“…Such a condition was introduced in [8] for p = 2 and generalized in [2] and [14] for p > 1, and it turns out that it is the natural p-Neumann boundary condition associated to (−∆) s p . Indeed, this boundary condition allows us to give a formulation of weak solutions of problem (1.1) in a variational way, see [2,8,14]. Moreover, in the case p = 2, it happens that among all functions in the associated fractional Sobolev space (to be defined below), the ones minimizing the Gagliardo seminorm, automatically satisfy the nonlocal Neumann boundary condition above, see [7].…”