On the smoothness of solutions to elliptic equations in domains with   H\"older boundary
  The dependence of the smoothness of variational solutions to the first boundary value problems for second order elliptic operators are studied. The results use Sobolev-Slobodetskii and Nikolskii-Besov spaces and their properties. Methods are based on real interpolation technique and generalization of Savar\'{e}-Nirenberg difference quotient technique. 
I.V. Tsylin
math.AP