S. A. Voitsekhovskii, V. L. Makarov, Yu. I. Rybak, “Estimates for the rate of convergence of the difference approximation of the Dirichlet problem for the equation $-\Delta u+\sum_{|\alpha|\le1}(-1)^{|\alpha|}D^\alpha q_\alpha(x)u=f(x)$ for $q_\alpha(x)\in W_\infty^{\lambda|\alpha|}(\Omega)$, $\lambda\in(0,1]$”, Differ. Uravn., 24:11 (1988), 1987–1994; Differ. Equ., 24:11 (1988), 1338–1344
