The unstable spectrum of the Navier-Stokes operator in the limit of   vanishing viscosity
  A general class of linear advective PDEs, whose leading order term is of viscous dissipative type, is considered. It is proved that beyond the limit of the essential spectrum of the underlying inviscid operator, the eigenvalues of the viscous operator, in the limit of vanishing viscosity, converge precisely to those of the inviscid operator. The general class of PDEs includes the equations of incompressible fluid dynamics. Hence eigenvalues of the Navier-Stokes operator converge in the inviscid limit to the eigenvalues of the Euler operator beyond the essential spectrum. 
Roman Shvydkoy and Susan Friedlander