Filippov, V. M. On a variational method in spaces with dominant mixed derivative. (English) Zbl 0709.35512 Proc. Steklov Inst. Math. 180, 265-266 (1989). MSC: 35L05 Wave equation 35L20 Initial-boundary value problems for second-order hyperbolic equations PDF BibTeX XML Cite \textit{V. M. Filippov}, Proc. Steklov Inst. Math. 180, 265--266 (1989; Zbl 0709.35512)