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Right-trivialized differential of the Lie-group exponential

Statement

Assume ACω. Let G be a finite-dimensional real Lie group with Lie algebra g. For Z,V∈g, right translation identifies the differential of the exponential with

d(Rexp⁡G(−Z))exp⁡GZ(d(exp⁡G)ZV)=∑n=0∞ad⁡Zn(V)(n+1)!.

The operator series converges absolutely in any norm and is denoted

ead⁡Z−Iad⁡Z(V),

with the displayed power series—not division by a possibly singular ad⁡Z—as its definition.

More generally, for every finite-dimensional real vector space and every endomorphism B, the linear-ODE exponential used here satisfies

etB=∑n=0∞tnBnn!,

with absolute convergence uniformly on compact t-intervals.

Facts & Assumptions

Given: ACω, a finite-dimensional real Lie group G, and Z,V∈g.

[F1]

Lie-group exponentials are smooth, and t↦exp⁡G(tW) is the integral curve of WL through the identity. The Lie-group exponential map is smooth with identity differential at zero. One-parameter subgroups are integral curves of left-invariant fields.

[F2]

Left and right translations and their differentials give the standard tangent trivializations. Left and right translations on a Lie group.

[F3]

Assuming countable choice, Ad⁡exp⁡G(tZ)=etad⁡Z, where the right side is the linear-ODE exponential. The Axiom of Countable Choice (ACω). Adjoint exponential identity.

[F4]

Linear matrix initial-value problems have unique solutions on compact intervals. Linear matrix ODEs have unique global solutions on a fixed interval.

[F7]

Differentials of smooth maps obey the chain rule. The chain rule for differentials of smooth maps.

Proof

technique · direct
1.1F4F5

Put B=ad⁡Z. In one basis, the operator series S(t)=∑n≥0tnBn/n! converges absolutely and uniformly on compact t-intervals, since its norm is bounded by the scalar exponential majorant ∑n≥0(∣t∣∥B∥)n/n!. Coordinatewise termwise differentiation gives S′(t)=BS(t) and S(0)=I. By [F4]--[F5], uniqueness therefore identifies S(t) with the linear-ODE exponential etB.

1.2F1F2F7algebra

Consider the smooth variation F(s,t)=exp⁡G(t(Z+sV)) and write γ(t)=F(0,t)=exp⁡G(tZ). Right-trivialize its variational field by ξ(t)=d(Rγ(t)−1)γ(t)(∂sF(0,t)). By [F1], ∂tF(s,t)=d(LF(s,t))e(Z+sV). Differentiate this identity in s, commute the two coordinate partial derivatives, and differentiate the right-trivialization using multiplication and inversion. The two terms containing Z cancel, leaving ξ′(t)=Ad⁡γ(t)V and ξ(0)=0.

2.1F3step 1.1

Consequently [F3] gives Ad⁡exp⁡G(tZ)=S(t) for every real t. This is the exact point where ACω is used.

3.1F5F6step 2.1step 1.2

By steps 2.1 and 1.2, ξ′(t)=S(t)V. The uniformly convergent series A(t)=∑n≥0tn+1Bn(V)/(n+1)! has A(0)=0 and, coordinatewise by [F5], A′(t)=S(t)V. Applying [F6] to ξ−A on [0,1] gives ξ(1)=A(1)=∑n≥0Bn(V)/(n+1)!.

4.1F2F7step 3.1

By the definition of F and the chain rule, ∂sF(0,1)=d(exp⁡G)ZV; by the definition of ξ, its value at one is the right translation of that vector by exp⁡G(−Z). Step 3.1 is therefore exactly the asserted formula.

5.1F1F2F3F4F5F6F7step 1.1step 2.1step 1.2step 3.1step 4.1∎

A Lie group is nonempty and boundaryless. In dimension zero both sides are the unique zero vector; in dimension one the bracket vanishes and the formula reduces to dRexp⁡(−Z)dexp⁡Z(V)=V. Singular ad⁡Z is allowed because the quotient notation means its entire power series. The variation uses the compact interval [0,1] including both endpoints. Countable choice is inherited through [F1] and [F3]; one basis and fixed vectors add no choice. No metric dependence or biconditional is asserted.

Depends on

Dependency tree · 2 levels

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