Originally publ. in: Proc. All-Union School in Diff. Eq. with Infinite Number of Variables and in Dyn. Syst. with Infinite Number of Degrees of Freedom, Dilhan 1973, 229-256, © Acad. Sc. Arm. SSR 1974 English transl.: Selecta Math. Sov. 5:4, 327-345, © Birkhäuser Boston, Inc. 1986 Reprint by Springer-Verlag Berlin Heidelberg 2014 with kind permission, DOI 10.1007/978-3-642-31031-7_32