@article{10_4153_CJM_1997_004_2,
     author = {Hu, Y. K. and Kopotun, K. A. and Yu, X. M.},
     title = {Constrained {Approximation} in {Sobolev} {Spaces}},
     journal = {Canadian journal of mathematics},
     pages = {74--99},
     year = {1997},
     volume = {49},
     number = {1},
     doi = {10.4153/CJM-1997-004-2},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1997-004-2/}
}