Minimal graded Lie algebras and representations of quadratic algebras

Abstract: Let $({\go g}_{0},B_{0})$ be a quadratic Lie algebra (i.e. a Lie algebra $\go{g}_{0}$ with a non degenerate symmetric invariant bilinear form $B_{0}$) and let $(\rho,V)$ be a finite dimensional representation of ${\go g}_{0}$. We define on $ \Gamma(\go{g}_{0}, B_{0}, V)=V^*\oplus {\go g}_{0}\oplus V$ a structure of local Lie algebra in the sense of Kac (\cite{Kac1}), where the bracket between $\go{g}_{0}$ and $V$ (resp. $V^*)$ is given by the representation $\rho$ (resp. $\rho^*$), and where the bracket between $V$ and $V^*$ depends on $B_{0}$ and $\rho$. This implies the existence of two $\Z$-graded Lie algebras ${\go g}_{max}(\Gamma(\go{g}_{0}, B_{0}, V))$ and ${\go g}_{min}(\Gamma(\go{g}_{0}, B_{0}, V))$ whose local part is $\Gamma(\go{g}_{0},B_{0}, V)$. We investigate these graded Lie algebras, more specifically in the case where ${\go g}_{0}$ is reductive. Roughly speaking, the map $(\go{g}_{0},B_{0}, V)\longmapsto {\go g}_{min}(\Gamma(\go{g}_{0}, B_{0}, V))$ a bijection between triplets and a class of graded Lie algebras. We show that the existence of "associated $\go {sl}_{2}$-triples" is equivalent to the existence of non trivial relative invariants on some orbit, and we define the "graded Lie algebras of symplectic type" which give rise to some dual pairs.