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

- Description
- Euclidean coordinates in (n + 1)-space
- n-ball
- Topological description
- Volume and surface area
- Examples
- Recurrences
- Closed forms
- Other relations
- Spherical coordinates
- Spherical volume and area elements
- Polyspherical coordinates
- Stereographic projection
- Generating random points
- Uniformly at random on the (n − 1)-sphere
- Uniformly at random within the n-ball
- Specific spheres
- Octahedral sphere
- See also
- Notes
- References
- External links

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

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 is a 2-sphere, often simply called a sphere,
- the three-dimensional boundary of a (four-dimensional) 4-ball is a 3-sphere,
- 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.

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.

The set of points in (*n* + 1)-space, (*x*_{1}, *x*_{2}, ..., *x*_{n+1}), that define an *n*-sphere, , is represented by the equation:

where **c** = (*c*_{1}, *c*_{2}, ..., *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

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,

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:

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 *S*^{n} = ℝ^{n} ∪ {∞}, 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 ℝ^{n}. This forms the basis for stereographic projection.^{ [1] }

*V _{n}*(

The constants *V _{n}* and

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

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

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 *n*th power of the radius, *R* (with different constants of proportionality that vary with *n*). We write *V*_{n}(*R*) = *V*_{n}R^{n} for the volume of the *n*-ball and *S*_{n}(*R*) = *S*_{n}R^{n} 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}*(

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

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

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

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

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

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

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

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

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

So,

We can also represent the unit (*n* + 2)-sphere as a union of products of a circle (1-sphere) with an *n*-sphere. Let *r* = cos *θ* and *r*^{2} + *R*^{2} = 1, so that *R* = sin *θ* and *dR* = cos *θ**dθ*. Then,

Since *S*_{1} = 2π *V*_{0}, the equation

holds for all *n*.

This completes the derivation of the recurrences:

Combining the recurrences, we see that

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

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

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

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

By multiplying *V _{n}* by

for the (*n* − 1)-dimensional volume of the sphere *S*^{n−1}.

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

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

where *S*_{0} = 2, *V*_{1} = 2, *S*_{1} = 2π and *V*_{2} = π.

The recurrence relation for *V*_{n} can also be proved via integration with 2-dimensional polar 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 *φ*_{1}, *φ*_{2}, ... *φ*_{n−1}, where the angles *φ*_{1}, *φ*_{2}, ... *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

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

where if *x _{k}* ≠ 0 for some

There are some special cases where the inverse transform is not unique; *φ _{k}* for any

To express the volume element of *n*-dimensional Euclidean space in terms of spherical coordinates, first observe that the Jacobian matrix of the transformation is:

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 *J*_{n} can be constructed from *J*_{n− 1} as follows. Except in column *n*, rows *n*− 1 and *n* of *J*_{n} are the same as row *n*− 1 of *J*_{n− 1}, but multiplied by an extra factor of cos φ_{n− 1} in row *n*− 1 and an extra factor of sin φ_{n− 1} in row *n*. In column *n*, rows *n*− 1 and *n* of *J*_{n} are the same as column *n*− 1 of row *n*− 1 of *J*_{n− 1}, but multiplied by extra factors of sin φ_{n− 1} in row *n*− 1 and cos φ_{n− 1} in row *n*, respectively. The determinant of *J*_{n} can be calculated by Laplace expansion in the final column. By the recursive description of *J*_{n}, the submatrix formed by deleting the entry at (*n*− 1, *n*) and its row and column almost equals *J*_{n− 1}, except that its last row is multiplied by sin φ_{n− 1}. Similarly, the submatrix formed by deleting the entry at (*n*, *n*) and its row and column almost equals *J*_{n− 1}, except that its last row is multiplied by cos φ_{n− 1}. Therefore the determinant of *J*_{n} is

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

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

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

for *j* = 1, 2,... *n* − 2, and the *e*^{isφj} for the angle *j* = *n* − 1 in concordance with the spherical harmonics.

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

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

This says that points in ℝ^{n} may be expressed by taking the ray starting at the origin and passing through **z** ∈ ℝ^{n− 1}, rotating it towards the first basis vector by θ, and traveling a distance *r* along the ray. Repeating this decomposition eventually leads to the standard spherical coordinate system.

Polyspherical coordinate systems arise from a generalization of this construction.^{ [3] } The space 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 *p* and *q* are positive integers such that *n* = *p* + *q*. Then ℝ^{n} = ℝ^{p}× ℝ^{q}. Using this decomposition, a point **x** ∈ ℝ^{n} may be written as

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

Here and are the unit vectors associated to **y** and **z**. This expresses **x** in terms of , , *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

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 and 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 ℝ^{n}, and its immediate children represent the first splitting into ℝ^{p} and ℝ^{q}. Leaf nodes correspond to Cartesian coordinates for 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 θ_{i}, taking the left branch introduces a factor of sin θ_{i} and taking the right branch introduces a factor of cos θ_{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 ℝ^{n} = ℝ^{p}× ℝ^{q} determines a subgroup

This is the subgroup that leaves each of the two factors 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 ℝ^{n} and the area measure on S^{n− 1} are products. There is one factor for each angle, and the volume measure on ℝ^{n} also has a factor for the radial coordinate. The area measure has the form:

where the factors *F*_{i} are determined by the tree. Similarly, the volume measure is

Suppose we have a node of the tree that corresponds to the decomposition ℝ^{n1 + n2} = ℝ^{n1}× ℝ^{n2} and that has angular coordinate θ. The corresponding factor *F* depends on the values of *n*_{1} and *n*_{2}. When the area measure is normalized so that the area of the sphere is 1, these factors are as follows. If *n*_{1} = *n*_{2} = 1, then

If *n*_{1} > 1 and *n*_{2} = 1, and if B denotes the beta function, then

If *n*_{1} = 1 and *n*_{2} > 1, then

Finally, if both *n*_{1} and *n*_{2} are greater than one, then

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,

Likewise, the stereographic projection of an *n*-sphere **S**^{n−1} of radius 1 will map to the (*n* − 1)-dimensional hyperplane ℝ^{n−1} perpendicular to the *x _{n}*-axis as

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** = (*x*_{1}, *x*_{2},... *x _{n}*). Now calculate the "radius" of this point:

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

An alternative given by Marsaglia is to uniformly randomly select a point **x** = (*x*_{1}, *x*_{2},... *x _{n}*) in the unit

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*^{.mw-parser-output .frac{white-space:nowrap}.mw-parser-output .frac .num,.mw-parser-output .frac .den{font-size:80%;line-height:0;vertical-align:super}.mw-parser-output .frac .den{vertical-align:sub}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}1⁄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 (*x*_{1},*x*_{2},...,*x*_{n+2}) is a point selected uniformly from the unit (*n* + 1)-sphere, then (*x*_{1},*x*_{2},...,*x*_{n}) is uniformly distributed within the unit *n*-ball (i.e., by simply discarding two coordinates).^{ [4] }

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.

- 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, ℝP
^{1}. Parallelizable. SO(2) = U(1). - 2-sphere
- Also known as the sphere. Complex structure; see Riemann sphere. Equivalent to the complex projective line,
**C**P^{1}. 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
- .
- 4-sphere
- Equivalent to the quaternionic projective line,
**H**P^{1}. SO(5)/SO(4). - 5-sphere
- Principal U(1)-bundle over
**C**P^{2}. 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) =
*G*_{2}/SU(3). The question of whether it has a complex structure is known as the*Hopf problem,*after Heinz Hopf.^{ [5] } - 7-sphere
- Topological quasigroup structure as the set of unit octonions. Principal Sp(1)-bundle over
*S*^{4}. Parallelizable. SO(8)/SO(7) = SU(4)/SU(3) = Sp(2)/Sp(1) = Spin(7)/*G*_{2}= 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
**O**P^{1}. - 23-sphere
- A highly dense sphere-packing is possible in 24-dimensional space, which is related to the unique qualities of the Leech lattice.

The **octahedral n-sphere** is defined similarly to the

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.^{ [6] } 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.

- ↑ James W. Vick (1994).
*Homology theory*, p. 60. Springer - ↑ 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. - ↑ 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. - ↑ 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.
- ↑ 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. - ↑ 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.

In physics, the **cross section** is a measure of the probability that a specific process will take place when some kind of radiant excitation intersects a localized phenomenon. For example, the Rutherford cross-section is a measure of probability that an alpha-particle will be deflected by a given angle during a collision with an atomic nucleus. Cross section is typically denoted *σ* (sigma) and is expressed in units of transverse area. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process.

In mathematics, the **polar coordinate system** is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point is called the *pole*, and the ray from the pole in the reference direction is the *polar axis*. The distance from the pole is called the *radial coordinate*, *radial distance* or simply *radius*, and the angle is called the *angular coordinate*, *polar angle*, or *azimuth*. The radial coordinate is often denoted by *r* or *ρ*, and the angular coordinate by *φ*, *θ*, or *t*. Angles in polar notation are generally expressed in either degrees or radians.

A **sphere** is a geometrical object in three-dimensional space that is the surface of a ball.

In mathematics, a **spherical coordinate system** is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the *radial distance* of that point from a fixed origin, its *polar angle* measured from a fixed zenith direction, and the *azimuthal angle* of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction on that plane. It can be seen as the three-dimensional version of the polar coordinate system.

In mathematics and physics, **Laplace's equation** is a second-order partial differential equation named after Pierre-Simon Laplace who first studied its properties. This is often written as

In mathematics, a **3-sphere**, or **glome**, is a higher-dimensional analogue of a sphere. It may be embedded in 4-dimensional Euclidean space as the set of points equidistant from a fixed central point. Analogous to how the boundary of a ball in three dimensions is an ordinary sphere, the boundary of a ball in four dimensions is a 3-sphere. A 3-sphere is an example of a 3-manifold and an n-sphere.

An **ellipsoid** is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation.

In mechanics and geometry, the **3D rotation group**, often denoted **SO(3)**, is the group of all rotations about the origin of three-dimensional Euclidean space under the operation of composition. By definition, a rotation about the origin is a transformation that preserves the origin, Euclidean distance, and orientation. Every non-trivial rotation is determined by its axis of rotation and its angle of rotation. Composing two rotations results in another rotation; every rotation has a unique inverse rotation; and the identity map satisfies the definition of a rotation. Owing to the above properties, the set of all rotations is a group under composition. Rotations are not commutative, making it a nonabelian group. Moreover, the rotation group has a natural structure as a manifold for which the group operations are smoothly differentiable; so it is in fact a Lie group. It is compact and has dimension 3.

In mathematics and physical science, **spherical harmonics** are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.

In 1851, George Gabriel Stokes derived an expression, now known as **Stokes law**, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by solving the Stokes flow limit for small Reynolds numbers of the Navier–Stokes equations.

In probability theory, the **Borel–Kolmogorov paradox** is a paradox relating to conditional probability with respect to an event of probability zero. It is named after Émile Borel and Andrey Kolmogorov.

This is a **table of orthonormalized spherical harmonics** that employ the Condon-Shortley phase up to degree * = 10. Some of these formulas give the "Cartesian" version. This assumes **x*, *y*, *z*, and *r* are related to and through the usual spherical-to-Cartesian coordinate transformation:

In calculus, the **Leibniz integral rule** for differentiation under the integral sign, named after Gottfried Leibniz, states that for an integral of the form

In mathematics, a **multiple integral** is a definite integral of a function of several real variables, for instance, *f*(*x*, *y*) or *f*(*x*, *y*, *z*). Integrals of a function of two variables over a region in are called **double integrals**, and integrals of a function of three variables over a region in are called **triple integrals**. For multiple integrals of a single-variable function, see the Cauchy formula for repeated integration.

In astronomy, **position angle** is the convention for measuring angles on the sky. The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the direction of the right ascension. In the standard (non-flipped) images, this is a counterclockwise measure relative to the axis into the direction of positive declination.

In geometric topology, the **Clifford torus** is the simplest and most symmetric flat embedding of the cartesian product of two circles *S*^{1}_{a} and *S*^{1}_{b}. It is named after William Kingdon Clifford. It resides in **R**^{4}, as opposed to in **R**^{3}. To see why **R**^{4} is necessary, note that if *S*^{1}_{a} and *S*^{1}_{b} each exists in its own independent embedding space **R**^{2}_{a} and **R**^{2}_{b}, the resulting product space will be **R**^{4} rather than **R**^{3}. The historically popular view that the cartesian product of two circles is an **R**^{3} torus in contrast requires the highly asymmetric application of a rotation operator to the second circle, since that circle will only have one independent axis *z* available to it after the first circle consumes *x* and *y*.

In mathematics, **vector spherical harmonics** (**VSH**) are an extension of the scalar spherical harmonics for use with vector fields. The components of the VSH are complex-valued functions expressed in the spherical coordinate basis vectors.

In fluid dynamics, the **Oseen equations** describe the flow of a viscous and incompressible fluid at small Reynolds numbers, as formulated by Carl Wilhelm Oseen in 1910. Oseen flow is an improved description of these flows, as compared to Stokes flow, with the (partial) inclusion of convective acceleration.

In four-dimensional geometry, the **spherinder**, or **spherical cylinder** or **spherical prism**, is a geometric object, defined as the Cartesian product of a 3-ball, radius *r*_{1} and a line segment of length 2*r*_{2}:

- 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).CS1 maint: postscript (link) - 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).CS1 maint: postscript (link) - 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.

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