Stability and instability of Navier boundary layers

We study the inviscid limit problem for the incompressible Navier-Stokes equation on a half-plane with a Navier boundary condition depending on the viscosity. On one hand, we prove the $L^2$ convergence of Leray solutions to the solution of the Euler equation. On the other hand, we show the nonlinear instability of WKB expansions in the stronger $L^{\infty}$ and $\dot{H}^s$ (s>1) norms.