  A regular elliptic boundary-value problem over a bounded domain with a smooth
boundary is studied. We prove that the operator of this problem is a Fredholm
one in the two-sided refined scale of the functional Hilbert spaces and
generates a complete collection of isomorphisms. Elements of this scale are the
isotropic spaces of Hormander-Volevich-Paneah and some its modifications. A
priori estimate for the solution is established and its regularity is
investigated.
