@article{MAN_1977__231_163047,
     author = {Man-Duen Choi and Tsit-Yuen Lam},
     title = {Extremal {Positive} {Semidefinite} {Forms.}},
     journal = {Mathematische Annalen},
     pages = {1--18},
     publisher = {mathdoc},
     volume = {231},
     year = {1977},
     zbl = {0347.15009},
     url = {http://geodesic.mathdoc.fr/item/MAN_1977__231_163047/}
}