$G ⊂ GL_n (\Bbb R)$ is an abelian connected Lie group, $\mathfrak{g}$ its Lie algebra and $exp_G : \mathfrak{g} → G$ the exponential map.

Prove that $Ker(exp_G )$ is discrete.

My attempt:

$Lie(Ker(exp_G))=Ker(d_0\ exp_G)$ since $exp_G$ is a morphism of topological groups $(\mathfrak{g},+) → (G,.)$, where $d_0$ is the differential at $0 \in \mathfrak{g}$.

Since $d_0\ exp_G$ is the map $X\to X$, its kernel is $\{0\}$, which gives $Lie(Ker(exp_G))=\{0\}$, therefore $Ker(exp_G)$ is discrete as $exp_G$ is a local homeomorphism at $0\in \mathfrak{g}$

Is this a sound proof?

Thank you for your help.


As the prove above isn't satisfactory, I am suggesting a new one:

$Ker\ exp_G = \{γ ∈ g : exp_G γ = e_G \}$

$exp_G$ being a local injection (by restriction of the exponential from $M_n(\Bbb R)$), we see that $0_{\mathfrak{g}}$ is isolated.

Translation being a homeomorphism of $Ker\ exp_G$, every element is isolated and $Ker\ exp_G$ is discrete.


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