“…has a crucial role in determining some properties of the fundamental solution of L k,µ,ν 2 . In the special case k = 0 (the so-called wave operator with scale-invariant damping and mass), it is known in the literature that the value of δ affects not only the fundamental solution of L 0,µ,ν 2 but also the critical exponents in the treatment of semilinear Cauchy problem associated with L 0,µ,ν 2 with power nonlinearity [11,16,17,1], nonlinearity of derivative type [18], and combined nonlinearity [4,5].…”