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Spectral invariants for finite dimensional Lie algebras

For a Lie algebra ${\mathcal L}$ with basis $\{x_1,x_2,\cdots,x_n\}$, its associated characteristic polynomial $Q_{\mathcal L}(z)$ is the determinant of the linear pencil $z_0I+z_1\text{ad} x_1+\cdots +z_n\text{ad} x_n.$ This paper shows that $Q_{\mathcal L}$ is invariant under the automorphism group $\text{Aut}({\mathcal L}).$ The zero variety and factorization of $Q_{\mathcal L}$ reflect the structure of ${\mathcal L}$. In the case ${\mathcal L}$ is solvable $Q_{\mathcal L}$ is known to be a product of linear factors. This fact gives rise to the definition of spectral matrix and the Poincaré polynomial for solvable Lie algebras. Application is given to $1$-dimensional extensions of nilpotent Lie algebras.