“…More recently, the program of Jones, Putterman and Roberts [14,15] has been provided with rigorous mathematical proofs (see the article [4] to review some of these progresses). For example, the non-existence of non-constant supersonic traveling waves (i.e., c > √ 2) of finite energy in dimension N ≥ 2 in [9], the non-existence of non-constant sonic traveling waves (i.e., c = √ 2) of finite energy in dimension two in [11] and the existence of non-constant axisymmetric traveling waves in [2,1,3,5,17,16].…”