# nLab sphere

Contents

### Context

#### Spheres

n-sphere

low dimensional n-spheres

# Contents

## Definition

### Finite-dimensional spheres

###### Definition

The $n$-dimensional unit sphere , or simply $n$-sphere, is the topological space given by the subset of the $(n+1)$-dimensional Cartesian space $\mathbb{R}^{n+1}$ consisting of all points $x$ whose distance from the origin is $1$

$S^n = \{ x: \mathbb{R}^{n+1} \;|\; \|x\| = 1 \} \,.$

The $n$-dimensional sphere of radius $r$ is

$S^n_r = \{ x: \mathbb{R}^{n+1} \;|\; \|x\| = r \} .$

Topologically, this is equivalent (homeomorphic) to the unit sphere for $r \gt 0$, or a point for $r = 0$.

This is naturally a smooth manifold of dimension $n$, with the smooth structure induced by the standard smooth structure on $\mathbb{R}^n$.

### Infinite dimensional spheres

One can also talk about the infinite-dimensional sphere in an arbitrary (possibly infinite-dimensional) normed vector space $V$:

$S(V) = \{ x: V \;\text{such that}\; {\|x\|} = 1 \} .$

If a locally convex topological vector space admits a continuous linear injection into a normed vector space, this can be used to define its sphere. If not, one can still define the sphere as a quotient of the space of non-zero vectors under the scalar action of $(0,\infty)$.

Homotopy theorists define $S^\infty$ to be the sphere in the (incomplete) normed vector space (traditionally with the $l^2$ norm) of infinite sequences almost all of whose values are $0$, which is the directed colimit of the $S^n$:

$S^{-1} \hookrightarrow S^0 \hookrightarrow S^1 \hookrightarrow S^2 \hookrightarrow \cdots S^\infty .$

In themselves, these provide nothing new to homotopy theory, as they are at least weakly contractible and usually contractible. However, they are a very useful source of big contractible spaces and so are often used as a starting point for making concrete models of classifying spaces.

If the vector space is a shift space, then contractibility is straightforward to prove.

###### Theorem

Let $V$ be a shift space of some order. Let $S V$ be its sphere (either via a norm or as the quotient of non-zero vectors). Then $S V$ is contractible.

###### Proof

Let $T \colon V \to V$ be a shift map. The idea is to homotop the sphere onto the image of $T$, and then down to a point.

It is simplest to start with the non-zero vectors, $V \setminus \{0\}$. As $T$ is injective, it restricts to a map from this space to itself which commutes with the scalar action of $(0,\infty)$. Define a homotopy $H \colon [0,1] \times (V \setminus \{0\}) \to V \setminus \{0\}$ by $H_t(v) = (1 - t)v + t T v$. It is clear that, assuming it is well-defined, it is a homotopy from the identity to $T$. To see that it is well-defined, we need to show that $H_t(v)$ is never zero. The only place where it could be zero would be on an eigenvector of $T$, but as $T$ is a shift map then it has none.

As $T$ is a shift map, it is not surjective and so we can pick some $v_0$ not in its image. Then we define a homotopy $G \colon [0,1] \times (V \setminus \{0\}) \to V \setminus \{0\}$ by $G_t(v) = (1 - t)T v + t v_0$. As $v_0$ is not in the image of $T$, this is well-defined on $V \setminus \{0\}$. Combining these two homotopies results in the desired contraction of $V \setminus \{0\}$.

If $V$ admits a suitable function defining a spherical subset (such as a norm) then we can modify the above to a contraction of the spherical subset simply by dividing out by this function. If not, as the homotopies above all commute with the scalar action of $(0,\infty)$, they descend to the definition of the sphere as the quotient of $V \setminus \{0\}$.

## Properties

### Coset space structure

###### Proposition

For $n \in \mathbb{N}$ the n-spheres are coset spaces of orthogonal groups

$S^n \;\simeq\; O(n+1)/O(n) \,.$

Similarly for the corresponding special orthogonal groups

$S^n \;\simeq\; SO(n+1)/SO(n)$

and spin groups

$S^n \;\simeq\; Spin(n+1)/Spin(n)$

and pin groups

$S^n \;\simeq\; Pin(n+1)/Pin(n) \,.$
###### Proof

Fix a unit vector in $\mathbb{R}^{n+1}$. Then its orbit under the defining $O(n+1)$-action on $\mathbb{R}^{n+1}$ is clearly the canonical embedding $S^n \hookrightarrow \mathbb{R}^{n+1}$. But precisely the subgroup of $O(n+1)$ that consists of rotations around the axis formed by that unit vector stabilizes it, and that subgroup is isomorphic to $O(n)$, hence $S^n \simeq O(n+1)/O(n)$.

Similarly, the analogous argument for unit spheres inside (the real vector spaces underlying) complex vector spaces, we have

###### Proposition

For $k \in \mathbb{N}$ the (2k+1)-sphere $S^{2k+1}$ is the coset space of special unitary groups:

$S^{2k+1} \;\simeq\; SU(k+1)/SU(k) \,.$

And still similarly, the analogous argument for unit spheres inside (the real vector spaces underlying) quaternionic vector spaces, we have

###### Proposition

For $k \in \mathbb{N}$, $k \geq 1$ the (4k-1)-sphere $S^{4k-1}$ is the coset space of quaternionic unitary groups:

$S^{4k-1} \;\simeq\; Sp(k)/Sp(k-1) \,.$

Generally:

###### Proposition

The connected Lie groups with effective transitive actions on n-spheres are precisely (up to isomorphism) the following:

with coset spaces

\begin{aligned} SO(n)/SO(n-1) & \simeq S^{n-1} \\ U(n)/U(n-1) & \simeq S^{2n-1} \\ SU(n)/SU(n-1) & \simeq S^{2n-1} \\ Sp(n)/Sp(n-1) & \simeq S^{4n-1} \\ Sp(n)\cdot SO(2)/Sp(n-1)\cdot SO(2) & \simeq S^{4n-1} \\ Sp(n)\cdot Sp(1)/Sp(n-1)\cdot Sp(1) & \simeq S^{4n-1} \\ G_2/SU(3) & \simeq S^6 \\ Spin(7)/G_2 & \simeq S^7 \\ Spin(9)/Spin(7) & \simeq S^{15} \end{aligned}

This goes back to Montogomery-Samelson 43, see
Gray-Green 70, p. 1-2

(also e.g Borel-Serre 53, 17.1)

###### Remark

The isomorphisms in Prop. and Prop. above hold in the category of topological spaces (homeomorphisms), but in fact also in the category of smooth manifolds (diffeomorphisms) and even in the category of Riemannian manifolds (isometries).

But the other coset space realizations of some n-spheres in Prop. are homeomorphisms, but not necessarily isometries (“squashed spheres”). There is also a double coset space realization which is not even a diffeomorphisms (“exotic sphere”, the Gromoll-Meyer sphere).

For more see 7-sphere – Coset space realization.

coset space-structures on n-spheres:

standard:
$S^{n-1} \simeq_{diff} SO(n)/SO(n-1)$this Prop.
$S^{2n-1} \simeq_{diff} SU(n)/SU(n-1)$this Prop.
$S^{4n-1} \simeq_{diff} Sp(n)/Sp(n-1)$this Prop.
exceptional:
$S^7 \simeq_{diff} Spin(7)/G_2$Spin(7)/G2 is the 7-sphere
$S^7 \simeq_{diff} Spin(6)/SU(3)$since Spin(6) $\simeq$ SU(4)
$S^7 \simeq_{diff} Spin(5)/SU(2)$since Sp(2) is Spin(5) and Sp(1) is SU(2), see Spin(5)/SU(2) is the 7-sphere
$S^6 \simeq_{diff} G_2/SU(3)$G2/SU(3) is the 6-sphere
$S^15 \simeq_{diff} Spin(9)/Spin(7)$Spin(9)/Spin(7) is the 15-sphere (from FSS 19, 3.4)

### Spin structure

###### Example

The $n$-sphere, for each $n \in \mathbb{N}$, carries a canonical spin structure, induced from its coset space-realization $S^n \simeq Spin(n+1)/Spin(n)$ (above), as a special case of the canonical $H$-structure on $G/H$ (this example).

Other ways to see this:

• Nikolai Nowaczyk, Theorem A.6.6 in: Dirac Eigenvalues of higher Multiplicity, Regensburg 2015 (arXiv:1501.04045)

• S. Gutt, Killing spinors on spheres and projective spaces, p. 238-248 in: A. Trautman, G. Furlan (eds.) Spinors in Geometry and Physics – Trieste 11-13 September 1986, World Scientific 1988 (doi:10.1142/9789814541510, GBooks, p. 243)

### Branched covers

Every $n$-dimensional PL manifold admits a branched covering of the n-sphere (Alexander 20).

By the Riemann existence theorem, every connected compact Riemann surface admits the structure of a branched cover by a holomorphic function to the Riemann sphere. See there at branched cover of the Riemann sphere. graphics grabbed from Chamseddine-Connes-Mukhanov 14, Figure 1, Connes 17, Figure 11

For 3-manifolds branched covering the 3-sphere see (Montesinos 74).

All PL 4-manifolds are simple branched covers of the 4-sphere (Piergallini 95, Iori-Piergallini 02).

But the n-torus for $n \geq 3$ is not a cyclic branched over of the n-sphere (Hirsch-Neumann 75)

### Iterated loop spaces

###### Proposition

(rational cohomology of iterated loop space of the 2k-sphere)

Let

$1 \leq D \lt n = 2k \in \mathbb{N}$

(hence two positive natural numbers, one of them required to be even and the other required to be smaller than the first) and consider the D-fold loop space $\Omega^D S^n$ of the n-sphere.

Its rational cohomology ring is the free graded-commutative algebra over $\mathbb{Q}$ on one generator $e_{n-D}$ of degree $n - D$ and one generator $a_{2n - D - 1}$ of degree $2n - D - 1$:

$H^\bullet \big( \Omega^D S^n , \mathbb{Q} \big) \;\simeq\; \mathbb{Q}\big[ e_{n - D}, a_{2n - D - 1} \big] \,.$

## Examples

### Formalization

Axiomatization of the homotopy type of the 1-sphere (the circle) and the 2-sphere, as higher inductive types, is in

Visualization of the idea of the construction for the 2-sphere is in

### Group actions on spheres

Discussion of free group actions on spheres by finite groups includes

The subgroups of SO(8) which act freely on $S^7$ have been classified in

• J. A. Wolf, Spaces of constant curvature, Publish or Perish, Boston, Third ed., 1974

and lifted to actions of Spin(8) in

• Sunil Gadhia, Supersymmetric quotients of M-theory and supergravity backgrounds, PhD thesis, School of Mathematics, University of Edinburgh, 2007 (spire:1393845)

Discussion of transitive actions on $n$-spheres by compact Lie groups:

• Deane Montgomery, Hans Samelson, Transformation Groups of Spheres, Annals of Mathematics Second Series, Vol. 44, No. 3 (Jul., 1943), pp. 454-470 (jstor:1968975)

• Alfred Gray, Paul S. Green, Sphere transitive structures and the triality automorphism, Pacific J. Math. Volume 34, Number 1 (1970), 83-96 (euclid:1102976640)

Further discussion of these actions is in

where they are related to the black M2-brane BPS-solutions of 11-dimensional supergravity at ADE-singularities.

### Geometric structures on spheres

Coset space structures on spheres:

The following to be handled with care:

### Embeddings of spheres

The (isotopy class of an) embedding of a circle (1-sphere) into the 3-sphere is a knot. Discussion of embeddings of spheres of more general dimensions into each other:

• André Haefliger, Differentiable Embeddings of $S^n$ in $S^{n+q}$ for $q \gt 2$, Annals of Mathematics Second Series, Vol. 83, No. 3 (May, 1966), pp. 402-436 (jstor:1970475)