We consider different fractional Neumann Laplacians
of order $sin(0,1)$ on domains $OmegasubsetR^n$, namely,
the Restricted Neumann Laplacian,
the Semirestricted Neumann
Laplacian and the Spectral Neumann Laplacian.
In particular, we are interested in the attainability of Sobolev constants for these operators when
Ω is a half-space.
