It is symbolic that in that same year of 1935, S.L. Sobolev, who was 26 years old that time, submitted to the editorial board of the journal "Matematicheskiy sbornik" his famous work [61] and published at the same time its brief version in "Doklady AN SSSR" [60]. This work laid foundations of a completely new outlook on the concept of function, unexpected even for N.N. Luzin — the concept of a generalized function (in
the framework of the notion of distribution introduced later). It is also symbolic that the work by Sobolev was devoted to the Cauchy problem for hyperbolic equations and, in particular, to the same vibrating string.