# Klein geometry

In mathematics, a Klein geometry is a type of geometry motivated by Felix Klein in his influential Erlangen program. More specifically, it is a homogeneous space X together with a transitive action on X by a Lie group G, which acts as the symmetry group of the geometry.

For background and motivation see the article on the Erlangen program.

## Formal definition

A Klein geometry is a pair (G, H) where G is a Lie group and H is a closed Lie subgroup of G such that the (left) coset space G/H is connected. The group G is called the principal group of the geometry and G/H is called the space of the geometry (or, by an abuse of terminology, simply the Klein geometry). The space X = G/H of a Klein geometry is a smooth manifold of dimension

dim X = dim G − dim H.

There is a natural smooth left action of G on X given by

$g.(aH) = (ga)H.$

Clearly, this action is transitive (take a = 1), so that one may then regard X as a homogeneous space for the action of G. The stabilizer of the identity coset HX is precisely the group H.

Given any connected smooth manifold X and a smooth transitive action by a Lie group G on X, we can construct an associated Klein geometry (G, H) by fixing a basepoint x0 in X and letting H be the stabilizer subgroup of x0 in G. The group H is necessarily a closed subgroup of G and X is naturally diffeomorphic to G/H.

Two Klein geometries (G1, H1) and (G2, H2) are geometrically isomorphic if there is a Lie group isomorphism φ : G1G2 so that φ(H1) = H2. In particular, if φ is conjugation by an element gG, we see that (G, H) and (G, gHg−1) are isomorphic. The Klein geometry associated to a homogeneous space X is then unique up to isomorphism (i.e. it is independent of the chosen basepoint x0).

## Bundle description

Given a Lie group G and closed subgroup H, there is natural right action of H on G given by right multiplication. This action is both free and proper. The orbits are simply the left cosets of H in G. One concludes that G has the structure of a smooth principal H-bundle over the left coset space G/H:

$H\to G\to G/H .$

## Types of Klein geometries

### Effective geometries

The action of G on X = G/H need not be effective. The kernel of a Klein geometry is defined to be the kernel of the action of G on X. It is given by

$K = \{k \in G : g^{-1}kg \in H\;\;\forall g \in G\}.$

The kernel K may also be described as the core of H in G (i.e. the largest subgroup of H that is normal in G). It is the group generated by all the normal subgroups of G that lie in H.

A Klein geometry is said to be effective if K = 1 and locally effective if K is discrete. If (G, H) is a Klein geometry with kernel K, then (G/K, H/K) is an effective Klein geometry canonically associated to (G, H).

### Geometrically oriented geometries

A Klein geometry (G, H) is geometrically oriented if G is connected. (This does not imply that G/H is an oriented manifold). If H is connected it follows that G is also connected (this is because G/H is assumed to be connected, and GG/H is a fibration).

Given any Klein geometry (G, H), there is a geometrically oriented geometry canonically associated to (G, H) with the same base space G/H. This is the geometry (G0, G0H) where G0 is the identity component of G. Note that G = G0 H.

### Reductive geometries

A Klein geometry (G, H) is said to be reductive and G/H a reductive homogeneous space if the Lie algebra $\mathfrak h$ of H has an H-invariant complement in $\mathfrak g$.

## Examples

In the following table, there is a description of the classical geometries, modeled as Klein geometries.

Projective geometry Conformal geometry on the sphere Hyperbolic geometry Elliptic geometry Spherical geometry Underlying space Transformation group G Subgroup H Invariants Real projective space $\mathbb{R}\mathrm{P}^n$ Projective group $\mathrm{PGL}(n+1)$ A subgroup $P$ fixing a flag $\{0\}\subset V_1\subset V_n$ Projective lines, cross-ratio Sphere $S^n$ Lorentz group of an $n+2$ dimensional space $\mathrm{O}(n+1,1)$ A subgroup $P$ fixing a line in the null cone of the Minkowski metric Generalized circles, angles Hyperbolic space $H(n)$, modelled e.g. as time-like lines in the Minkowski space $\R^{1,n}$ Orthochronous Lorentz group $\mathrm{O}(1,n)/\mathrm{O}(1)$ $\mathrm{O}(1)\times \mathrm{O}(n)$ Lines, circles, distances, angles Elliptic space, modelled e.g. as the lines through the origin in Euclidean space $\mathbb{R}^{n+1}$ $\mathrm{O}(n+1)/\mathrm{O}(1)$ Lines, circles, distances, angles Sphere $S^n$ Orthogonal group $\mathrm{O}(n+1)$ Orthogonal group $\mathrm{O}(n)$ Lines (great circles), circles, distances of points, angles Affine space $A(n)\simeq\R^n$ Affine group $\mathrm{Aff}(n)\simeq \R^n \rtimes \mathrm{GL}(n)$ General linear group $\mathrm{GL}(n)$ Lines, quotient of surface areas of geometric shapes, center of mass of triangles Euclidean space $E(n)$ Euclidean group $\mathrm{Euc}(n)\simeq \R^n \rtimes \mathrm{O}(n)$ Orthogonal group $\mathrm{O}(n)$ Distances of points, angles of vectors, areas

## References

• R. W. Sharpe (1997). Differential Geometry: Cartan's Generalization of Klein's Erlangen Program. Springer-Verlag. ISBN 0-387-94732-9.