Abstract Non-perfect separably closed fields are stable, and not superstable. As a result, not all types can be ranked. We develop here a new tool, a “semi-rank”, which takes values in the non-negative reals, and gives a sufficient condition for forking of types. This semi-rank is built up from a transcendence function, analogous to the one considered by Kolehin in the context of differentially closed fields. It yields some orthogonality and stratification results.

Abstract We give a complete description of minimal groups infinitely definable in separably closed fields of finite degree of imperfection. In particular we answer positively the question of the existence of such a group with infinite transcendence degree (i.e., a minimal group with non thin generic).

Abstract We consider separably closed fields of characteristic p > 0 and fixed imperfection degree as modules over a skew polynomial ring. We axiomatize the corresponding theory and we show that it is complete and that it admits quantifier elimination in the usual module language augmented with additive functions which are the analog of the p-component functions.

Abstract An Ax-Kochen-Ershov principle for intermediate structures between valued groups and valued fields. We will consider structures that we call valued B-groups and which are of the form 〈G, B, *, υ〉 where – G is an abelian group, – B is an ordered group, – υ is a valuation denned on G taking its values in B, – * is an action of B on G satisfying: ∀x ϵ G ∀ b ∈ B υ(x * b) = ν(x) · b. The analysis of Kaplanski for valued fields can be adapted to our context and allows us to formulate an Ax-Kochen-Ershov principle for valued B-groups: we axiomatise those which are in some sense existentially closed and also obtain many of their model-theoretical properties. Let us mention some applications: 1. Assume that υ(x) = υ(nx) for every integer n ≠ 0 and x ϵ G, B is solvable and acts on G in such a way that, for the induced action, Z[B] ∖ {0} embeds in the automorphism group of G. Then 〈G, B, *, υ〉 is decidable if and only if B is decidable as an ordered group. 2. Given a field k and an ordered group B, we consider the generalised power series field k((B)) endowed with its canonical valuation. We consider also the following structure: where k((B))+ is the additive group of k((B)), S is a unary predicate interpreting {Tb ∣ b ϵB}, and ×↾k((B))×S is the multiplication restricted to k((B)) × S, structure which is a reduct of the valued field k((B)) with its canonical cross section. Then our result implies that if B is solvable and decidable as an ordered group, then M is decidable. 3. A valued B–group has a residual group and our Ax-Kochen-Ershov principle remains valid in the context of expansions of residual group and value group. In particular, by adding a residual order we obtain new examples of solvable ordered groups having a decidable theory.

We prove the undecidability of a very large class of restricted and unrestricted wreath products (Theorem 1.2), and of some skew fields of power series (Section2). Both undecidabilities are obtained by interpreting some enrichments of twisted wreath products, which are themselves proved to be undecidable (Proposition 1.1). We consider division rings of power series in various languages: We show (Theorem 2.8) that every power series division ring k((B)), whose field of constants k is commutative and whose ordered group of exponents is noncommutative with a convex center, is undecidable in every extension of the language of rings where the valuation and the ordered group B are definable. For certain k and B we prove here the undecidability of the structure where X↾k((B))xB is the restriction of the multiplication to k((B)) Χ B,and γ is a given conjugation of k((B)). This shows that we cannot hope to improve our previous result, a sort of Ax-Kochen-Ershov principle for power series division rings, which ensures that is decidable for every decidable solvable B.

Abstract We study the model theory of fields k carrying a henselian valuation with real closed residue field. We give a criteria for elementary equivalence and elementary inclusion of such fields involving the value group of a not necessarily definable valuation. This allows us to translate theories of such fields to theories of ordered abelian groups, and we study the properties of this translation. We also characterize the first-order definable convex subgroups of a given ordered abelian group and prove that the definable real valuation rings of k are in correspondence with the definable convex subgroups of the value group of a certain real valuation of k.

Abstract We study the structures (K ⊂ Kr), where K is an ordered field and Kr its real closure, in the language of ordered fields with an additional unary predicate for the subfield K. Two such structures (K ⊂ Kr) and (L ⊂ Lr) are not necessarily elementary equivalent when K and L are. But with some saturation assumption on K and L, then the two structures become equivalent, and we give a description of the complete theory.

Abstract The theory of immediate pairs of Henselian valued fields, with a given residual theory (of characteristic zero) and a given theory of valuation group (nonzero), is undecidable and has completions.

Abstract A chain-closed field is defined as a chainable field (i.e. a real field such that, for all n ∈ N, ΣK2n+2 ≠ ΣK2n) which does not admit any “faithful” algebraic extension, and can also be seen as a field having a Henselian valuation ν such that the residue field K/ν is real closed and the value group νK is odd divisible with ∣νK/2νK∣ = 2. If K admits only one such valuation, we show that f ∈ K(X) is in ΣK(X)2n for any real algebraic extension L of K,“f(L) ⊆ ΣL2n” holds. The conclusion is also true for K = R((t))(a chainable but not chain-closed field), and in the case n = 1 it holds for several variables and any real field K.

Abstract Baur a défini la notion d'extension séparée de corps valués et montré que toute extension d'un corps maximal est séparée. Nous prouvons que, si (K, υ) est henselien et de caractéristique résiduelle nulle, alors (K, υ) ⊂ (L, w) est séparée ssi L est linéairement disjoint sur K de toute extension immédiate de K. Separated and immediate extensions of valued fields. The notion of separated extension of valued fields was introduced by Baur. He showed that extensions of maximal fields are separated. We prove that, when (K, υ) is Henselian with residual characteristic 0, then (K, υ) ⊂ (L, w) is separated iff L is linearly disjoint over K from each immediate extension of K.

Abstract Consider k((G)) in the language of valued fields enriched with a unary predicate for the set of constants and another one for the cross-section. For perfect k, this structure is undecidable if it does not satisfy Kaplansky's conditions.

Abstract L. van den Dries proved that the theory of n-valued rings has a model companion. We show here that this result is still true when the valuation rings are required to satisfy given inclusion relations (we restrict ourselves to the case of residual characteristic zero).

Abstract C. U. Jensen suggested the following construction, starting from a fieldK: and asked when two fieldsKαandKβare equivalent. We give a complete answer in the case of a fieldKof characteristic 0.

Un espace ultramétrique est un ensemble muni d'une distance à valeurs dans un ordre total avec premier élément et pour laquelle tout triangle est isocèle avex deux grands côtés égaux. Les deux cas importants d'espaces ultramétriques sont: (1) les corps valués lorsqu'on ne considère que leur structure métrique, et (2) les ensembles Aλ, où λ est un ensemble bien ordonné, munis de la distance d(α, β) = inf {γ ∈ λ; α(γ) ≠ β(γ)} si α ≠ β et d(α, β) = 0 sinon; distance à valeurs dans l'ordre inverse de λ enrichi d'un premier élément 0. Nous étudions ces structures dans un langage comportant un seul type de variables, les points de l'espace, et un prédicat à quatre places traduisant l'ordre sur les distances Nous définissons la notion d'espace riche, qui est la modèle-complétion relative à un ensemble des distances fixé: un espace est riche si et seulement s'il est existentiellement clos dans toute extension qui n'ajoute pas de nouvelle distance. Les deux exemples précédemment donnés, espaces Aλ et corps valués, fournissent des espaces riches. La suite de l'article s'attache à la description des espaces riches.