@article{MZ_1996__221_1_174842,
     author = {D.E. Edmunds and A.A. Ilyn},
     title = {Approximation numbers of embeddings of {Sobolev} spaces.},
     journal = {Mathematische Zeitschrift},
     pages = {177--188},
     publisher = {mathdoc},
     volume = {221},
     number = {1},
     year = {1996},
     url = {http://geodesic.mathdoc.fr/item/MZ_1996__221_1_174842/}
}