On a Sobolev critical problem for the superposition of a local and nonlocal operator with the "wrong sign''

We study a critical problem for an operator of mixed order obtained by the superposition of a Laplacian with a fractional Laplacian. The main novelty is that we consider a mixed operator of the form $-\mathrm{\Delta }-\gamma (-\mathrm{\Delta }{)}^s$, namely we suppose that the fractional Laplacian has the ``wrong sign’’ and can be seen as a nonlocal perturbation of the purely local case, which is needed to produce a nontrivial solution of the critical problem.