On the interior Bernoulli free boundary problem for the fractional Laplacian on an interval

Abstract We study the structure of solutions of the interior Bernoulli free boundary problem for $$(-\Delta )^{\alpha /2}$$ ( - Δ ) α / 2 on an interval D with parameter $$\lambda > 0$$ λ > 0 . In particular, we show that there exists a constant $$\lambda _{\alpha ,D} > 0$$ λ α , D > 0 (called the Bernoulli constant) such that the problem has no solution for $$\lambda \in (0,\lambda _{\alpha ,D})$$ λ ∈ ( 0 , λ α , D ) , at least one solution for $$\lambda = \lambda _{\alpha ,D}$$ λ = λ α , D and at least two solutions for $$\lambda > \lambda _{\alpha ,D}$$ λ > λ α , D . We also study the interior Bernoulli problem for the fractional Laplacian for an interval with one free boundary point. We discuss the connection of the Bernoulli problem with the corresponding variational problem and present some conjectures. In particular, we show for $$\alpha = 1$$ α = 1 that there exists solutions of the interior Bernoulli free boundary problem for $$(-\Delta )^{\alpha /2}$$ ( - Δ ) α / 2 on an interval which are not minimizers of the corresponding variational problem.