I am reading a paper in fractional elliptic. It is true that $(-\Delta)^{-s}$ is continuous from $H^{-s}$ to $H^{s}$. However, in the following argument, I cannot find a good reference for it:

[Fractional Paper][1]


  [1]: https://i.sstatic.net/2n37Y.jpg

Can anyone explain or give me some references about the isomorphism? 

Here is the full article: https://www.sciencedirect.com/science/article/pii/S002203961400196X