The Wikipedia entry for quantum groups states that a quantum group $U_{q}\left(\mathfrak{g}\right)$ has an infinite formal sum that plays the role of an $R$ matrix, which is the product of two factors,

$$ q^{\eta\sum_{i}t_{\lambda_{i}}\otimes t_{\mu_{i}}} $$

and an infinite formal such, where $\lambda_j$ is a basis for the dual space to the Cartan subalgebra, $\mu_j$ is the dual basis, $\eta=\pm1$, and $t_\lambda$ are the Cartan generators.

I find this expression to be convenient and useful, but I have searched the references for the Wikipedia page and numerous other material on quantum group algebras, and did not find anything sufficiently similar.

I would appreciate a reference for the $q^{\eta\sum_{i}t_{\lambda_{i}}\otimes t_{\mu_{i}}}$ expression.