“…On the other hand, in the undamped case µ = 0 (that is, for the semilinear wave equation with speed of propagation t −k ) the exponent max{p 0 (k, n), p 1 (k, n)} is consistent with the result for the generalized semilinear Tricomi equation (i.e., the semilinear wave equation with speed of propagation t ℓ , where ℓ > 0) obtained in the recent works [13,14,15,21].…”