1. Lecture Notes;E. Becker,1978
2. E. Becker, Summen n-ter Potenzen in Körpern, J. Reine Angew. Math.307/308, 8–30 (1979)
3. E. Becker, R. Berr, F. Delon, D. Gondard, Hilbert’s 17-th problem for sums of 2n-th powers, preprint
4. E. Becker, D. Gondard, On rings admitting orderings and 2-primary orderings of higher level, Manuscripta Math.65, 63–82 (1989)
5. R. Berr, Null- and Positivstellensätze for generalized real closed fields, preprint