Algebra Seminar

Speaker: Franz-Viktor Kuhlmann (Saskatchewan)

"Notes on tame and extremal valued fields"

Time: 14:30

Room: MC 107

In the year 2003 I first heard of the notion of extremal valued fields when Yuri Ershov gave a talk at a conference in Tehran. He proved that algebraically complete discretely valued fields are extremal. However, the proof contained a mistake, and it turned out in 2009 through an observation by Sergej Starchenko that Ershov's original definition leads to all extremal fields being algebraically closed. In joint work with Salih Durhan (formerly Azgin) and Florian Pop, we chose a more appropriate definition and then characterized extremal valued fields in several important cases.

We call a valued field $(K,v)$ extremal if for all natural numbers $n$ and all polynomials $f$ in $K[X_1,...,X_n]$, the set {$f(a_1,...,a_n) | a_1,...,a_n$ in the valuation ring} has a maximum (which is allowed to be infinity, which is the case if $f$ has a zero in the valuation ring). This is such a natural property of valued fields that it is in fact surprising that it has apparently not been studied much earlier. It is also an important property because Ershov's original statement is true under the revised definition, which implies that in particular all Laurent Series Fields over finite fields are extremal. As it is a deep open problem whether these fields have a decidable elementary theory and as we are therefore looking for complete recursive axiomatizations, it is important to know the elementary properties of them well. That these fields are extremal seems to be an important ingredient in the determination of their structure theory, which in turn is an essential tool in the proof of model theoretic properties.

Further, it came to us as a surprise that extremality is closely connected with Pop's notion of "large fields". Also the notion of tame valued fields plays a crucial role in the characterization of extremal fields; both large and tame valued fields have the right model theoretic properties. A valued field $K$ with algebraic closure $K^{ac}$ is tame if it is henselian and the ramification field of the extension $K^{ac}|K$ coincides with the algebraic closure.

In my talk I will introduce the above notions, try to explain their meaning and importance also to the non-expert, and discuss in detail what is known about extremal fields and how the properties of large and of tame fields appear in the proofs of the characterizations we give. I will also discuss the connections with additive polynomials in the case of positive characteristic. Finally, I will present some challenging open problems whose solution may have an impact on the above mentioned problem for Laurent Series Fields over finite fields.