@article{GDZPPN002314908, title = {Extremal Positive Semidefinite Forms.}, journal = {Mathematische Annalen}, volume = {229}, year = {1977}, pages = {1-18}, zbl = {0347.15009}, url = {http://dml.mathdoc.fr/item/GDZPPN002314908} }
Choi, Man-Duen; Lam, Tsit-Yuen. Extremal Positive Semidefinite Forms.. Mathematische Annalen, Tome 229 (1977) pp. 1-18. http://gdmltest.u-ga.fr/item/GDZPPN002314908/