[UNDELETED]

The question
https://mathoverflow.net/questions/275624/checking-concavity-of-a-highly-non-linear-function

seems to have been closed over-hastily; those with sufficient rep can see fedja's comments, and I am prepared to believe quite readily that if fedja thinks it is non-trivial, it is non-trivial. 

I would like to nominate this for re-opening.