Franz-Viktor Kuhlmann's

# Abstracts of Talks

Talk in the Honorary Colloquium on Occasion of Wilfried Buchholz' 60th Birthday at the Mathematical Institute of the University of Munich, April 4 and 5, 2008:

## What is the connection between resolution of singularities and decidability?

Abstract:
Completeness and decidability of mathematical theories are a main subject in model theory. They have been of particular interest in model theoretic algebra. There are nice examples from field theory that by now may well be called classical''. Model theoretical results for algebraically closed, real closed and p-adically closed fields have found interesting applications: Hilbert's 17. Problem, Nullstellensaetze, description of positive definite polynomials and their p-adic analogues. In the year 1965 Ax and Kochen generated much interest in model theory through their proof of a correct version of Artin's Conjecture about non-trivial zeros of forms over the p-adic numbers. Since then one of the best known open problems in model theoretic algebra is whether the elementary theory of the field Fp((t)) of formal Laurent series over the field with p elements is decidable. Although this field looks so similar to the field Qp of p-adic numbers and the theory of the latter has been shown by Ax, Kochen and Ershov to be decidable, several excellent model theorists tried in vein to solve this problem. But this is not due to a lack of knowledge in model theory. In contrast to Qp, the field Fp((t)) is a valued field of positive characteristic, and we simply do not know enough about the structure of such valued fields. In this way, model theoretical questions have stimulated new research in a classical area of algebra: valuation theory. Again in 1965, another famous theorem was proved: Hironaka showed resolution of singularities for all algebraic varieties over fields of characteristic 0. Since then also for this theorem its analogue in positive characteristic has remained an open problem, in spite of all attacks from excellent algebraic geometers. Since Zariski it is known that the local version of resolution of singularities, called local uniformisation'', is of valuation theoretical nature. Yet it was a quite unexpected finding that t he decidability problem and the problem of local uniformisation both are based on the same valuation theretical problem: the defect. In the presence of defect, the classification of valued fields up to elementary equivalence, relative to their invariants (value group and residue field), breaks down. Another good indication for the connection between the two problems is the work of Denef and Schoutens. They show that if resolution of singularities in positive characteristic holds, then at least the existential elementary theory of Fp((t)) is decidable.

Talk at the CUNY Graduate center, New York, November 2006:

Abstract:
Want to deal with the model theory of valued fields in positive characteristic? Or prove local uniformization (a local form of resolution of singularities) in positive characteristic? Then meet your enemy! There are bad places and valuations, and they are the main obstruction in the above mentioned projects. We show how the defect, a phenomenon only appearing for valuations of positive residue characteristic, affects the structure theory of valued function fields, which in turn is crucial for the proof of Ax-Kochen-Ershov Principles and of local uniformization. We produce lots of defects and construct very complicated bad valuations on very simple fields, namely rational function fields. But not only in positive characteristic, bad valuations on rational function fields are interesting. Integer parts of real closed fields are of interest for model theorists because they are precisely the models of open induction. Mourgues and Ressayre have shown that every real closed field admits an integer part. On the other hand, Boughattas has constructed an ordered field that does not admit any integer part. We construct orderings on rational function fields that do not admit integer parts, and valuations on rational function fields that do not admit subrings which are complements to the valuation ring. This shows that field arithmetic structure on fields as "simple" as rational function fields can be very complicated.

Talk in the Model Theory and Applications to Algebra and Analysis Programme at the Isaac Newton Institute for Mathematical Sciences, June 2005:

Abstract:
I give a survey on additive polynomials, their role in valuation theory and algebraic geometry and what we know and do not know about them. I show their connection with the defect of valued field extensions and the meaning of the defect for the model theory of valued fields and for local uniformization in positive (residue) characteristic. I mention a classification of Artin-Schreier extensions and its applications. I also talk about maximality properties of valued fields and Ershov's notion of extremal fields.

List of open problems and references for this talk: (dvi file) --- (postscript file) --- (pdf file)

## What can the theory of valued fields say about local uniformization?

Abstract:
The problem of local uniformization can be reformulated as a problem about the structure of valued function fields. I will quickly review this reformulation.

One part of the problem is elimination of ramification. This is harder in positive characteristic because there, you also have to deal with wild ramification, and the main obstacle turns out to be the defect of valued field extensions. I will give examples of non-trivial defect. Then I will show why certain valuations (called "Abhyankar valuations") always admit a positive solution, and describe the valuation theoretical theorems used for the solution.

In order to generalize these theorems, we have to learn more about the defect. A first step is to classify Artin-Schreier extensions with non-trivial defect, which I have done in a recent preprint. Based on this classification, there is joint work in progress with O. Piltant on the higher ramification groups of such extensions. Future work shall also lead to a better understanding of defect extensions generated by additive polynomials other than the Artin-Schreier polynomial.

International Congress on Nonstandard Models of Arithmetic and Analysis at the University of Pisa, June 24 to 28, 2004:

## Dense subfields and integer parts

Abstract:
Answering a question asked at the Conference Logic, Algebra and Arithmetic'' in Teheran 2003, We show that every real closed (or more generally, henselian) non-archimedean ordered field L admits a proper dense subfield. This is interesting because any integer part of K will also be an integer part of L. If L is small'', then L|K must be algebraic. However, there are examples of trdeg L|K being any pre-assigned cardinal. In particular, if the natural valuation on L has no coarsest non-trivial coarsening, then trdeg L|K can be any countable cardinal. At the same time our construction provides a counterexample to the following conjecture: If a valuation v has no coarsest non-trivial coarsening and if the residue field with respect to every coarsening is henselian, then v is henselian. We will also discuss the existence of integer parts and of truncation closed embeddings in power series fields. We show that Boughattas' ordered field which admits no integer part is not henselian. Finally, we show that there is a real closed field that has larger cardinality than the quotient fields of all of its integer parts. It is an open question whether there is a real closed field that is larger than the quotient fields of all of its integer parts, but has the same cardinality.

Seminaire de Structures Algebriques Ordonnees, Univ. Paris 7, France, May 2004:

## Classification of Artin-Schreier defect extensions, and characterizations of algebraically maximal and defectless fields

Abstract:
We classify Artin-Schreier extensions of valued fields with non-trivial defect according to whether they are connected with purely inseparable extensions with non-trivial defect, or not. We use this classification to show that in positive characteristic, a valued field is algebraically complete if and only if it has no proper immediate algebraic extension and every finite purely inseparable extension is defectless. This result is an important tool for the construction of algebraically complete fields. We also use the result to show that extremal fields are algebraically complete. A valued field (K,v) is called extremal if for all polynomials f in several variables the value set vf(K^n) has a maximum. Restricting this condition to certain classes of polynomials yields further interesting properties. In that way, we give characterizations of algebraically maximal and inseparably defectless fields. Finally, we give a second characterization of algebraically complete fields, in terms of their completion. As an example by Cutkosky and Piltant shows, a certain property called relative resolution may work with one type of Artin Schreier defect extensions, but not with the other. This connection with algebraic geometry has to be investigated further.

This work was strongly inspired by the first part of Francoise Delon's thesis. Some results are generalized, some others are put in a larger perspective.

Seminaire de Structures Algebriques Ordonnees, Univ. Paris 7, France, April 2004:

## Extensions of valuations to rational function fields

Abstract:
We classify all possible extensions of a valuation from a ground field K to a rational function field in one or several variables over K. We determine which value groups and residue fields can appear, and we show how to construct extensions having these value groups and residue fields. In particular, we give constructions of extensions whose corresponding value group and residue field extensions are not finitely generated. One can even construct valuations on rational function fields in two variables which admit an infinite tower of degree p extensions with defect p. Such nasty valuations constitute a serious empediment for local uniformization in positive characteristic, so it is important to study their structure in detail. In the case of a rational function field K(x) in one variable, we consider the relative algebraic closure of K in the henselization of K(x) with respect to the given extension, and we show that this can be any countably generated separable-algebraic extension of K. In the tame case'', we show how to determine this relative algebraic closure. These methods can be applied to power series fields and to the p-adics.

Workshop and Conference on Logic, Algebra and Arithmetic at the Institute for Theoretical Physics and Mathematics, Tehran, October 2003:

## Additive polynomials and their role in the model theory of power series fields over finite fields and in local uniformization

Abstract:
A polynomial f over an infinite field K is called additive if f(a+b)=f(a)+f(b) for all a,b\in K. If the characteristic of K is 0, then the only additive polynomials are of the form cx with c in K. But if the characteristic is p>0, then for instance, X^p and the Artin-Schreier-polynomial X^p-X are additive. I will explain the particular role that additive polynomials play in the model theory of power series fields over finite fields. This is tightly connected with the structure theory of valued function fields, which in turn also plays a crucial role for the problem of local uniformization. The latter is a local form of resolution of singularities and therefore of valuation theoretical nature. While local uniformization has been proved in characteristic 0 by Zariski in 1940, the positive characteristic case is still open, and only special cases have been solved. In all of these solutions, additive polynomials play a role. Finally, I will sketch some results on the classification of certain Artin-Schreier-extensions of valued fields and their connection with recent work of Cutkosky and Piltant on resolution of singularities in positive characteristic.

Canadian Mathematical Society Summer 2001 Meeting, Saskatoon, June 2-4, Model Theoretic Algebra Session

## Additive polynomials and the model theory of valued fields in positive characteristic

Abstract:
Let Fp((t)) denote the field of formal Laurent series over the field with p elements; it carries the t-adic valuation vt. A well known open problem in model theoretic algebra is to find a complete recursive axiom system for the elementary theory of Fp((t)). This would yield that it is decidable. A similar result was shown by Ax-Kochen and Ershov for the p-adics Qp. An adaptation of their axiom system to the case of Fp((t)) is: henselian defectless valued field of characteristic p with value group a Z-group and residue field Fp''.

We showed in 1989 that this axiom system is not complete. In 1998 we developed an elementary axiom scheme which holds in maximal valued fields of positive characteristic and describes valuation theoretic properties of additive polynomials on such fields. This axiom scheme is independent of the above axioms. This work has appeared in the JSL. In 1999, in joint work with L. van den Dries, we developed a particularly simple version of this axiom scheme for Fp((t)), using the fact that this field is locally compact. This work is to appear in the Canad. Math. Bull.

Festkolloqium and Conference on Algebra, Model Theory and Theoretical Physics celebrating Rüdiger Göbel's 60th Birthday, University of Essen, February 2001

## Functional equations in lexicographic products (joint work with Salma Kuhlmann and Saharon Shelah)

Abstract:
Let Gamma be a chain (= a totally ordered set), and Delta a chain with distinguished element 0Delta. Given a mapping s: Gamma -> Delta, define the support of s to be the set {gamma in Gamma; s(gamma) not= 0Delta}. The lexicographic power DeltaGamma (in base 0Delta) is the set
{s: Gamma -> Delta ; support(s) is wellordered}
ordered lexicographically. In DeltaGamma, we denote by 0 the sequence with empty support. We characterize solutions Gamma to the following functional equations:
(DeltaGamma)<=0 order isomorphic to Gamma
DeltaGamma order isomorphic to Gamma
(DeltaGamma)<0 order isomorphic to Gamma
and discuss cases (and their applications) where such solutions do not exist.

University of Oldenburg, Germany, February 2001

## Über das Problem der Desingularisierung in positiver Charakteristik

Abstract:
Im Jahre 1965 bewies Hironaka Desingularisierung für alle algebraischen Varietä:ten über Körpern der Charakteristik 0. Das entsprechende Problem für positive Charakteristik ist bis zum heutigen Tage offen. Es wurde lediglich bis zur Dimension 3 von Abhyankar gelöst. De Jong bewies eine schwächere Aussage ("desingularization by alteration"): hier nimmt man eine endliche Erweiterung des Funktionenkörpers in Kauf.

Der ursprüngliche Ansatz von Zariski ist, zunächst einzelne Singularitäten aufzulösen, und die Lösungen dann zusammenzusetzen. Aber was bedeutet es, lokal zu desingularisieren? Dies führt direkt zum bewertungstheoretischen Begriff der "Stelle". Im Jahre 1939 bewies Zariski mit bewertungstheoretischen Methoden, dass einzelne Singularitäten in Charakteristik 0 immer aufgelöst werden können ("Local Uniformization Theorem"). Für algebraische Flächen in Charakteristik 0 konnte er dann die lokalen Lösungen zu einer globalen Desingularisierung zusammensetzen. Auch Abhyankar folgt diesem Weg, während aber Hironakas Ansatz nicht bewertungstheoretisch ist.

In den letzten Jahren hat der bewertungstheoretische Ansatz wieder an Interesse gewonnen. In meinem Vortrag werde ich zeigen, welche Rolle Hensel's Lemma in der lokalen Uniformisierung spielt, und welche neuen Ergebnisse für positive Charakteristik die Bewertungstheorie liefern kann. Ich werde das bewertungstheoretische Phänomen vorstellen, das für die Schwierigkeiten in positiver Charakteristik (in der Desingularisierung wie auch in der Modelltheorie der bewerteten Körper) verantwortlich ist. Schliesslich werde ich eine Art "bewertungstheoretisches Arbeitsprogramm" skizzieren, mit dem man das Problem der Desingularisierung in positiver Charakteristik neu in Angriff nehmen könnte.

[1] Franz-Viktor Kuhlmann: Valuation theoretic and model theoretic aspects of local uniformization, in:
Resolution of Singularities - A Research Textbook in Tribute to Oscar Zariski.
Herwig Hauser, Joseph Lipman, Frans Oort, Adolfo Quiros (eds.), Progress in Mathematics Vol.181, Birkhäuser Verlag Basel (2000), 381-456

[2] Franz-Viktor Kuhlmann und Peter Roquette:
Abhyankar places admit local uniformization in any characteristic, in Vorbereitung
(dvi file) --- (postscript file)

[3] Franz-Viktor Kuhlmann: Every place admits local uniformization in a finite extension of the function field, in Vorbereitung

Canadian Mathematical Society Winter Meeting, Vancouver, December 2000

## On local uniformization in arbitrary characteristic

Abstract:
In 1939, Zariski proved the Local Uniformization Theorem for places of algebraic function fields over ground fields of characteristic 0. Later, he used this theorem to prove resolution of singularities for surfaces in characteristic 0. Apart from Abhyankar's results for dimension up to 3 and de Jong's desingularization by alteration, not much has been known for positive characteristic.

We prove that every place of an algebraic function field F|K of arbitrary characteristic admits local uniformization, provided that the sum of the rational rank of its value group and the transcendence degree of its residue field over K is equal to the transcendence degree of F|K (we call such places Abhyankar places). Further, we show that finitely many such places admit simultaneous local uniformization if they have isomorphic value groups. Since Abhyankar places lie dense in the Zariski space of all places of F|K with respect to the patch topology, simultaneous local uniformization of any finite number of them might open a way to pass from local uniformization to resolution of singularities.

Further, we prove that every place of an algebraic function field F|K of arbitrary characteristic admits local uniformization in a finite extension F' of F. This fact actually follows from de Jong's result. But we can show in addition that F'|F can be chosen to be Galois. Alternatively, F'|F can be chosen to satisfy a valuation theoretical condition which is very natural in positive characteristic. Our proofs are based solely on valuation theoretical theorems, which are of fundamental importance in positive characteristic.

We also indicate certain analogues of our results for the arithmetic case.

[1] Franz-Viktor Kuhlmann: On local uniformization in arbitrary characteristic, The Fields Institute Preprint Series, Toronto (July 1997)
(postscript file)

[2] Franz-Viktor Kuhlmann and Peter Roquette:
Abhyankar places admit local uniformization in any characteristic, in preparation
(dvi file) --- (postscript file)

[3] Franz-Viktor Kuhlmann: Every place admits local uniformization in a finite extension of the function field, in preparation

Model Theory Meeting, Oberwolfach, January 2000

## Additive polynomials and the elementary properties of Fp((t))

The abstract is available as dvi file or as postscript file.

[1] Kuhlmann, F.-V.: Elementary properties of power series fields over finite fields, to appear in J. Symb. Logic; prepublication in: Structures Algebriques Ordonnees, Seminaire Paris VII (1997) (postscript file)

Talk at the conference "Model Theory of Henselian Valued Fields", Edinburgh, May 1999

## Quantifier elimination and Ax-Kochen-Ershov principles for henselian fields

Abstract:
I shall summarize the Ax-Kochen-Ershov principles and the relative quantifier elimination results which are known for henselian valued fields. Ax-Kochen-Ershov principles are known for the following classes:
1. henselian fields of residue characteristic 0,
2. henselian finitely ramified fields (includes formally p-adic fields of fixed p-rank),
3. tame fields (includes algebraically closed fields and, more generally, algebraically maximal Kaplansky fields).
Tame fields are henselian valued fields K for which the ramification field of the extension K^sep|K is algebraically closed; here, K^sep denotes the separable-algebraic closure of K. The model theory of tame fields has nice applications to the theory of places of algebraic function fields in arbitrary characteristic.

All tame fields are perfect. For imperfect valued fields, not many positive results are known; I shall sketch the problems one encounters when dealing with them. Similar problems appear for fields of mixed characteristic which are not finitely ramified, like certain infinite algebraic extensions of the p-adics.

Relative QE in various forms is known for all above mentioned classes of valued fields, except for the tame fields in general. I shall describe a form of QE which was invented by Basarab [1] and refined by myself [3]. The idea is to show QE relative to a structure naturally associated to every valued fields, which carries more information than value group and residue field. I called this structure amc-structure'' since it encodes additive and multiplicative congruences which hold in the field. If O and M denote valuation ring and ideal of K, then the amc-structure (of level 0) is the residue field O/M and the group Kx/(1+M) together with a natural group homomorphism (O/M)x -> Kx/(1+M) whose cokernel is the value group. To cover the case of tame fields (or imperfect fields), one would have to add something which takes care of the structure induced by additive polynomials in the field. At present, there is no solution known for this problem; I shall describe the responsible stumbling block in detail.

For henselian fields of mixed characteristic, one has to use amc-structures of higher level; I shall give their definition. The class of all henselian fields of mixed characteristic admits QE relative to these structures, because they take care of the essential positive characteristic part of the fields. In special cases, one would like to get away with much less complex reducts of the amc-structures. And indeed, the Macintyre power predicates turn out to be such a reduct suitable for the case of the p-adics.

For a summary of QE for the p-adics via cell decomposition (and its use to show the rationality of the Igusa local zeta functions), I recommend [4].

[1] Basarab, S. A.: Relative elimination of quantifiers for Henselian valued fields, Annals of Pure and Applied Logic 53 (1991), 51-74

[2] Basarab, S. A. - Kuhlmann, F.-V.: An isomorphism theorem for henselian algebraic extensions of valued fields, manuscripta mathematica 77 (1992), 113-126
(postscript file) --- [Abstract]

[3] Kuhlmann, F.-V.: Quantifier elimination for henselian fields relative to additive and multiplicative congruences, Israel Journal of Mathematics 85 (1994), 277-306
(postscript file)

[4] Pas, J.: Some applications of uniform p-adic cell decomposition, Asterisque 198-200 (1991), 265-271

AMS Annual Meeting, January 1999, San Antonio, Special Session on Model Theory

## Local uniformization and the model theory of valued fields in positive characteristic

Abstract:
Recently, it has turned out that fundamental results of valuation theory can be used to prove new theorems in the model theory of valued fields as well as about local uniformization of algebraic varieties; cf. [1]. We sketch these results and indicate how they are used to prove Ax-Kochen-Ershov principles via embedding lemmas. We present our results about perfect valued fields of positive characteristic which were obtained in this way ([2]). We apply them to gain new insight into the structure of the Zariski-Riemann manifold, which links local uniformization to desingularization ([3]). Finally, we discuss the relation between desingularization in positive characteristic and the open problem whether the power series field Fp((t)) (which is not a perfect field!) is decidable. We point out specific aspects of the model theoretic problem that do not appear in the local uniformization problem, and vice versa. E.g., getting rid of tame ramification is essential for local uniformization, but not for embedding problems. Vice versa, local uniformization in positive characteristic (if it exists) would have to be generalized (!) in order to give a solution to our embedding problems. Indeed, we need a relative version which works over non-trivially valued ground fields (where the valuation will in general not be discrete!). We also need additional information about the transcendence basis chosen in the uniformization. This demonstrates that the problems from the model theory of non-perfect valued fields lead to the formulation of very interesting generalizations of local uniformization. However, an example originally given to show that the model theory of Fp((t)) is a hard problem ([4]) also sets a limit to these generalizations.

[1] Kuhlmann, F.-V.: Valuation theoretic and model theoretic aspects of local uniformization, to appear in the Proceedings of the Tirol Conference on Resolution of Singularities 1997 (postscript file)

[2] Kuhlmann, F.-V.: The model theory of tame valued fields, in preparation

[3] Kuhlmann, F.-V.: On places of algebraic function fields in arbitrary characteristic, in preparation

[4] Kuhlmann, F.-V.: Elementary properties of power series fields over finite fields, to appear in J. Symb. Logic; prepublication in: Structures Algebriques Ordonnees, Seminaire Paris VII (1997) (postscript file)

AMS Annual Meeting, January 1999, San Antonio, Special Session on Singularities

## A valuation theoretic approach to local uniformization

Abstract:
Using a purely valuation theoretic approach, we have proved a valuative version'' of de Jong's theorem. It states that every place of a function field F|K admits local uniformization on a finite normal extension of F ([2]). Our approach has the following advantages:

1) It allows us to prove local improvements'': for certain places, only a Galois extension is needed. In special (but important) cases, the place admits local uniformization already on F. Furthermore, instead of asking for normal extensions, one can control the extension of the value group and of the residue field associated to the extension of F ([1],[3]). This is important for certain applications, which we describe briefly. We state our results in detail and discuss the open problems and technical difficulties.

2) It connects the problem of local uniformization in positive characteristic to the open problems in the model theory of valued fields (cf. our talk in the model theory section). We sketch the fundamental valuation theoretic results which we have applied to the problem of local uniformization ([1],[2],[3]) as well as to the model theory of valued fields in positive characteristic: the theory of algebraic valuation independence, Kaplansky's theory of immediate extensions of valued fields, the generalized Grauert--Remmert stability theorem, the structure theory of valued function fields (henselian rationality''). We indicate analogues of these results in the recent research connected with desingularization.

Finally, we give the definition of relative local uniformization which works with arbitrary valued field extensions. This constitutes an interesting new notion in valuation theory and a nice tool to prove local uniformization results (because it is transitive). We also discuss its connection to ramification theory.

[1] Kuhlmann, F.-V.: On local uniformization in arbitrary characteristic, The Fields Institute Preprint Series, Toronto (July 1997) (postscript file)

[2] Kuhlmann, F.-V.: On local uniformization in arbitrary characteristic I, submitted (postscript file)

[3] Kuhlmann, F.-V.: On local uniformization in arbitrary characteristic II, preprint, Saskatoon (1998)

Colloquium, Department of Mathematics and Statistics, University of Saskatchewan, Saskatoon, December 1998

## Large Fields

Abstract:
A field K is called "large" if every smooth curve over K has infinitely many K-rational points, provided it has at least one. This notion was introduced by Florian Pop in an Annals paper in 1996. There he deals with problems of inverse Galois theory (a short description will be given in the talk). Among other results, Pop proves a theorem which "approximates" the Shafarevich Conjecture (which states that the absolute Galois group of the maximal cyclotomic extension of the field of rationals is profinite free).

There are several properties of fields which are equivalent to "large". Some of them are tightly connected to my own work of the early 1990's. I state these properties and give an idea of how the equivalence is proved. It turns out that there is a nice relation to properties that origin from model theoretic algebra. These express that K is "existentially closed" in suitable extensions L, that is, every elementary sentence asserting the existence of certain elements will hold in K, provided it holds in L. We show the connection of this notion with the existence of rational points and rational places.

There are many large fields. Basic examples are the algebraically closed, real closed and p-adically closed fields (and then PAC, PRC, PpC fields and fields with universal local-global principles). I explain what "existentially closed" means for the first three examples, by the Nullstellensatz framework.

Finally, I describe a new result about large fields which can be derived in two different ways, either from my results about local uniformization, or from my theory of the space of all (rational) places of an algebraic function field.

Model Theory Meeting, Oberwolfach, October 1998

## Rational place = existentially closed?

Abstract:
Take F|K to be a function field and P a place of F which is trivial on K. If FP=K then P is called a rational place. We say that (F|K,P) is weakly uniformizable if there is a model of F|K on which P is centered at a smooth point. The following is well-known: If K is existentially closed in F, then F|K admits a rational place which is weakly uniformizable. For the converse, one has:

Theorem 1: Assume that K admits a henselian valuation w (or, more generally, that K is a large field''). Assume further that F|K admits a rational place P.
a) If K is perfect, then K is existentially closed in F.
b) If P is weakly uniformizable, then K is existentially closed in F.

Part b) is well-known. Which places are weakly uniformizable? Local uniformization is not known in positive characteristic, but we can give a partial answer:

Theorem 2 (K. 1997):
Rational Abhyankar places of rank 1 and rational discrete places are weakly uniformizable.

We call a place an Abhyankar place if it satisfies equality in the Abhyankar inequality, i.e., if

trdeg F|K = rational rank vP F + trdeg FP|K,

where rational rank vP F is the rational rank of the value group of P.

Theorem 3 (K. 1997):
If K is perfect and P is a rational place of F|K, then there is a finite extension F|F and an extension of P to F such that P is still a rational place of F|K and (F|K,P) is weakly uniformizable.

Since K is existentially closed in F if it is existentially closed in F, this proves part a) of Theorem 1 via part b). For vP denoting the valuation associated with P, and vPow its composition with w, we demonstrate that a certain kind of existential sentences which hold in (F,vPow) will also hold in (K,w) (although in general it cannot be expected that (K,w) be existentially closed in (F,vPow)). To prove this by an embedding lemma, we use a result about the density of certain nice'' places in the space of all rational places of F|K, with respect to an existentially constructible topology''. Part b) of Theorem 1 can be proved in a similar way, also in the case of large fields.