Let $g$ be a Kac-Moody algebra with a symmetrizable Cartan matrix, and let $\{u\_j\}$ and $\{u^j\}$ be bases of $g$ dual with respect to a nondegenerate invariant bilinear form $(\cdot|\cdot)$ on $g$, and consistent with the triangular decomposition of $g$. Let $L(\Lambda)$ be an integrable representation of $g$ with highest weight $\Lambda$, and let $v\_\Lambda$ be its highest weight vector. Denote the Casimir $\Omega=\sum\_j u\_j\otimes u^j$.



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> **Question.** Why $\Omega(v\_\Lambda\otimes v\_\Lambda)=(\Lambda|\Lambda)v\_\Lambda\otimes v\_\Lambda$? Could someone
> give some explanation or some references?
> 
> 
> 



Action of the Casimir on highest weight modules for Kac-Moody algebra
You should be a bit careful, as this isn't precisely the action of the Casimir on $v \otimes v$, but instead follows from it.


For each positive root $\alpha$, let $e\_\alpha^{(1)}, \dots, e\_\alpha^{(n\_\alpha)}$ be a basis of the root space $\mathfrak{g}\_\alpha$, and let $\{f\_\alpha^{(i)}\}$ be the corresponding dual space for $\mathfrak{g}\_{-\alpha}$. Define $\Omega\_\alpha:= \sum\_{i=1}^{n\_\alpha} f\_\alpha^i e\_\alpha^i$. Also, let $\{h\_i, h^i\}$ be a dual basis for $\mathfrak{h}$, and set $\Omega\_0:= \sum\_i h\_ih^i$. Then the Casimir element is given by
$$ \Omega= 2\nu^{-1}(\rho)+\Omega\_0 +2\sum\_{\alpha \in \Delta^+} \Omega\_\alpha$$
where $\rho$ is a Weyl vector defined by $\rho(\alpha\_i^\vee)=1$ for all simple coroots $\alpha\_i^\vee$, and $\nu: \mathfrak{h} \to \mathfrak{h}^\ast$ is the isomorphism determined by the nondegenerate bilinear form.


A key fact about the Casimir operator is that, for any $v \in L(\mu)$ for any dominant weight $\mu$, we have that $\Omega(v)=(\mu | \mu+2\rho)v$. Now, by just expanding out over the tensor product using the above definition of $\Omega$, we get for any $v \in L(\Lambda)$
$$
\Omega(v \otimes v) = (\Omega(v)) \otimes v + v \otimes (\Omega(v)) +2 \sum\_{\alpha \in \Delta \sqcup \{0\}} \sum\_{i=1}^{n\_\alpha} e\_\alpha^i(v) \otimes f\_\alpha^i(v).
$$


Now, if $v \in G(v\_\Lambda)$ is a vector in the $G$-orbit of $v\_\Lambda$ (one such example is precisely $v\_\Lambda$), we have $v \otimes v \in L(2\Lambda)$. Then the left-hand side is precisely $(2\Lambda | 2\Lambda +2\rho) (v\otimes v)$. The right-hand side is similarly
$$
2(\Lambda | \Lambda+2\rho)(v \otimes v) + 2 (\text{the term you are interested in}).$$


Solving for the term you want gives precisely $(\Lambda | \Lambda)(v\otimes v)$.


