@article{700323,
     title = {$N$-widths for singularly perturbed problems},
     booktitle = {Proceedings of Equadiff 10},
     series = {GDML\_Books},
     publisher = {Masaryk University},
     address = {Brno},
     year = {2002},
     pages = {197-205},
     mrnumber = {MR1981538},
     zbl = {1005.41009},
     url = {http://dml.mathdoc.fr/item/700323}
}

Stynes, Martin; Kellogg, R. Bruce. $N$-widths for singularly perturbed problems, dans Proceedings of Equadiff 10, GDML_Books,  (2002), pp. 197-205. http://gdmltest.u-ga.fr/item/700323/