36 research outputs found

### On a question of Abraham Robinson's

In this note we give a negative answer to Abraham Robinson's question whether
a finitely generated extension of an undecidable field is always undecidable.
We construct 'natural' undecidable fields of transcendence degree 1 over Q all
of whose proper finite extensions are decidable. We also construct undecidable
algebraic extensions of Q that allow decidable finite extensions

### On the "Section Conjecture" in anabelian geometry

Let X be a smooth projective curve of genus >1 over a field K which is
finitely generated over the rationals. The section conjecture in Grothendieck's
anabelian geometry says that the sections of the canonical projection from the
arithmetic fundamental group of X onto the absolute Galois group of K are (up
to conjugation) in one-to-one correspondence with K-rational points of X. The
birational variant conjectures a similar correspondence where the fundamental
group is replaced by the absolute Galois group of the function field K(X).
The present paper proves the birational section conjecture for all X when K
is replaced e.g. by the field of p-adic numbers. It disproves both conjectures
for the field of real or p-adic algebraic numbers. And it gives a purely group
theoretic characterization of the sections induced by K-rational points of X in
the birational setting over almost arbitrary fields.
As a biproduct we obtain Galois theoretic criteria for radical solvability of
polynomial equations in more than one variable, and for a field to be PAC, to
be large, or to be Hilbertian.Comment: 21 pages, late

### An existential 0-definition of F_q[[t]] in F_q((t))

We show that the valuation ring F_q[[t]] in the local field F_q((t)) is
existentially definable in the language of rings with no parameters. The method
is to use the definition of the henselian topology following the work of
Prestel-Ziegler to give an existential-F_q-definable bounded neighbouhood of 0.
Then we `tweak' this set by subtracting, taking roots, and applying Hensel's
Lemma in order to find an existential-F_q-definable subset of F_q[[t]] which
contains tF_q[[t]]. Finally, we use the fact that F_q is defined by the formula
x^q-x=0 to extend the definition to the whole of F_q[[t]] and to rid the
definition of parameters.
Several extensions of the theorem are obtained, notably an existential
0-definition of the valuation ring of a non-trivial valuation with divisible
value group.Comment: 9 page

### Definable henselian valuations

In this note we investigate the question whether a henselian valued field
carries a non-trivial 0-definable henselian valuation (in the language of
rings). It follows from the work of Prestel and Ziegler that there are
henselian valued fields which do not admit a 0-definable non-trivial henselian
valuation. We give conditions on the residue field which ensure the existence
of a parameter-free definiton. In particular, we show that a henselian valued
field admits a non-trivial 0-definable valuation when the residue field is
separably closed or sufficiently non-henselian, or when the absolute Galois
group of the (residue) field is non-universal.Comment: 14 pages, revised versio

### Free product of absolute Galois groups

The free profinite product of finitely many absolute Galois group is an
absolute Galois group

### Recovering p-adic valuations from pro-p Galois groups

Let (Formula presented.) be a field with (Formula presented.), where (Formula presented.) denotes the maximal pro-2 quotient of the absolute Galois group of a field (Formula presented.). We prove that then (Formula presented.) admits a (non-trivial) valuation (Formula presented.) which is 2-henselian and has residue field (Formula presented.). Furthermore, (Formula presented.) is a minimal positive element in the value group (Formula presented.) and (Formula presented.). This forms the first positive result on a more general conjecture about recovering (Formula presented.) -adic valuations from pro- (Formula presented.) Galois groups which we formulate precisely. As an application, we show how this result can be used to easily obtain number-theoretic information, by giving an independent proof of a strong version of the birational section conjecture for smooth, complete curves (Formula presented.) over (Formula presented.), as well as an analogue for varieties