Hi Mr. Yang, Let me be more clear. I want to know some references when the ambient manifold is smooth (for instance $\mathbb{R}^n$) and the submanifolds are Sobolev regular. My motivation was the simplest case, where the submanifold is in fact a hypersurface of $\mathbb{R}^n$, the boundary of a domain(the 'classic' PDE overview). This kind of issue usually appear when working with regularity theory for PDE's. Cheers, Luís.