# Centraliser of $\Delta U$ in $U\otimes U$

Let $$U$$ be a universal enveloping algebra of a simple Lie algebra $$\mathfrak{g}$$ over $$\mathbb{C}$$. What is a good reference for the centralizer of $$\Delta (\mathfrak{g})$$ in $$U \otimes U$$ ? Here $$\Delta$$ is the usual coproduct, the diagonal.