Kinetic-fluid boundary layers and acoustic limit for the Boltzmann equation with general Maxwell reflection boundary condition

We prove the acoustic limit from the Boltzmann equation with hard sphere collisions and the Maxwell reflection boundary condition. Our construction of solutions includes the interior fluid part and Knudsen-viscous coupled boundary layers. The main novelty is that the accommodation coefficient is in the full range $ 0<\alpha\leq 1 $. The previous works in the context of classical solutions only considered the simplest specular reflection boundary condition, i.e., $ \alpha = 0 $. The mechanism of the derivation of fluid boundary conditions in the case $ \alpha = O(1) $ is quite different from the cases $ \alpha = 0 $ or $ \alpha = o(1) $. This rigorously justifies the corresponding formal analysis in Sone's books [33,34]. In particular, this is a smooth solution analogue of [24], in which the renormalized solution was considered and the boundary layers were not visible.