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Figure 2 Proof. From the Strong Maximum Principle and the Hopf Boundary Lemma for parabolic equations [7], it follows immediately that u;(x, t) >0 in Q for t>0, i=1, 2. Hence E(u(t)) is well-defined for all t>0. After some tedious calculations, we have It is easy to see that the first integral in the right-hand side of (4.18) is non- positive if