@article{10_1090_S0894_0347_05_00506_0,
     author = {Landsberg, J.},
     title = {The border rank of the multiplication of 2{\texttimes}2 matrices is seven},
     journal = {Journal of the American Mathematical Society},
     pages = {447--459},
     year = {2006},
     volume = {19},
     number = {2},
     doi = {10.1090/S0894-0347-05-00506-0},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-05-00506-0/}
}