Asymptotic formulae are provided for the number of representations of a natural number as the sum of four and of three squares that are pairwise coprime.
@article{bwmeta1.element.bwnjournal-article-doi-10_4064-ba62-3-3, author = {J\"org Br\"udern}, title = {Sums of Squares Coprime in Pairs}, journal = {Bulletin of the Polish Academy of Sciences. Mathematics}, volume = {62}, year = {2014}, pages = {215-231}, zbl = {06390259}, language = {en}, url = {http://dml.mathdoc.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ba62-3-3} }
Jörg Brüdern. Sums of Squares Coprime in Pairs. Bulletin of the Polish Academy of Sciences. Mathematics, Tome 62 (2014) pp. 215-231. http://gdmltest.u-ga.fr/item/bwmeta1.element.bwnjournal-article-doi-10_4064-ba62-3-3/