@article{GDZPPN002211602,
title = {Hilbert's 17-th problem for sums of 2n-th powers.},
journal = {Journal f\"ur die reine und angewandte Mathematik},
volume = {450},
year = {1994},
pages = {139-158},
zbl = {0807.14045},
url = {http://dml.mathdoc.fr/item/GDZPPN002211602}
}
Becker, E.; Berr, R.; Delon, F.; Gondard, D. Hilbert's 17-th problem for sums of 2n-th powers.. Journal für die reine und angewandte Mathematik, Tome 450 (1994) pp. 139-158. http://gdmltest.u-ga.fr/item/GDZPPN002211602/