@article{10_1017_S0017089503001320,
     author = {AKEMANN, CHARLES A. and ANDERSON, JOEL},
     title = {THE {SPECTRAL} {SCALE} {AND} {THE} $k${{\textendash}NUMERICAL} {RANGE}},
     journal = {Glasgow mathematical journal},
     pages = {225--238},
     year = {2003},
     volume = {45},
     number = {2},
     doi = {10.1017/S0017089503001320},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089503001320/}
}