@article{MAN_1974__210_162624,
     author = {C.V. Hutton},
     title = {On the {Approximation} {Numbers} of an {Operator} and {Its} {Adjoint.}},
     journal = {Mathematische Annalen},
     pages = {277--280},
     publisher = {mathdoc},
     volume = {210},
     year = {1974},
     zbl = {0272.47017},
     url = {http://geodesic.mathdoc.fr/item/MAN_1974__210_162624/}
}