“…34 In the case of the functions U and V, which are 2-periodic in v, the fixed set will be M * = {u, v, t : sin v = 0, sin t = 0}. 34 The functions U and V can also satisfy the conditions System (1.1) then has two fixed sets. We will call the solution u = (t), v = (t) of system (1.1) a 2k-periodic motion, k ∈ N, if (1.2) Note that, in mechanical systems, the solution (1.2) describes both the oscillatory motion (m = 0) and the rotational motions (m = 0).…”