@article{10_1090_S0894_0347_2012_00733_4,
     author = {Blekherman, Grigoriy},
     title = {Nonnegative polynomials and sums of squares},
     journal = {Journal of the American Mathematical Society},
     pages = {617--635},
     publisher = {mathdoc},
     volume = {25},
     number = {3},
     year = {2012},
     doi = {10.1090/S0894-0347-2012-00733-4},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-2012-00733-4/}
}