', a [[line segment]], is the interior of a 0-sphere.
* A {{tmath|2}}-''ball'', a [[Disk (mathematics)|disk]], is the interior of a [[circle]] ({{tmath|1}}-sphere).
* A {{tmath|3}}-''ball'', an ordinary [[Ball (mathematics)|ball]], is the interior of a [[sphere]] ({{tmath|2}}-sphere).
* A {{tmath|4}}-''ball'' is the interior of a [[3-sphere|{{math|3}}-sphere]], etc.

===Topological description===
[[Topology|Topologically]], an {{tmath|n}}-sphere can be constructed as a [[Alexandroff extension|one-point compactification]] of {{tmath|n}}-dimensional Euclidean space.  Briefly, the {{tmath|n}}-sphere can be described as {{tmath|1=S^n = \R^n \cup \{ \infty \} }}, which is {{tmath|n}}-dimensional Euclidean space plus a single point representing infinity in all directions.
In particular, if a single point is removed from an {{tmath|n}}-sphere, it becomes [[Homeomorphism|homeomorphic]] to <math>\R^n</math>. This forms the basis for [[stereographic projection]].<ref>James W. Vick (1994). ''Homology theory'', p. 60. Springer</ref>

==Volume and area==
{{see also|Volume of an n-ball|Unit sphere#Volume and area}}

Let {{tmath|S_{n-1} }} be the surface area of the unit {{tmath|(n-1)}}-sphere of radius {{tmath|1}} embedded in {{tmath|n}}-dimensional Euclidean space, and let {{tmath|V_n}} be the volume of its interior, the unit {{tmath|n}}-ball. The surface area of an arbitrary {{tmath|(n-1)}}-sphere is proportional to the {{tmath|(n-1)}}st power of the radius, and the volume of an arbitrary {{tmath|n}}-ball is proportional to the {{tmath|n}}th power of the radius.

[[File:hypersphere_volume_and_surface_area_graphs.svg|thumb|right|Graphs of [[Volume|volumes]] ({{tmath|V_n}}) and [[Surface area|surface areas]] ({{tmath|S_{n-1} }}) of {{mvar|n}}-balls of radius {{math|1}}.]]

The {{tmath|0}}-ball is sometimes defined as a single point. The {{tmath|0}}-dimensional [[Hausdorff measure]] is the number of points in a set. So
: <math>V_0=1.</math>

A unit {{tmath|1}}-ball is a line segment whose points have a single coordinate in the interval {{tmath|[-1, 1]}} of length {{tmath|2}}, and the {{tmath|0}}-sphere consists of its two end-points, with coordinate {{tmath|\{ -1, 1 \} }}.
:<math>S_0 = 2, \quad V_1 = 2.</math>

A unit {{tmath|1}}-sphere is the [[unit circle]] in the Euclidean plane, and its interior is the [[unit disk]] ({{tmath|2}}-ball).
:<math>S_1 = 2\pi, \quad V_2 = \pi .</math>

The interior of a [[sphere|2-sphere]] in [[three-dimensional space]] is the unit {{tmath|3}}-ball.
:<math>S_2 = 4\pi, \quad V_3 = \tfrac{4}{3} \pi.</math>

In general, {{tmath|S_{n-1} }} and {{tmath|V_n}} are given in closed form by the expressions

:<math>
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)}
</math>

where {{tmath|\Gamma}} is the [[gamma function]]. Note that {{tmath|\Gamma}}'s values at half-integers contain a factor of {{tmath|\sqrt{\pi} }} that cancels out the factor in the numerator.

As {{tmath|n}} tends to infinity, the volume of the unit {{tmath|n}}-ball (ratio between the volume of an {{tmath|n}}-ball of radius {{tmath|1}} and an [[hypercube| {{tmath|n}}-cube]] of side length {{tmath|1}}) tends to zero.<ref name="jst">{{Cite journal |last1=Smith |first1=David J. |last2=Vamanamurthy |first2=Mavina K. |year=1989 |title=How Small Is a Unit Ball? |url=https://www.jstor.org/stable/2690391 |journal=Mathematics Magazine |volume=62 |issue=2 |pages=101–107 |doi=10.1080/0025570X.1989.11977419 |jstor=2690391}}</ref> 

===Recurrences===
The ''surface area'', or properly the {{tmath|n}}-dimensional volume, of the {{tmath|n}}-sphere at the boundary of the {{tmath|(n+1)}}-ball of radius {{tmath|R}} is related to the volume of the ball by the differential equation
:<math>S_{n}R^{n}=\frac{dV_{n+1}R^{n+1}}{dR}={(n+1)V_{n+1}R^{n}}.</math>

Equivalently, representing the unit {{tmath|n}}-ball as a union of concentric {{tmath|(n-1)}}-sphere ''[[spherical shell|shells]]'',
:<math>V_{n+1} = \int_0^1 S_{n}r^{n}\,dr = \frac{1}{n+1}S_n.</math>

We can also represent the unit {{tmath|(n+2)}}-sphere as a union of products of a circle ({{tmath|1}}-sphere) with an {{tmath|n}}-sphere. Then {{tmath|S_{n+2} {{=}} 2\pi V_{n+1} }}. Since {{tmath|S_1 {{=}} 2\pi V_0}}, the equation
:<math>S_{n+1} =  2\pi V_{n}</math>
holds for all {{tmath|n}}. Along with the base cases {{tmath|S_0 {{=}} 2}}, {{tmath|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 {{tmath|n}}-dimensional Euclidean space which is analogous to the [[Spherical coordinates|spherical coordinate system]] defined for {{tmath|3}}-dimensional Euclidean space, in which the coordinates consist of a radial coordinate {{tmath|r}}, and {{tmath|n-1}} angular coordinates {{tmath|\varphi_1, \varphi_2, \ldots, \varphi_{n-1} }}, where the angles {{tmath|\varphi_1, \varphi_2, \ldots, \varphi_{n-2} }} range over {{tmath|[0, \pi]}} radians (or {{tmath|[0, 180]}} degrees) and {{tmath|\varphi_{n-1} }} ranges over {{tmath|[0, 2\pi)}} radians (or {{tmath|[0, 360)}} degrees). If {{tmath|x_i}} are the Cartesian coordinates, then we may compute {{tmath|x_1, \ldots, x_n }} from {{tmath|r, \varphi_1, \ldots, \varphi_{n-1} }} with:<ref>{{cite journal |last1=Blumenson |first1=L. E. |title=A Derivation of n-Dimensional Spherical Coordinates |journal=The American Mathematical Monthly |date=1960 |volume=67 |issue=1 |pages=63–66 |jstor=2308932 |doi=10.2307/2308932 }}</ref>{{efn|1=Formally, this formula is only correct for {{tmath|n>3}}. For {{tmath|n-3}}, the line beginning with {{tmath|x_3 {{=}} \cdots }} must be omitted, and for {{tmath|n {{=}} 2}}, the formula for [[polar coordinates]] must be used. The case {{tmath|n {{=}} 1}} reduces to {{tmath|x{{=}} r}}. Using [[capital-pi notation]] and the usual convention for the [[empty product]], a formula valid for {{tmath|n\ge 2}} is given by {{tmath|1=\textstyle x_n = r\prod_{i=1}^{n-1} \sin \varphi_i }} and {{tmath|1=\textstyle x_k =r \cos \varphi_k\prod_{i=1}^{k-1} \sin \varphi_i }} for {{tmath| k {{=}} 1, \ldots, n-1}}.}}

:<math>\begin{align}
  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{align}</math>
Except in the special cases described below, the inverse transformation is unique:

:<math>
\begin{align}
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{align}
</math>

where {{math|[[atan2]]}} is the two-argument arctangent function.

There are some special cases where the inverse transform is not unique; {{tmath|\varphi_k}} for any {{tmath|k}} will be ambiguous whenever all of {{tmath|x_k, x_{k+1}, \ldots x_n}} are zero; in this case {{tmath|\varphi_k}} may be chosen to be zero. (For example, for the {{tmath|2}}-sphere, when the polar angle is {{tmath|0}} or {{tmath|\pi}} then the point is one of the poles, zenith or nadir, and the choice of azimuthal angle is arbitrary.)

===Spherical volume and area elements===
The arc length element is<math display="block">d s^2=d r^2+\sum_{k=1}^{n-1} r^2\left(\prod_{m=1}^{k-1} \sin ^2\left(\varphi_m\right)\right) d \varphi_k^2</math>To express the [[volume element]] of {{tmath|n}}-dimensional Euclidean space in terms of spherical coordinates, let {{tmath|s_k {{=}} \sin \varphi_k}} and {{tmath|c_k {{=}} \cos \varphi_k}} for concision, then observe that the [[Jacobian matrix and determinant|Jacobian matrix]] of the transformation is:
:<math>
J_n =
\begin{pmatrix} 
c_1                        &-rs_1              &0        &0 &\cdots &0      \\
s_1c_2                     &rc_1c_2            &-rs_1s_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}.
</math>

The determinant of this matrix can be calculated by induction.  When {{tmath|n {{=}} 2}}, a straightforward computation shows that the determinant is {{tmath|r}}.  For larger {{tmath|n}}, observe that {{tmath|J_n}} can be constructed from {{tmath|J_{n-1} }} as follows.  Except in column {{tmath|n}}, rows {{tmath|n-1}} and {{tmath|n}} of {{tmath|J_n}} are the same as row {{tmath|n-1}} of {{tmath|J_{n-1} }}, but multiplied by an extra factor of {{tmath|\cos \varphi_{n-1} }} in row {{tmath|n-1}} and an extra factor of {{tmath|\sin \varphi_{n-1} }} in row {{tmath|n}}.  In column {{tmath|n}}, rows {{tmath|n-1}} and {{tmath|n}} of {{tmath|J_n}} are the same as column {{tmath|n-1}} of row {{tmath|n-1}} of {{tmath|J_{n-1} }}, but multiplied by extra factors of {{tmath|\sin \varphi_{n-1} }} in row {{tmath|n-1}} and {{tmath|\cos \varphi_{n-1} }} in row {{tmath|n}}, respectively.  The determinant of {{tmath|J_n}} can be calculated by [[Laplace expansion]] in the final column.  By the recursive description of {{tmath|J_n}}, the submatrix formed by deleting the entry at {{tmath|(n-1, n)}} and its row and column almost equals {{tmath|J_{n-1} }}, except that its last row is multiplied by {{tmath|\sin \varphi_{n-1} }}.  Similarly, the submatrix formed by deleting the entry at {{tmath|(n, n)}} and its row and column almost equals {{tmath|J_{n-1} }}, except that its last row is multiplied by {{tmath|\cos \varphi_{n-1} }}.  Therefore the determinant of {{tmath|J_n}} is
:<math>\begin{align}
|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{align}</math>
Induction then gives a [[closed-form expression]] for the volume element in spherical coordinates
:<math>\begin{align}
d^nV &= 
\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{align}</math>
The formula for the volume of the {{tmath|n}}-ball can be derived from this by integration.

Similarly the surface area element of the {{tmath|(n-1)}}-sphere of radius {{tmath|r}}, which generalizes the [[area element]] of the {{tmath|2}}-sphere, is given by

: <math>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}.</math>

The natural choice of an orthogonal basis over the angular coordinates is a product of [[Gegenbauer polynomial|ultraspherical polynomials]],

: <math>\begin{align}
& {} \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{align}</math>

for {{tmath|j {{=}} 1, 2, \ldots, n-2}}, and the {{tmath|e^{is\varphi_j} }} for the angle {{tmath|j {{=}} n-1}} in concordance with the [[spherical harmonics]].

=== Polyspherical coordinates ===
The standard spherical coordinate system arises from writing {{tmath|\R^n}} as the product {{tmath|\R \times \R^{n-1} }}.  These two factors may be related using polar coordinates.  For each point {{tmath|\mathbf x}} of <math>\R^n</math>, the standard Cartesian coordinates
:<math>\mathbf{x} = (x_1, \dots, x_n) = (y_1, z_1, \dots, z_{n-1}) = (y_1, \mathbf{z})</math>
can be transformed into a mixed polar–Cartesian coordinate system:
:<math>\mathbf{x} = (r\sin\theta, (r\cos\theta)\hat\mathbf{z}).</math>
This says that points in {{tmath|\R^n}} may be expressed by taking the ray starting at the origin and passing through <math>\hat\mathbf{z}=\mathbf{z}/\lVert\mathbf{z}\rVert\in S^{n-2}</math>, rotating it towards <math>(1,0,\dots,0)</math> by <math>\theta=\arcsin y_1/r</math>, and traveling a distance <math>r=\lVert\mathbf{x}\rVert</math> along the ray.  Repeating this decomposition eventually leads to the standard spherical coordinate system.

Polyspherical coordinate systems arise from a generalization of this construction.<ref>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.</ref> The space {{tmath|\R^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 {{tmath|p}} and {{tmath|q}} are positive integers such that {{tmath|n {{=}} p + q}}.  Then {{tmath|\R^n {{=}} \R^p \times \R^q}}.  Using this decomposition, a point {{tmath| x \in \R^n }} may be written as
:<math>\mathbf{x} = (x_1, \dots, x_n) = (y_1, \dots, y_p, z_1, \dots, z_q) = (\mathbf{y}, \mathbf{z}).</math>
This can be transformed into a mixed polar–Cartesian coordinate system by writing:
:<math>\mathbf{x} = ((r\sin \theta)\hat\mathbf{y}, (r\cos \theta)\hat\mathbf{z}).</math>
Here <math>\hat\mathbf{y}</math> and <math>\hat\mathbf{z}</math> are the unit vectors associated to {{tmath| \mathbf y}} and {{tmath|\mathbf z}}.  This expresses {{tmath|\mathbf x}} in terms of {{tmath| \hat\mathbf{y} \in S^{p-1} }}, {{tmath| \hat\mathbf{z} \in S^{q-1} }}, {{tmath|r \geq 0}}, and an angle {{tmath|\theta}}.  It can be shown that the domain of {{tmath|\theta}} is {{tmath|[0, 2\pi)}} if {{tmath|1= p = q = 1}}, {{tmath|[0, \pi]}} if exactly one of {{tmath|p}} and {{tmath|q}} is {{tmath|1}}, and {{tmath|[0, \pi/2]}} if neither {{tmath|p}} nor {{tmath|q}} are {{tmath|1}}.  The inverse transformation is
:<math>\begin{align}
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{align}</math>

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 <math>\hat\mathbf{y}</math> and <math>\hat\mathbf{z}</math> are spheres, so the coordinates of a polyspherical coordinate system are a non-negative radius and {{tmath|n-1}} angles.  The possible polyspherical coordinate systems correspond to binary trees with {{tmath|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 {{tmath|\R^n}}, and its immediate children represent the first splitting into {{tmath|\R^p}} and {{tmath|\R^q}}.  Leaf nodes correspond to Cartesian coordinates for {{tmath|S^{n-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 {{tmath|\theta_i}}, taking the left branch introduces a factor of {{tmath|\sin \theta_i}} and taking the right branch introduces a factor of {{tmath|\cos \theta_i}}.  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 {{tmath|\R^n {{=}} \R^p \times \R^q }} determines a subgroup
:<math>\operatorname{SO}_p(\R) \times \operatorname{SO}_q(\R) \subseteq \operatorname{SO}_n(\R).</math>
This is the subgroup that leaves each of the two factors <math>S^{p-1} \times S^{q-1} \subseteq S^{n-1}</math> 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 {{tmath|\R^n}} and the area measure on {{tmath|S^{n-1} }} are products.  There is one factor for each angle, and the volume measure on {{tmath|\R^n}} also has a factor for the radial coordinate.  The area measure has the form:
:<math>dA_{n-1} = \prod_{i=1}^{n-1} F_i(\theta_i)\,d\theta_i,</math>
where the factors {{tmath|F_i}} are determined by the tree.  Similarly, the volume measure is
:<math>dV_n = r^{n-1}\,dr\,\prod_{i=1}^{n-1} F_i(\theta_i)\,d\theta_i.</math>
Suppose we have a node of the tree that corresponds to the decomposition {{tmath|\R^{n_1 + n_2} {{=}} \R^{n_1} \times \R^{n_2} }} and that has angular coordinate {{tmath|\theta}}.  The corresponding factor {{tmath|F}} depends on the values of {{tmath|n_1}} and {{tmath|n_2}}.  When the area measure is normalized so that the area of the sphere is {{tmath|1}}, these factors are as follows.  If {{tmath|1=n_1 = n_2 = 1}}, then
:<math>F(\theta) = \frac{d\theta}{2\pi}.</math>
If {{tmath|n_1 > 1}} and {{tmath|n_2 {{=}} 1}}, and if {{tmath|\Beta}} denotes the [[beta function]], then
:<math>F(\theta) = \frac{\sin^{n_1 - 1}\theta}{\Beta(\frac{n_1}{2}, \frac{1}{2})}\,d\theta.</math>
If {{tmath|n_1 {{=}} 1}} and {{tmath|n_2 > 1}}, then
:<math>F(\theta) = \frac{\cos^{n_2 - 1}\theta}{\Beta(\frac{1}{2}, \frac{n_2}{2})}\,d\theta.</math>
Finally, if both {{tmath|n_1}} and {{tmath|n_2}} are greater than one, then
:<math>F(\theta) = \frac{(\sin^{n_1 - 1}\theta)(\cos^{n_2 - 1}\theta)}{\frac{1}{2}\Beta(\frac{n_1}{2}, \frac{n_2}{2})}\,d\theta.</math>

== Stereographic projection ==

{{main|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 {{tmath|n}}-sphere can be mapped onto an {{tmath|n}}-dimensional [[hyperplane]] by the {{tmath|n}}-dimensional version of the stereographic projection. For example, the point {{tmath|[x, y, z]}} on a two-dimensional sphere of radius {{tmath|1}} maps to the point {{tmath|\bigl[ \tfrac{x}{1-z}, \tfrac{y}{1-z} \bigr] }} on the {{tmath|xy}}-plane. In other words,
:<math>[x,y,z] \mapsto \left[\frac{x}{1-z},\frac{y}{1-z}\right].</math>

Likewise, the stereographic projection of an {{tmath|n}}-sphere {{tmath|S^n}} of radius {{tmath|1}} will map to the {{tmath|(n-1)}}-dimensional hyperplane {{tmath|\R^{n-1} }} perpendicular to the {{tmath|x_n}}-axis as
:<math>[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].</math>

== Probability distributions ==

=== Uniformly at random on the {{math|(''n'' − 1)}}-sphere ===
[[File:2sphere-uniform.png|thumbnail|A set of points drawn from a uniform distribution on the surface of a unit {{math|2}}-sphere, generated using Marsaglia's algorithm.]]

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

Generate an {{tmath|n}}-dimensional vector of [[normal distribution|normal deviates]] (it suffices to use {{tmath|N(0, 1)}}, although in fact the choice of the variance is arbitrary), {{tmath|\mathbf x {{=}} (x_1, x_2, \ldots, x_n)}}. Now calculate the "radius" of this point:

:<math>r=\sqrt{x_1^2+x_2^2+\cdots+x_n^2}.</math>

The vector {{tmath|\tfrac 1r \mathbf x}} is uniformly distributed over the surface of the unit {{tmath|n}}-ball.

An alternative given by Marsaglia is to uniformly randomly select a point {{tmath|\mathbf x {{=}} (x_1, x_2, \ldots, x_n)}} in the unit [[hypercube|{{mvar|n}}-cube]] by sampling each {{tmath|x_i}} independently from the [[continuous uniform distribution|uniform distribution]] over {{tmath|(-1, 1)}}, computing {{tmath|r}} as above, and rejecting the point and resampling if {{tmath|r \geq 1}} (i.e., if the point is not in the {{tmath|n}}-ball), and when a point in the ball is obtained scaling it up to the spherical surface by the factor {{tmath|\tfrac 1r }}; then again {{tmath|\tfrac 1r \mathbf x}} is uniformly distributed over the surface of the unit {{tmath|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 <math>10^{-24}</math> 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 {{tmath|(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 {{tmath|n}}-ball.  If {{tmath|u}} is a number generated uniformly at random from the interval {{tmath|[0, 1]}} and {{tmath|\mathbf x}} is a point selected uniformly at random from the unit {{tmath|(n-1)}}-sphere, then {{tmath|u^{1/n} \mathbf x}} is uniformly distributed within the unit {{tmath|n}}-ball.

Alternatively, points may be sampled uniformly from within the unit {{tmath|n}}-ball by a reduction from the unit {{tmath|(n+1)}}-sphere. In particular, if {{tmath|(x_1, x_2, \ldots, x_{n+2})}} is a point selected uniformly from the unit {{tmath|(n+1)}}-sphere, then {{tmath|(x_1, x_2, \ldots, x_n)}} is uniformly distributed within the unit {{tmath|n}}-ball (i.e., by simply discarding two coordinates).<ref>{{cite report|first1=Aaron R. | last1=Voelker | first2=Jan | last2=Gosmann | first3=Terrence C. | last3=Stewart | title=Efficiently sampling vectors and coordinates from the n-sphere and n-ball | year=2017 | publisher=Centre for Theoretical Neuroscience | url=http://compneuro.uwaterloo.ca/publications/voelker2017.html | doi=10.13140/RG.2.2.15829.01767/1}}</ref>

If {{tmath|n}} is sufficiently large, most of the volume of the {{tmath|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 {{tmath|y {{=}} x_1^2 }} be the square of the first coordinate of a point sampled uniformly at random from the {{tmath|(n-1)}}-sphere, then its probability density function, for <math>y\in [0, 1]</math>, is 

<math display="block">
\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}.
</math>

Let <math>z = y/N</math> be the appropriately scaled version, then at the <math>N\to \infty</math> limit, the probability density function of <math>z</math> converges to <math> (2\pi ze^z)^{-1/2}</math>. This is sometimes called the Porter–Thomas distribution.<ref>{{Citation |last=Livan |first=Giacomo |title=One Pager on Eigenvectors |date=2018 |url=https://doi.org/10.1007/978-3-319-70885-0_9 |work=Introduction to Random Matrices: Theory and Practice |pages=65–66 |editor-last=Livan |editor-first=Giacomo |access-date=2023-05-19 |series=SpringerBriefs in Mathematical Physics |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-70885-0_9 |isbn=978-3-319-70885-0 |last2=Novaes |first2=Marcel |last3=Vivo |first3=Pierpaolo |editor2-last=Novaes |editor2-first=Marcel |editor3-last=Vivo |editor3-first=Pierpaolo}}</ref>

==Specific spheres==
; {{math|0}}-sphere : The pair of points {{tmath| \{ \pm R\} }} with the [[discrete topology]] for some {{tmath|R > 0}}. The only sphere that is not [[path-connected]]. [[Parallelizable]].
; [[1-sphere|{{math|1}}-sphere]] : Commonly called a [[circle]]. Has a nontrivial fundamental group. Abelian Lie group structure {{math|[[U(1)]]}}; the [[circle group]]. [[Homeomorphic]] to the [[real projective line]]. [[Parallelizable]]
; [[2-sphere|{{math|2}}-sphere]] : Commonly simply called a [[sphere]]. For its complex structure, see ''[[Riemann sphere]]''. Homeomorphic to the [[complex projective line]]
; [[3-sphere|{{math|3}}-sphere]] : Parallelizable, [[principal bundle|principal]] [[circle bundle|{{math|U(1)}}-bundle]] [[Hopf fibration|over]] the {{tmath|2}}-sphere, [[Lie group]] structure {{math|[[Sp(1)]]}} = {{math|[[SU(2)]]}}.
; {{math|4}}-sphere : Homeomorphic to the [[quaternionic projective line]], {{tmath|\mathbf{HP}^1}}. {{tmath|\operatorname{SO}(5) / \operatorname{SO}(4)}}.
; {{math|5}}-sphere : [[principal bundle|Principal]] [[circle bundle|{{math|''U''(1)}}-bundle]] over the [[complex projective space]] {{tmath|\mathbf{CP}^2 }}. {{tmath|1=\operatorname{SO}(6) / \operatorname{SO}(5) = \operatorname{SU}(3) / \operatorname{SU}(2)}}. It is [[undecidable problem|undecidable]] whether a given {{tmath|n}}-dimensional manifold is homeomorphic to {{tmath|S^n}} for {{tmath|n \geq 5}}.<ref>{{citation|title=Classical Topology and Combinatorial Group Theory|volume=72|series=Graduate Texts in Mathematics|first=John|last=Stillwell|authorlink=John Stillwell|publisher=Springer|year=1993|isbn=9780387979700|page=247|url=https://books.google.com/books?id=265lbM42REMC&pg=PA247}}.</ref>
; {{math|6}}-sphere : Possesses an [[almost complex structure]] coming from the set of pure unit [[octonion]]s. {{tmath|1=\operatorname{SO}(7) / \operatorname{SO}(6) = G_2 / \operatorname{SU}(3)}}. The question of whether it has a [[Complex manifold|complex structure]] is known as the ''Hopf problem,'' after [[Heinz Hopf]].<ref>{{cite journal|last1=Agricola |first1=Ilka |author-link1=Ilka Agricola |first2=Giovanni |last2=Bazzoni |first3=Oliver |last3=Goertsches |first4=Panagiotis |last4=Konstantis |first5=Sönke |last5=Rollenske |title=On the history of the Hopf problem |arxiv=1708.01068 |journal=Differential Geometry and Its Applications |year=2018 |volume=57 |pages=1–9|doi=10.1016/j.difgeo.2017.10.014 |s2cid=119297359 }}</ref>
; {{math|7}}-sphere : Topological [[quasigroup]] structure as the set of unit [[octonion]]s. Principal {{tmath|\operatorname{Sp}(1)}}-bundle over {{tmath|S^4}}. [[Parallelizable]]. {{tmath|1=\operatorname{SO}(8) / \operatorname{SO}(7) = \operatorname{SU}(4) / \operatorname{SU}(3) = \operatorname{Sp}(2) / \operatorname{Sp}(1) = \operatorname{Spin}(7) / G_2 = \operatorname{Spin}(6) / \operatorname{SU}(3)}}. The {{tmath|7}}-sphere is of particular interest since it was in this dimension that the first [[exotic sphere]]s were discovered.
; {{math|8}}-sphere : Homeomorphic to the octonionic projective line {{tmath|\mathbf{OP}^1}}.
; {{math|23}}-sphere : A highly dense [[sphere-packing]] is possible in {{tmath|24}}-dimensional space, which is related to the unique qualities of the [[Leech lattice]].

== Octahedral sphere ==
The '''octahedral {{tmath|n}}-sphere''' is defined similarly to the {{tmath|n}}-sphere but using the [[1 norm|{{math|1}}-norm]]

:<math> S^n = \left\{ x \in \R^{n+1} : \left\| x \right\|_1 = 1 \right\}</math>

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

The octahedral {{tmath|1}}-sphere is a square (without its interior). The octahedral {{tmath|2}}-sphere is a regular [[octahedron]]; hence the name. The octahedral {{tmath|n}}-sphere is the [[topological join]] of {{tmath|n+1}} pairs of isolated points.<ref name=":7">{{Cite journal|last=Meshulam|first=Roy|date=2001-01-01|title=The Clique Complex and Hypergraph Matching|journal=Combinatorica|language=en|volume=21|issue=1|pages=89–94|doi=10.1007/s004930170006|s2cid=207006642|issn=1439-6912}}</ref> 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==
*{{Annotated link|Conformal geometry}}
*{{Annotated link|Exotic sphere}}
*{{Annotated link|Homology sphere}}
*{{Annotated link|Homotopy groups of spheres}}
*{{Annotated link|Inversive geometry}}
*{{Annotated link|Möbius transformation}}

== Notes ==
{{notelist}}
{{Reflist}}

== References ==
* {{cite journal|last1=Marsaglia|first1=G.|title=Choosing a Point from the Surface of a Sphere|journal=Annals of Mathematical Statistics|volume=43|pages=645–646|year=1972|doi=10.1214/aoms/1177692644|issue=2|doi-access=free}}
* {{cite journal|first1=Greg|last1=Huber|title=Gamma function derivation of n-sphere volumes|journal=Amer. Math. Monthly|volume=89|year=1982|pages=301–302|mr=1539933 |jstor=2321716|issue=5|doi=10.2307/2321716
}}
* {{Cite book | last1=Weeks | first1=Jeffrey R. | author1-link=Jeffrey Weeks (mathematician) | title=The Shape of Space: how to visualize surfaces and three-dimensional manifolds | publisher=Marcel Dekker | isbn=978-0-8247-7437-0 | year=1985| postscript=&nbsp;(Chapter 14: The Hypersphere).}}
*{{cite journal|first1=E. G. |last1=Kalnins| first2=W. |last2=Miller|title=Separation of variables on n-dimensionsional Riemannian manifolds. I. the n-sphere S_n and Euclidean n-sparce R_n|doi=10.1063/1.527088| journal=J. Math. Phys.|year=1986|  pages=1721-1746|volume=27|doi-access=free|hdl=10289/1219|hdl-access=free}}
* {{cite book | last1=Flanders | first1=Harley | title=Differential forms with applications to the physical sciences | publisher=[[Dover Publications]] | location=New York | isbn=978-0-486-66169-8 | year=1989}}
* {{Cite book | last1=Moura | first1=Eduarda | last2=Henderson | first2=David G. | title=Experiencing geometry: on plane and sphere | url=https://archive.org/details/experiencinggeom0000hend | publisher=[[Prentice Hall]] | isbn=978-0-13-373770-7 | year=1996| postscript=&nbsp;(Chapter 20: 3-spheres and hyperbolic 3-spaces). | url-access=registration }}
* {{cite journal|first1=Nir | last1=Barnea | title=Hyperspherical functions with arbitrary permutational symmetry: Reverse construction
|year=1999 | journal = Phys. Rev. A | doi=10.1103/PhysRevA.59.1135
|volume=59 | number=2|pages=1135–1146| bibcode=1999PhRvA..59.1135B }}

==External links==
* {{MathWorld|title=Hypersphere|urlname=Hypersphere}}

{{Dimension topics}}
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[[Category:Multi-dimensional geometry]]
[[Category:Spheres]]