Kuptsov, L. P. Averaging operators and partial differential equations. (English) Zbl 0709.35508 Proc. Steklov Inst. Math. 180, 168-170 (1989). MSC: 35J70 Degenerate elliptic equations 35B99 Qualitative properties of solutions to partial differential equations 35K25 Higher-order parabolic equations PDF BibTeX XML Cite \textit{L. P. Kuptsov}, Proc. Steklov Inst. Math. 180, 168--170 (1989; Zbl 0709.35508)