“…Ruying Xue considered problem (1) with the nonlinearity f (u) = u k+1 , k = 1, 2, • • • , and proved the global existence for the small initial data and k > 4. The problem (1) with much more general nonlinear case f (u) = ±|u| p and |u| p−1 u was then considered in [15], and also by the potential well theory [4], [9], [8], [22], in the sub-critical initial energy case, i.e. E(0) ≤ d, the global and non-global solutions were classified in term of initial data.…”