@article{10_1090_S0894_0347_06_00529_7,
     author = {Freedman, Michael and Lov\'asz, L\'aszl\'o and Schrijver, Alexander},
     title = {Reflection positivity, rank connectivity, and homomorphism of graphs},
     journal = {Journal of the American Mathematical Society},
     pages = {37--51},
     year = {2007},
     volume = {20},
     number = {1},
     doi = {10.1090/S0894-0347-06-00529-7},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-06-00529-7/}
}