@article{10_1090_S0894_0347_2011_00694_2,
     author = {Hrube\r{A}{\textexclamdown}, Pavel and Wigderson, Avi and Yehudayoff, Amir},
     title = {Non-commutative circuits and the sum-of-squares problem},
     journal = {Journal of the American Mathematical Society},
     pages = {871--898},
     publisher = {mathdoc},
     volume = {24},
     number = {3},
     year = {2011},
     doi = {10.1090/S0894-0347-2011-00694-2},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-2011-00694-2/}
}