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Oct 12, 2018 at 7:50 comment added Mateusz Kwaśnicki @VishnuM: I do not quite remember the problem, but I think the simplest way is to make the counter-example explicit: set $u(x,y) = (1-x^2-y^2)(1+2x^2)$ (it has three critical points in the unit disk), $f(x,y)=1$ and $Lu = (2+32x^2+4y^2)^{-1}(u_{xx}+2 u_{yy})$. However, this does not meet your condition about the coefficients having no humps.
Oct 12, 2018 at 4:39 comment added Andre of Astora How could I make the counterexample you gave rigorous?
Sep 9, 2018 at 15:16 comment added Mateusz Kwaśnicki @VishnuM: Thanks for the update. I do not think I can contribute more; I suppose one should have a look at the original dissertation to figure out what the author had on mind.
Sep 8, 2018 at 13:01 comment added Andre of Astora I have added a condition in the question that hopefully solves this problem.
Sep 8, 2018 at 10:02 comment added Andre of Astora I had a specific PDE in mind, but wanted to prove a more general result since my PDE was quite messy. However an additional condition on $L$ would be that the coefficient functions have nonzero gradient in the $\Omega$.
Sep 8, 2018 at 9:04 history answered Mateusz Kwaśnicki CC BY-SA 4.0