“…However, the justification of this formal boundary expansion is a challenging problem. The studies on steady Prandtl equations and linearized steady Prandtl equations can be found in [3,14,33,41,42] and the validity of Prandtl boundary layer expansion has also some important progresses, see [11,15,16,17,12,8,19,9]. Moreover, the stability in Sobolev space for some class shear flow of Prandtl type has been studied in [3,2].…”