This analysis characterizes fractional Sobolev spaces using Lusin area functions and Littlewood–Paley functions, suggesting new options for these spaces.
In this paper, the authors characterize the Sobolev spaces (Formula presented.) with (Formula presented.) and (Formula presented.) via a generalized Lusin area function and its corresponding Littlewood–Paley (Formula presented.)-function. The range (Formula presented.) is also proved to be nearly sharp in the sense that these new characterizations are not true when (Formula presented.) and (Formula presented.). Moreover, in the endpoint case (Formula presented.), the authors also obtain some weak type estimates. Since these generalized Littlewood–Paley functions are of wide generality, these results provide some new choices for introducing the notions of fractional Sobolev spaces on metric measure spaces.
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Sato et al. (2017) studied this question.
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