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TheoremStatement: AI-adaptedProof: AI-adaptedPipeline-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-09-14
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The Lie-group exponential map is smooth with identity differential at zero

Statement

Assume ACω. For a finite-dimensional real Lie group G, the exponential map

exp⁡G:g=TeG⟶G

is smooth, satisfies exp⁡G(0)=e, and, under the canonical identification T0g≃g, has differential

d(exp⁡G)0=id⁡g.

The countable-choice assumption is used exactly through the supplied smooth tangent-bundle trivialization and one-parameter-subgroup results.

Facts & Assumptions

Given: ACω and a finite-dimensional real Lie group G with identity e and Lie algebra g=TeG.

[F1]

ACω is countable choice. The Axiom of Countable Choice (ACω).

[F2]

The exponential map is the total map exp⁡G(Y)=γY(1). Exponential map of a Lie group.

[F3]

Assuming ACω, the map (g,Y)↦d(Lg)eY is a smooth vector-bundle trivialization G×g→TG. Translations are diffeomorphisms and their differentials trivialize the tangent bundle.

[F4]

Assuming ACω, every Y∈g determines a unique global one-parameter subgroup γY, and γY′(t)=d(LγY(t))eY. One-parameter subgroups are integral curves of left-invariant fields.

[F5]

The maximal flow of a smooth vector field has open domain, is smooth on that domain, and is uniquely determined by its maximal integral curves. The fundamental theorem on flows.

[F6]

The scaling identity is γY(a)=exp⁡G(aY) for all a∈R. Exponential scales one-parameter subgroups.

[F7]

Every one-parameter subgroup satisfies γ(0)=e. One-parameter subgroup of a Lie group.

[F8]

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

Proof

technique · direct
1.1F3construct

On the product manifold M=G×g, define X(g,Y):=(d(Lg)eY,0)∈TgG×TYg=T(g,Y)M. Its first component is the smooth map supplied by [F3], and its second component is the zero field on the vector space g. Hence X is a smooth vector field on M.

2.1F3F4F5F8step 1.1algebra

For (g,Y)∈M, define cg,Y:R→M by cg,Y(t)=(gγY(t),Y). By [F4], [F8], and Lg∘LγY(t)=LgγY(t), cg,Y′(t)=(d(Lg)γY(t)d(LγY(t))eY,0)=(d(LgγY(t))eY,0)=Xcg,Y(t). Thus cg,Y is an integral curve of X through (g,Y) and is defined on all of R. Every maximal integral curve of X is therefore global. By [F5], the global flow is smooth and is Ψ(t,(g,Y))=(gγY(t),Y).

3.1F2F5step 2.1

Restricting this smooth flow to (t,(e,Y)) and projecting to G shows that (t,Y)↦γY(t) is smooth. Restricting further to t=1 gives the map Y↦γY(1)=exp⁡G(Y), which is smooth by [F2].

4.1F4F6F7F8step 3.1algebra

By [F6] with a=0 and [F7], exp⁡G(0)=γX(0)=e. Fix X∈g and consider qX(a)=aX. Applying [F8] to exp⁡G∘qX and then [F6] gives d(exp⁡G)0(X)=dda∣a=0exp⁡G(aX)=dda∣a=0γX(a)=γX′(0)=X. Hence d(exp⁡G)0 is the identity under T0g≃g.

5.1F1F2F3F4F5F6F7F8step 1.1step 2.1step 3.1step 4.1∎

Lie groups are nonempty and boundaryless. If dim⁡G=0, then g=0, the exponential maps the unique vector to e, and its differential is the identity of the zero space; in dimension one the proof is unchanged. The global curves in step 2.1 remove finite-time endpoint issues. No metric or nondegeneracy condition occurs. The only choice use is the stated ACω, inherited through [F2]--[F4] and especially the smooth tangent-bundle trivialization [F3]; forming one product field and restricting its flow adds no choice. No biconditional is asserted.

Depends on

Dependency tree · 2 levels

Sources