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Sep 20, 2016 at 11:48 history bounty ended CommunityBot
Sep 17, 2016 at 21:49 vote accept aglearner
Sep 14, 2016 at 19:39 comment added Jaap Eldering My feeling is that there always exists a critical point, because you have directions along $x$ where $f$ increases and along $y$ where it decreases. This creates some kind of saddle configuration, that I think should always contain a critical point. Maybe Morse theory or some adaptation of the (finite-dimensional) mountain pass lemma may help you here, but I don't know these topics well.
Sep 13, 2016 at 21:10 comment added aglearner Thanks Jaap! Indeed this looks like a counterexample. Do you think you can elaborate this example further so that $F$ does not have any critical point in $[-1,1]\times (-1,1)$?
Sep 13, 2016 at 16:19 history answered Jaap Eldering CC BY-SA 3.0