  This is a continuation and completion of the program (initiated in
\cite{GrN1,GrN2}) to derive pointwise estimates on the Green function and sharp
bounds on the semigroup of linearized Navier-Stokes around a generic stationary
boundary layer profile. This is done via a spectral analysis approach and a
careful study of the Orr-Sommerfeld equations, or equivalently the
Navier-Stokes resolvent operator $(\lambda - L)^{-1}$. The earlier work
(\cite{GrN1,GrN2}) treats the Orr-Sommerfeld equations away from critical
layers: this is the case when the phase velocity is away from the range of the
background profile or when $\lambda$ is away from the Euler continuous
spectrum. In this paper, we study the critical case: the Orr-Sommerfeld
equations near critical layers, providing pointwise estimates on the Green
function as well as carefully studying the Dunford's contour integral near the
critical layers.
  As an application, we obtain pointwise estimates on the Green function and
sharp bounds on the semigroup of the linearized Navier-Stokes problem near
monotonic boundary layers that are spectrally stable to the Euler equations,
complementing \cite{GrN1,GrN2} where unstable profiles are considered.
