  The Rayleigh equation, which is the linearized Euler equations near a shear
flow in vorticity formulation, is a key ingredient in the study of the long
time behavior of solutions of linearized Euler equations, in the study of the
linear stability of shear flows for Navier-Stokes equations and in particular
in the construction of the so called Tollmien-Schlichting waves. It is also a
key ingredient in the study of vorticity depletion.
  In this article we locally describe the solutions of Rayleigh equation near
critical points of any order of degeneracy, and link their values on the
boundary with their behaviors at infinity.
