1. Hereditarily pythagorean fields and orderings of higher levels;Becker,1978
2. Summen n-ter Potenzen in Körpern;Becker;J. Reine Angew. Math.,1979
3. The real holomorphy ring and sums of 2n-th powers;Becker,1982
4. Extended Artin-Schreier theory of fields;Becker;Rocky Mountain J. Math.,1984
5. Hilbert's 17th problem for sums of 2nth powers;Becker;J. Reine Angew. Math.,1994