“…Firstly, Lemma 2.1 and u = g * y, g ≥ 0 imply y(t, x) and u(t, x) have the same sign with y 0 (x). Moreover, from the proof of Theorem 5.1 in [10], we have u x (t, x) ≥ −|µ 0 |. Now note that given t ∈ [0, T ), there is a ξ(t) ∈ S such that u x (t, ξ(t)) = 0 by the periodicity of u to x-variable.…”