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PropositionStatement: AI-adaptedProof: AI-adaptedPipeline-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-09-14
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Adjoint exponential identity

Statement

Assume ACω. Let G be a finite-dimensional real Lie group with Lie algebra g. For every X∈g,

Ad⁡exp⁡GX=ead⁡X.

Here, for B∈End⁡(g), the linear-ODE exponential etB denotes the unique solution EB(t) of

EB′(t)=B∘EB(t),EB(0)=I,

and eB=EB(1). The countable-choice assumption is inherited exactly from the supplied exponential and dAd⁡=ad⁡ results.

Facts & Assumptions

Given: ACω, a finite-dimensional real Lie group G with Lie algebra g, and X∈g.

[F1]

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

[F2]

The adjoint map is a smooth representation, so Ad⁡gh=Ad⁡gAd⁡h and Ad⁡e=I. Adjoint is a smooth Lie-group representation.

[F3]

The unique one-parameter subgroup with initial velocity X is the curve t↦exp⁡G(tX). One-parameter subgroups are exactly exponentials. Exponential scales one-parameter subgroups.

[F4]

Under TIGL⁡(g)=End⁡(g), d(Ad⁡)eX=ad⁡X. The differential of Ad is ad.

[F5]

A linear matrix initial-value problem on a compact interval has a unique solution on the whole interval. Linear matrix ODEs have unique global solutions on a fixed interval.

Proof

technique · direct
1.1F2F3F4

Put A(t)=Ad⁡exp⁡G(tX). By [F2]--[F3], A is smooth, A(0)=I, and A(t+s)=A(t)A(s)=A(s)A(t). The chain rule and [F4] give A′(0)=d(Ad⁡)eX=ad⁡X.

2.1step 1.1algebra

Fix t and differentiate A(t+s)=A(s)A(t) with respect to s at zero. Using step 1.1 gives A′(t)=ad⁡X∘A(t) and A(0)=I.

3.1F5step 2.1

In one fixed basis of g, [F5] gives a unique solution of E′(t)=ad⁡XE(t), E(0)=I, on every compact interval containing zero. Solutions on overlapping intervals agree by uniqueness, so they define the global linear-ODE exponential E(t)=etad⁡X without any arbitrary selection. Step 2.1 and uniqueness give A(t)=E(t) on every such interval. Evaluating at t=1 yields Ad⁡exp⁡GX=ead⁡X.

4.1F1F2F3F4F5step 1.1step 2.1step 3.1∎

A Lie group is nonempty and boundaryless. If dim⁡G=0, both sides are the unique endomorphism of the zero space; in dimension one, ad⁡X=0 and the ODE gives both sides equal to I. Degenerate adjoint endomorphisms are allowed. The compact-interval solutions glue globally, so there is no endpoint issue, and no metric occurs. The only choice use is the stated ACω inherited through [F3]--[F4]; one finite basis and uniquely determined ODE solutions add no choice. No biconditional is asserted.

Depends on

Dependency tree · 2 levels

Sources