@article{FORUM_1997__9_2_141803,
     author = {Winfried Sickel},
     title = {Necessary conditions on composition operators acting on {Sobolev} spaces of fractional order. {The} critical case 1 ... s ... n/p.},
     journal = {Forum mathematicum},
     pages = {267--302},
     publisher = {mathdoc},
     volume = {9},
     number = {2},
     year = {1997},
     zbl = {0898.46032},
     url = {http://geodesic.mathdoc.fr/item/FORUM_1997__9_2_141803/}
}