@article{10_4153_CMB_2015_038_9,
     author = {He, Ziyi and Yang, Dachun and Yuan, Wen},
     title = {Littlewood{\textendash}Paley {Characterizations} of {Second-Order} {Sobolev} {Spaces} via {Averages} on {Balls}},
     journal = {Canadian mathematical bulletin},
     pages = {104--118},
     year = {2016},
     volume = {59},
     number = {1},
     doi = {10.4153/CMB-2015-038-9},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-2015-038-9/}
}