Fractional Adams-Moser-Trudinger type inequalities

Extending several works, we prove a general Adams Moser Trudinger type inequality for the embedding of Bessel-potential spaces (r2) into Orlicz spaces for an arbitrary domain,r2 with finite measure. In particular we prove

sup (u is an element of Hn/p,p (Omega), parallel to(-Delta)n/2p u parallel to LP(Omega)<= 1) integral Omega (E alpha n,p broken vertical bar u broken vertical bar p/p-1dx <= Cn,p broken vertical bar Omega broken vertical bar,)

for a positive constant amp whose sharpness we also prove. We further extend this result to the case of Lorentz-spaces (i.e. (-Delta) u is an element of L-(P,L-q)). The proofs are simple, as they use Green functions for fractional Laplace operators and suitable cut-off procedures to reduce the fractional results to the sharp estimate on the Riesz potential proven by Adams and its generalization proven by Xiao and Zhai. We also discuss an application to the problem of prescribing the Q-curvature and some open problems.