Abstract
We consider the self-adjoint operator L (the Friedrichs extension) associated with the closable form \({a_m}[u,f] = \int_\Omega {\left( {\sum\nolimits_{\left| \alpha \right| = m} {{\rho ^2}(x){D^\alpha }u\overline {{D^\alpha }f} + v_m^2(x)u\bar f} } \right)dx}\) in the space L 2, ω where Ω ⊂ ℝ n is a domain, \(f,u \in C_0^\infty (\Omega )\) , and m ∈ ℕ. Here v s ( x ) = ρ ( x ) h -s ( x ), s > 0, and the positive functions ρ (·) and h (·) satisfy some special conditions. The space L 2, ω is a weighted Hilbert space with a locally integrable weight ω ( x ) positive almost everywhere in Ω. For s > 0 and 1 < p < ∞, we introduce the weighted space of potentials \(H_p^s(\rho ,{v_s})\) . For s = m ∈ ℕ and p = 2, the space \(H_2^m(\rho ,{v_m})\) is the weighted Sobolev space \(H_2^m(\rho ,{v_m})\) with the equivalent norm \(\left\| {f;W_2^m(\rho ,{v_m})} \right\| = \sqrt {{a_m}[f,f]}\) . Descriptions are given of the interpolation spaces H s obtained by the complex and real interpolation methods from the pair \(\left\{ {H_p^{{s_0}}(\rho ,{v_{{s_0}}}), H_p^{{s_1}}(\rho ,{v_{{s_1}}})} \right\}\) , 0 < s 0 < s 1 . Estimates are derived for the linear widths and Kolmogorov widths of the compact sets \(\mathcal{F} = B{H_s}\bigcap {{L^{ - 1}}(B{L_{2,\omega }})}\) , s > 0 (where BX is the unit ball in a space X ).