“…Since we do not have sublinearity for the weak derivatives of the Hardy-Littlewood maximal function, the continuity of M : W ,p (R n ) → W ,p (R n ) for < p < ∞ is a rmatively a nontrivial issue, which was addressed by Luiro [23] and later extensions were given in [24]. We can consult [2,4,5,7,25,26] for the endpoint Sobolev regularity of maximal operators, as well as [17,20] for the regularity properties of maximal operators on other smooth function spaces, such as Triebel-Lizorkin spaces, fractional Sobolev spaces and Besov spaces. It should be pointed out that the commutativity with translations for maximal operators plays a key role in deducing the boundedness of maximal operators on W ,p (R n ).…”