  We investigate regular elliptic boundary-value problems in bounded domains
and show the Fredholm property for the related operators in an extended scale
formed by inner product Sobolev spaces (of arbitrary real orders) and
corresponding interpolation Hilbert spaces. In particular, we can deal with
boundary data with arbitrary low regularity. In addition, we show interpolation
properties for the extended scale, embedding results, and global and local a
priori estimates for solutions to the problems under investigation. The results
are applied to elliptic problems with homogeneous right-hand side and to
elliptic problems with rough boundary data in Nikoskii spaces, which allows us
to treat some cases of white noise on the boundary.
