@article{JRAM_1994__450_153618,
     author = {E. Becker and R. Berr and F. Delon and D. Gondard},
     title = {Hilbert's 17-th problem for sums of 2n-th powers.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {139--158},
     publisher = {mathdoc},
     volume = {450},
     year = {1994},
     zbl = {0807.14045},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1994__450_153618/}
}