 Langevin function

The Langevin function is the mean projection of a freely orientable three-dimensional unit vector weighted by $e^{\xi \cos \theta }$. Integrating over solid angle gives $Z_1=4\pi \sinh \xi /\xi$, and differentiation gives $⟨\cos \theta ⟩={\partial }_{\xi }\log Z_1=\coth \xi -1/\xi$. It behaves as $\xi /3$ at small argument and $1-1/\xi$ at large positive argument.