Exponential map

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There are two different (but closely related) notions of an exponential map in differential geometry, both of which generalize the ordinary exponential function of mathematical analysis.

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[edit] Lie theory

The exponential map is a fundamental construction in the theory of Lie groups. It is a map from the Lie algebra of a Lie group to the group which allows one to completely recapture the local group structure from the Lie algebra. The existence of the exponential map is one of the primary justifications for the study of Lie groups at the level of Lie algebras.

The ordinary exponential function of mathematical analysis may be viewed as a special case of the exponential map when G is the multiplicative group of positive real numbers (whose Lie algebra is the additive group of all real numbers). The Lie-theoretic exponential map satisfies many properties analogous to those the ordinary exponential function, however, it also differs in many important respects.

[edit] Definition

Let G be a Lie group and \mathfrak g be its Lie algebra (thought of as the tangent space to the identity element of G). The exponential map is a map

\exp\colon \mathfrak g \to G

given by exp(X) = γ(1) where

\gamma\colon \mathbb R \to G

is the unique one-parameter subgroup of G whose tangent vector at the identity is equal to X. It follows easily from the chain rule that exp(tX) = γ(t). The map γ may be constructed as the integral curve of either the right- or left-invariant vector field associated with X. That the integral curve exists for all real parameters follows by right- or left-translating the solution near zero.

If G is a matrix Lie group, then the exponential map coincides with the matrix exponential and is given by the ordinary series expansion:

\exp X = \sum_{k=0}^\infty\frac{X^k}{k!} = I + X + \frac{1}{2}X^2 + \frac{1}{6}X^3 + \cdots.

[edit] Properties

  • For all X\in\mathfrak g, the map γ(t) = exp(tX) is the unique one-parameter subgroup of G whose tangent vector at the identity is X. It follows that:
    • \exp(t+s)X = (\exp tX)(\exp sX)\,
    • \exp(-X) = (\exp X)^{-1}\,
  • The exponential map \exp\colon \mathfrak g \to G is a smooth map. Its derivative at the identity, \exp_{*}\colon \mathfrak g \to \mathfrak g, is the identity map (with the usual identifications). The exponential map, therefore, restricts to a diffeomorphism from some neighborhood of 0 in \mathfrak g to a neighborhood of 1 in G.
  • The image of the exponential map always lies in the identity component of G. When G is compact, the exponential map is surjective onto the identity component.
  • The map γ(t) = exp(tX) is the integral curve through the identity of both the right- and left-invariant vector fields associated to X.
  • The integral curve through g\in G of the left-invariant vector field XL associated to X is given by gexp(tX). Likewise, the integral curve through g of the right-invariant vector field XR is given by exp(tX)g. It follows that the flows ξL,R generated by the vector fields XL,R are given by:
    • \xi^L_t = R_{\exp tX}
    • \xi^R_t = L_{\exp tX}.
    Since these flows are globally defined, every left- and right-invariant vector field on G is complete.
  • Let \phi\colon G \to H be a Lie group homomorphism and let φ * be its derivative at the identity. Then the following diagram commutes:
  • In particular, when applied to the adjoint action of a group G we have
    • g(\exp X)g^{-1} = \exp(\mathrm{Ad}_gX)\,
    • \mathrm{Ad}_{\exp X} = \exp(\mathrm{ad}_X)\,

[edit] Riemannian geometry

In Riemannian geometry, an exponential map is a map from a subset of a tangent space TpM of a Riemannian manifold M to M itself.

[edit] Definition

For v ∈ TpM, there is a unique geodesic γv satisfying γv(0) = p such that the tangent vector γ′v(0) = v. Then the corresponding exponential map is defined by expp(v) = γv(1). In general, the exponential map really is only locally defined, that is, it only takes a small neighborhood of the origin at TpM, to a neighborhood of p in the manifold (this is simply due to the fact that it relies on the theorem on existence and uniqueness of ODEs which is local in nature).

[edit] Properties

Intuitively speaking, the exponential map takes a given tangent vector to the manifold, runs along the geodesic starting at that point and going in that direction, for a unit time. Since v corresponds to the velocity vector of the geodesic, the actual (Riemannian) distance traveled will be dependent on that. We can also reparametrize geodesics to be unit speed, so equivalently we can define expp(v) = β(|v|) where β is the unit-speed geodesic (geodesic parameterized by arc length) going in the direction of v. As we vary the tangent vector v we will get, when applying expp, different points on M which are within some distance from the base point p—this is perhaps one of the most concrete ways of demonstrating that the tangent space to a manifold is a kind of "linearization" of the manifold.

The Hopf-Rinow theorem asserts that it is possible to define the exponential map on the whole tangent space if and only if the manifold is complete as a metric space (which justifies the usual term geodesically complete for a manifold having an exponential map with this property). In particular, compact manifolds are geodesically complete. However even if expp is defined on the whole tangent space, it will in general not be a global diffeomorphism. However, its differential at the origin of the tangent space is the identity map and so, by the inverse function theorem we can find a neighborhood of the origin of TpM on which the exponential map is an embedding (i.e. the exponential map is a local diffeomorphism). The radius of the largest ball about the origin in TpM that can be mapped diffeomorphically via expp is called the injectivity radius of M at p.

An important property of the exponential map is the following lemma of Gauss (yet another Gauss's lemma): given any tangent vector v in the domain of definition of expp, and another vector w based at the tip of v (hence w is actually in the double-tangent space Tv(TpM)) and orthogonal to v, remains orthogonal to v when pushed forward via the exponential map. This means, in particular, that the boundary sphere of a small ball about the origin in TpM is orthogonal to the geodesics in M determined by those vectors (i.e. the geodesics are radial). This motivates the definition of geodesic normal coordinates on a Riemannian manifold.

The exponential map is also useful in relating the abstract definition of curvature to the more concrete realization of it originally conceived by Riemann himself—the sectional curvature is intuitively defined as the Gaussian curvature of some surface (i.e. a slicing of the manifold by a 2-dimensional submanifold) through the point p in consideration. Via the exponential map, it now can be precisely defined as the Gaussian curvature of a surface through p determined by the image under expp of a 2-dimensional subspace of TpM.

[edit] Relationships

The two notions of the exponential map coincide in the case of Lie groups equipped with bi-invariant metrics (i.e. Riemannian metrics invariant under left and right translation). In this case the geodesics through the identity are precisely the one-parameter subgroups of G.

Take the example that gives the "honest" exponential map. Consider the positive real numbers R+, a Lie group under the usual multiplication. Then each tangent space is just R. On each copy of R at the point y, we introduce the modified inner product

<u,v>y = uv/y2

(multiplying them as usual real numbers but scaling by y2). (This is what makes the metric left-invariant, for left multiplication by a factor will just pull out of the inner product, twice-canceling the square in the denominator).

Consider the point 1 ∈ R+, and x ∈ R an element of the tangent space at 1. The usual straight line emanating from 1, namely y(t) = 1 + xt covers the same path as a geodesic, of course, except we have to reparametrize so as to get a curve with constant speed ("constant speed", remember, is not going to be the ordinary constant speed, because we're using this funny metric). To do this we reparametrize by arc length (the integral of the length of the tangent vector in the norm |.|y induced by the modified metric):

s(t) = \int_0^t |x|_{y(\tau)} d\tau = \int_0^t \frac{|x|}{1 + \tau x} d\tau = |x| \int_0^t \frac{d\tau}{1 + \tau x} = \frac{|x|}{x} \ln|1 + tx|

and after inverting the function to obtain t as a function of s, we substitute and get

y(s) = esx/|x|.

Now using the unit speed definition, we have

exp1(x) = y(|x|1) = y(|x|),

giving the expected ex.

The Riemannian distance defined by this is simply

dist(a,b) = |ln(b/a)|,

a metric which should be familiar to anyone who has drawn graphs on log paper.

[edit] See also

[edit] References

  • Manfredo P. do Carmo, Riemannian Geometry, Birkhäuser (1992). ISBN 0-8176-3490-8. See Chapter 3.
  • Jeff Cheeger and David G. Ebin, Comparison Theorems in Riemannian Geometry, Elsevier (1975). See Chapter 1, Sections 2 and 3.
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