Unit sphere

Unit ball (red) and unit sphere (blue) for the Euclidean norm in two dimensions.

In mathematics, a unit sphere is a sphere of unit radius: the set of points at Euclidean distance 1 from some center point in three-dimensional space. More generally, the unit Template:Tmath-sphere is an [[n-sphere|Template:Tmath-sphere]] of unit radius in Template:Tmath-dimensional Euclidean space; the unit circle is a special case, the unit Template:Tmath-sphere in the plane. An (open) unit ball is the region inside of a unit sphere, the set of points of distance less than 1 from the center.

A unit sphere or unit ball with center at the origin of the space is called the (canonical) unit sphere or the (canonical) unit ball. Any arbitrary sphere can be transformed to the unit sphere by a combination of translation and scaling, so the study of spheres in general can often be reduced to the study of the unit sphere.

The unit sphere is often used as a model for spherical geometry because it has constant sectional curvature of 1, which simplifies calculations. In trigonometry, circular arc length on the unit circle is called radians and used for measuring angular distance; in spherical trigonometry surface area on the unit sphere is called steradians and used for measuring solid angle.

In more general contexts, a unit sphere is the set of points of distance 1 from a fixed central point, where different norms can be used as general notions of "distance", and an (open) unit ball is the region inside.

Some 1-spheres: Template:Math is the norm for Euclidean space.

Unit spheres and balls in Euclidean space

In Euclidean space of Template:Tmath dimensions, the Template:Tmath-dimensional unit sphere is the set of all points $(x_1,\dots ,x_n)$ which satisfy the equation $$x_1^2+x_2^2+\cdots +x_n^2=1.$$

The open unit Template:Tmath-ball is the set of all points satisfying the inequality $$x_1^2+x_2^2+\cdots +x_n^2<1,$$ and closed unit Template:Tmath-ball is the set of all points satisfying the inequality $$x_1^2+x_2^2+\cdots +x_n^2\le 1.$$

Volume and area

Graphs of volumes (Template:Mvar) and surface areas (Template:Mvar) of unit Template:Mvar-balls

The classical equation of a unit sphere is that of the ellipsoid with a radius of 1 and no alterations to the Template:Tmath-, Template:Tmath-, or Template:Tmath- axes: $$x^2+y^2+z^2=1$$

The volume of the unit ball in Euclidean Template:Tmath-space, and the surface area of the unit sphere, appear in many important formulas of analysis. The volume of the unit Template:Tmath-ball, which we denote $V_n,$ can be expressed by making use of the gamma function. It is $$V_n=\frac{{\pi }^{n/2}}{\mathrm{\Gamma }(1+n/2)}=\{\begin{array}{cc}{\pi }^{n/2}/(n/2)!& \mathrm{i}\mathrm{f}n\ge 0\mathrm{i}\mathrm{s}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\\ 2(2\pi {)}^{(n-1)/2}/n!!& \mathrm{i}\mathrm{f}n\ge 0\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{d}\mathrm{d},\end{array}$$ where $n!!$ is the double factorial.

The hypervolume of the Template:Tmath-dimensional unit sphere (i.e., the "area" of the boundary of the Template:Tmath-dimensional unit ball), which we denote $A_{n-1},$ can be expressed as $$A_{n-1}=n{V}_n=\frac{n{\pi }^{n/2}}{\mathrm{\Gamma }(1+n/2)}=\frac{2{\pi }^{n/2}}{\mathrm{\Gamma }(n/2)}=\{\begin{array}{cc}2{\pi }^{n/2}/(n/2-1)!& \mathrm{i}\mathrm{f}n\ge 1\mathrm{i}\mathrm{s}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{n}\\ 2(2\pi {)}^{(n-1)/2}/(n-2)!!& \mathrm{i}\mathrm{f}n\ge 1\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{d}\mathrm{d}\mathrm{.}\end{array}$$ For example, $A_0=2$ is the "area" of the boundary of the unit ball $[-1,1]\subset ℝ$, which simply counts the two points. Then $A_1=2\pi$ is the "area" of the boundary of the unit disc, which is the circumference of the unit circle. $A_2=4\pi$ is the area of the boundary of the unit ball $\{x\in {ℝ}^3:x_1^2+x_2^2+x_3^2\le 1\}$, which is the surface area of the unit sphere $\{x\in {ℝ}^3:x_1^2+x_2^2+x_3^2=1\}$.

The surface areas and the volumes for some values of Template:Tmath are as follows:

Template:Tmath	$A_{n-1}$ (surface area)		$V_n$ (volume)
0			$(1/0!){\pi }^0$	1
1	$1(2^1/1!!){\pi }^0$	2	$(2^1/1!!){\pi }^0$	2
2	$2(1/1!){\pi }^1=2\pi$	6.283	$(1/1!){\pi }^1=\pi$	3.141
3	$3(2^2/3!!){\pi }^1=4\pi$	12.57	$(2^2/3!!){\pi }^1=(4/3)\pi$	4.189
4	$4(1/2!){\pi }^2=2{\pi }^2$	19.74	$(1/2!){\pi }^2=(1/2){\pi }^2$	4.935
5	$5(2^3/5!!){\pi }^2=(8/3){\pi }^2$	26.32	$(2^3/5!!){\pi }^2=(8/15){\pi }^2$	5.264
6	$6(1/3!){\pi }^3={\pi }^3$	31.01	$(1/3!){\pi }^3=(1/6){\pi }^3$	5.168
7	$7(2^4/7!!){\pi }^3=(16/15){\pi }^3$	33.07	$(2^4/7!!){\pi }^3=(16/105){\pi }^3$	4.725
8	$8(1/4!){\pi }^4=(1/3){\pi }^4$	32.47	$(1/4!){\pi }^4=(1/24){\pi }^4$	4.059
9	$9(2^5/9!!){\pi }^4=(32/105){\pi }^4$	29.69	$(2^5/9!!){\pi }^4=(32/945){\pi }^4$	3.299
10	$10(1/5!){\pi }^5=(1/12){\pi }^5$	25.50	$(1/5!){\pi }^5=(1/120){\pi }^5$	2.550

where the decimal expanded values for $n\ge 2$ are rounded to the displayed precision.

Recursion

The $A_n$ values satisfy the recursion: $$A_0=2$$ $$A_1=2\pi$$ $$A_n=\frac{2\pi }{n-1}{A}_{n-2}$$ for Template:Tmath.

The $V_n$ values satisfy the recursion: $$V_0=1$$ $$V_1=2$$ $$V_n=\frac{2\pi }{n}{V}_{n-2}$$ for Template:Tmath.

Non-negative real-valued dimensions

The value $2^{-n}{V}_n={\pi }^{n/2}/2^n\mathrm{\Gamma }(1+\frac{1}{2}n)$ at non-negative real values of Template:Tmath is sometimes used for normalization of Hausdorff measure.

Other radii

The surface area of an Template:Tmath-sphere with radius Template:Tmath is $A_{n-1}{r}^{n-1}$ and the volume of an Template:Tmath-ball with radius Template:Tmath is $V_n{r}^n.$ For instance, the area is $A_2=4\pi r^2$ for the two-dimensional surface of the three-dimensional ball of radius $r.$ The volume is $V_3=\frac{4}{3}\pi r^3$ for the three-dimensional ball of radius Template:Tmath.

Unit balls in normed vector spaces

The open unit ball of a normed vector space Template:Tmath with the norm $\Vert \cdot \Vert$ is given by $$\{x\in V:\Vert x\Vert <1\}$$

It is the topological interior of the closed unit ball of $(V,\Vert \cdot \Vert ):$ $$\{x\in V:\Vert x\Vert \le 1\}$$

The latter is the disjoint union of the former and their common border, the unit sphere of $(V,\Vert \cdot \Vert ):$ $$\{x\in V:\Vert x\Vert =1\}$$

The "shape" of the unit ball is entirely dependent on the chosen norm; it may well have "corners", and for example may look like $[-1,1{]}^n$ in the case of the max-norm in Template:Tmath. One obtains a naturally round ball as the unit ball pertaining to the usual Hilbert space norm, based in the finite-dimensional case on the Euclidean distance; its boundary is what is usually meant by the unit sphere.

Let $x=(x_1,\mathrm{...}{x}_n)\in {ℝ}^n.$ Define the usual Template:Tmath-norm for $p\ge 1$ as: $$\Vert x{\Vert }_p=(\sum _{k=1}^n|x_k{|}^p{)}^{1/p}$$

Then $\Vert x{\Vert }_2$ is the usual Hilbert space norm. $\Vert x{\Vert }_1$ is called the Hamming norm, or Template:Tmath-norm. The condition $p\ge 1$ is necessary in the definition of the ${\ell }_p$ norm, as the unit ball in any normed space must be convex as a consequence of the triangle inequality. Let $\Vert x{\Vert }_{\mathrm{\infty }}$ denote the max-norm or Template:Tmath-norm of Template:Tmath.

Note that for the one-dimensional circumferences $C_p$ of the two-dimensional unit balls, we have: $$C_1=4\sqrt{2}$$ is the minimum value. $$C_2=2\pi$$ $$C_{\mathrm{\infty }}=8$$ is the maximum value.

Generalizations

Metric spaces

All three of the above definitions can be straightforwardly generalized to a metric space, with respect to a chosen origin. However, topological considerations (interior, closure, border) need not apply in the same way (e.g., in ultrametric spaces, all of the three are simultaneously open and closed sets), and the unit sphere may even be empty in some metric spaces.

Quadratic forms

If Template:Tmath is a linear space with a real quadratic form $F:V\to ℝ,$ then $\{p\in V:F(p)=1\}$ may be called the unit sphere or unit quasi-sphere of $V.$ For example, the quadratic form Template:Tmath, when set equal to one, produces the unit hyperbola, which plays the role of the "unit circle" in the plane of split-complex numbers. Similarly, the quadratic form $x^2$ yields a pair of lines for the unit sphere in the dual number plane.

See also

• Ball
• [[n-sphere|Template:Tmath-sphere]]
• Sphere
• Superellipse
• Unit circle
• Unit disk
• Unit tangent bundle
• Unit square

Notes and references

• Mahlon M. Day (1958) Normed Linear Spaces, page 24, Springer-Verlag.
• Template:Citation. Reviewed in Newsletter of the European Mathematical Society 64 (June 2007), p. 57. This book is organized as a list of distances of many types, each with a brief description.

External links

• Template:Mathworld