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Jun 8, 2023 at 16:55 comment added Fawen90 @leomonsaingeon Further, how could you show that $\Delta a z \le 0$?
Jun 7, 2023 at 13:02 comment added Fawen90 Dear Leo, as the linear maximum principle (applied to $z$) that you mentioned at the end, could you please provide a reference? Further, the $a$ appearing here is the function $a=a(x,t)$ or the constant $a=a(x_0,t_0)$? Many thanks for the clarification
Apr 22, 2014 at 22:22 comment added leo monsaingeon No problem, you're very welcome!
Apr 22, 2014 at 19:46 vote accept riem
Apr 21, 2014 at 16:37 comment added leo monsaingeon One last comment: the "$a=\frac{F(u)-F(v)}{u-v}$" trick is preciesly what makes the duality method work for very weak solutions, see the proof of theorem 6.5 (with the extra difficulty that one first has to truncate to avoid the degenerate levelset $u=0$).
Apr 21, 2014 at 16:31 comment added leo monsaingeon I updated my previous answer
Apr 21, 2014 at 16:31 history edited leo monsaingeon CC BY-SA 3.0
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Apr 21, 2014 at 16:06 comment added riem Thank you Leo. I in fact was reading the very book. The author claims on page 95 the topic of my OP (near equation 5.27) but does not say why it is true. The theorem you cited I believe is more general (i.e. the weak formulation is different) so I believe whatever the comparison principle the author refers to on page 94 probably is simpler than the one on page 132 hence my question.
Apr 21, 2014 at 15:46 history answered leo monsaingeon CC BY-SA 3.0