Given a group $G$ and a subgroup $H$, then their coset object is the quotient $G/H$, hence the set of equivalence classes of elements of $G$ where two are regarded as equivalent if they differ by right multiplication with an element in $H$.
If $G$ is a topological group, then the quotient is a topological space and usually called the coset space. This is in particular a homogeneous space, see there for more.
In a category $C$, for $G$ a group object and $H \hookrightarrow G$ a subgroup object, the left/right object of cosets is the object of orbits of $G$ under left/right multiplication by $H$.
Explicitly, the left coset space $G/H$ coequalizes the parallel morphisms
where $\mu$ is (the inclusion $H\times G \hookrightarrow G\times G$ composed with) the group multiplication.
Simiarly, the right coset space $H\backslash G$ coequalizes the parallel morphisms
Specializing the above definition to the case where $C$ is the well-pointed topos $Set$, given an element $g$ of $G$, its orbit $g H$ is an element of $G/H$ and is called a left coset.
Using comprehension, we can write
Similarly there is a coset on the right $H \backslash G$.
If $H \hookrightarrow G$ is an inclusion of Lie groups then the quotient $G/H$ is also called a Klein geometry.
More generally, given an (∞,1)-topos $\mathbf{H}$ and a homomorphism of ∞-group ojects $H \to G$, hence equivalently a morphism of their deloopings $\mathbf{B}H \to \mathbf{B}G$, then the homotopy quotient $G/H$ is given by the homotopy fiber of this map
See at ∞-action for more on this definition. See at higher Klein geometry and higher Cartan geometry for the corresponding concepts of higher geometry.
The coset inherits the structure of a group if $H$ is a normal subgroup.
Unless $G$ is abelian, considering both left and right coset spaces provide different information.
For $X$ a smooth manifold and $G$ a compact Lie group equipped with a free smooth action on $X$, then the quotient projection
is a $G$-principal bundle (hence in particular a Serre fibration).
This is originally due to (Gleason 50). See e.g. (Cohen, theorem 1.3)
For $G$ a Lie group and $H \subset G$ a compact subgroup, then the coset quotient projection
is an $H$-principal bundle (hence in particular a Serre fibration).
This is originally due to (Samelson 41).
For $G$ a compact Lie group and $K \subset H \subset G$ closed subgroups, then the projection map
is a locally trivial $H/K$-fiber bundle (hence in particular a Serre fibration).
Observe that the projection map in question is equivalently
(where on the left we form the Cartesian product and then divide out the diagonal action by $H$). This exhibits it as the $H/K$-fiber bundle associated to the $H$-principal bundle of corollary .
(coset space coprojections with local sections)
Let $G$ be a topological group and $H \subset G$ a subgroup.
Sufficient conditions for the coset space coprojection $G \overset{q}{\to} G/H$ to admit local sections, in that there is an open cover $\underset{i \in I}{\sqcup}U_i \to G/H$ and a continuous section $\sigma_{\mathcal{U}}$ of the pullback of $q$ to the cover,
include the following:
$G$ is any topological group
and $H$ is a compact Lie group
(in particular for $H$ a closed subgroup if $G$ itself is a compact Lie group, since closed subspaces of compact Hausdorff spaces are equivalently compact subspaces).
or:
The underlying topological space of $G$ is
(e.g. if $G$ is a Lie group)
and $H \subset G$ is a closed subgroup.
In geometric homotopy theory (in an (∞,1)-topos), for $H \longrightarrow G$ any homomorphisms of ∞-group objects, then the natural projection $G \longrightarrow G/H$, generally realizes $G$ as an $H$-principal ∞-bundle over $G/H$. This is exhibited by a homotopy pullback of the form
where $\mathbf{B}H$ is the delooping groupoid of $H$. This also equivalently exhibits the ∞-action of $H$ on $G$ (see there for more).
By the pasting law for homotopy pullbacks then we get the homotopy pullback
which exhibits the coset as the homotopy fiber of $\mathbf{B}H \to \mathbf{B}G$.
The n-spheres are coset spaces of orthogonal groups:
The odd-dimensional spheres are also coset spaces of unitary groups:
Regarding the first statement:
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 $S^n \simeq O(n+1)/O(n)$.
The second statement follows by the same kind of reasoning:
Clearly $U(n+1)$ acts transitively on the unit sphere $S^{2n+1}$ in $\mathbb{C}^{n+1}$. It remains to see that its stabilizer subgroup of any point on this sphere is $U(n)$. If we take the point with coordinates $(1,0, 0, \cdots,0)$ and regard elements of $U(n+1)$ as matrices, then the stabilizer subgroup consists of matrices of the block diagonal form
where $A \in U(n)$.
There are also various exceptional realizations of spheres as coset spaces. For instance:
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 |
see also Spin(8)-subgroups and reductions
homotopy fibers of homotopy pullbacks of classifying spaces:
(from FSS 19, 3.4)
Consider $K \hookrightarrow H \hookrightarrow G$ two consecutive group inclusions with their induced coset quotient projections
When $G/K \to G/H$ is a Serre fibration, for instance in the situation of prop. (so that this is indeed a homotopy fiber sequence with respect to the classical model structure on topological spaces) then it induces the corresponding long exact sequence of homotopy groups
Consider a sequence of inclusions of orthogonal groups of the form
Then by example we have that $O(n+1)/O(n) \simeq S^n$ is the n-sphere and by corollary the quotient map is a Serre fibration. Hence there is a long exact sequence of homotopy groups of the form
Now for $q \lt n$ then $\pi_q(S^n) = 0$ and hence in this range we have isomorphisms
Hans Samelson, Beiträge zur Topologie der Gruppenmannigfaltigkeiten, Ann. of Math. 2, 42, (1941), 1091 - 1137. (jstor:1970463, doi:10.2307/1970463)
Andrew Gleason, Spaces with a compact Lie group of transformations, Proc. of A.M.S 1, (1950), 35 - 43 (jstor:2032430, doi:10.2307/2032430)
Norman Steenrod, Section I.7 of: The topology of fibre bundles, Princeton Mathematical Series 14, Princeton Univ. Press, 1951.
Paul Mostert, Local Cross Sections in Locally Compact Groups, Proceedings of the American Mathematical Society, Vol. 4, No. 4 (Aug., 1953), pp.645-649 (jstor:2032540, doi:10.2307/2032540)
Glen Bredon, Section I.4 of: Introduction to compact transformation groups, Academic Press 1972 (ISBN 9780080873596, pdf)
R. Cohen, Topology of fiber bundles, Lecture notes (pdf)
Last revised on April 8, 2021 at 01:57:30. See the history of this page for a list of all contributions to it.