A note on computable real fields

1970 ◽  
Vol 35 (2) ◽  
pp. 239-241 ◽  
Author(s):  
E. W. Madison
Keyword(s):  
Algebraic Closure ◽  
Algebraic Extension ◽  
Real Field ◽  
Ordered Fields ◽  
Real Closure ◽  
Carry Over

It is well known that every field (formally, real field ) has an algebraic closure (real-closure ). This is to say is an algebraic extension of which is algebraically closed (real-closed). Of course, certain properties of carry over to . In particular, M. O. Rabin has proved in [3] that the algebraic closure—which is of course unique up to isomorphism—of a computable field is computable. The purpose of this note is to establish an analogue of Rabin's theorem for formally real fields. It is clear that a direct analogue can be formulated only in the case of ordered fields, for otherwise there may be many (nonisomorphic) such .

scholarly journals Ordered fields dense in their real closure and definable convex valuations

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Lothar Sebastian Krapp ◽  
Salma Kuhlmann ◽  
Gabriel Lehéricy
Keyword(s):  
Ordered Fields ◽  
Real Closure

Abstract In this paper, we undertake a systematic model- and valuation-theoretic study of the class of ordered fields which are dense in their real closure. We apply this study to determine definable henselian valuations on ordered fields, in the language of ordered rings. In light of our results, we re-examine the Shelah–Hasson Conjecture (specialized to ordered fields) and provide an example limiting its valuation-theoretic conclusions.

scholarly journals DECIDABLE ALGEBRAIC FIELDS

2017 ◽  
Vol 82 (2) ◽  
pp. 474-488
Author(s):  
MOSHE JARDEN ◽  
ALEXANDRA SHLAPENTOKH
Keyword(s):  
Positive Integer ◽  
Elementary Theory ◽  
Algebraic Closure ◽  
Profinite Group ◽  
Algebraic Extension ◽  
Existential Theory ◽  
Free Profinite Group ◽  
Image Position ◽  
Algebraic Fields ◽  
Primitive Recursive

AbstractWe discuss the connection between decidability of a theory of a large algebraic extensions of ${\Bbb Q}$ and the recursiveness of the field as a subset of a fixed algebraic closure. In particular, we prove that if an algebraic extension K of ${\Bbb Q}$ has a decidable existential theory, then within any fixed algebraic closure $\widetilde{\Bbb Q}$ of ${\Bbb Q}$, the field K must be conjugate over ${\Bbb Q}$ to a field which is recursive as a subset of the algebraic closure. We also show that for each positive integer e there are infinitely many e-tuples $\sigma \in {\text{Gal}}\left( {\Bbb Q} \right)^e $ such that the field $\widetilde{\Bbb Q}\left( \sigma \right)$ is primitive recursive in $\widetilde{\Bbb Q}$ and its elementary theory is primitive recursively decidable. Moreover, $\widetilde{\Bbb Q}\left( \sigma \right)$ is PAC and ${\text{Gal}}\left( {\widetilde{\Bbb Q}\left( \sigma \right)} \right)$ is isomorphic to the free profinite group on e generators.

Pfaffian differential equations over exponential o-minimal structures

2002 ◽  
Vol 67 (1) ◽  
pp. 438-448 ◽  
Author(s):  
Chris Miller ◽  
Patrick Speissegger
Keyword(s):  
Asymptotic Behavior ◽  
Differential Equations ◽  
Asymptotic Integration ◽  
Real Field ◽  
Asymptotic Behavior Of Solutions ◽  
The Real ◽  
Unary Function ◽  
Ordered Fields ◽  
Component Function

In this paper, we continue investigations into the asymptotic behavior of solutions of differential equations over o-minimal structures.Let ℜ be an expansion of the real field (ℝ, +, ·).A differentiable map F = (F1,…, F1): (a, b) → ℝi is ℜ-Pfaffian if there exists G: ℝ1+l → ℝl definable in ℜ such that F′(t) = G(t, F(t)) for all t ∈ (a, b) and each component function Gi: ℝ1+l → ℝ is independent of the last l − i variables (i = 1, …, l). If ℜ is o-minimal and F: (a, b) → ℝl is ℜ-Pfaffian, then (ℜ, F) is o-minimal (Proposition 7). We say that F: ℝ → ℝl is ultimately ℜ-Pfaffian if there exists r ∈ ℝ such that the restriction F ↾(r, ∞) is ℜ-Pfaffian. (In general, ultimately abbreviates “for all sufficiently large positive arguments”.)The structure ℜ is closed under asymptotic integration if for each ultimately non-zero unary (that is, ℝ → ℝ) function f definable in ℜ there is an ultimately differentiable unary function g definable in ℜ such that limt→+∞[g′(t)/f(t)] = 1- If ℜ is closed under asymptotic integration, then ℜ is o-minimal and defines ex: ℝ → ℝ (Proposition 2).Note that the above definitions make sense for expansions of arbitrary ordered fields.

Counting points of given height that generate a quadratic extension of a function field

2015 ◽  
Vol 11 (02) ◽  
pp. 569-592 ◽  
Author(s):  
David Kettlestrings ◽  
Jeffrey Lin Thunder
Keyword(s):  
Finite Field ◽  
Projective Space ◽  
Function Field ◽  
Asymptotic Estimate ◽  
Algebraic Closure ◽  
Quadratic Extension ◽  
Rational Functions ◽  
Rational Numbers ◽  
Algebraic Extension ◽  
Counting Points

Let K be a finite algebraic extension of the field of rational functions in one indeterminate over a finite field and let [Formula: see text] denote an algebraic closure of K. We count points in projective space [Formula: see text] with given height and generating a quadratic extension of K. If n > 2, we derive an asymptotic estimate for the number of such points as the height tends to infinity. Such estimates are analogous to previous results of Schmidt where the field K is replaced by the field of rational numbers ℚ.

XVIIème problème de Hilbert sur les corps chaîne-clos

1991 ◽  
Vol 56 (3) ◽  
pp. 853-861
Author(s):  
Françoise Delon et Danielle Gondard
Keyword(s):  
Residue Field ◽  
Algebraic Extension ◽  
Real Field ◽  
Closed Field ◽  
Several Variables ◽  
A Chain ◽  
Henselian Valuation

AbstractA 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.

W-Groups under Quadratic Extensions of Fields

2000 ◽  
Vol 52 (4) ◽  
pp. 833-848 ◽  
Author(s):  
Ján Mináč ◽  
Tara L. Smith
Keyword(s):  
General Setting ◽  
Quadratic Extension ◽  
Field Extension ◽  
Galois Groups ◽  
Real Field ◽  
Quadratic Extensions ◽  
Finite Subgroups ◽  
Carry Over ◽  
Rigid Element ◽  
Pythagorean Field

AbstractTo each field F of characteristic not 2, one can associate a certain Galois group , the so-called W-group of F, which carries essentially the same information as the Witt ring W(F) of F. In this paperwe investigate the connection between and (√a), where F(√a) is a proper quadratic extension of F. We obtain a precise description in the case when F is a pythagorean formally real field and a = −1, and show that the W-group of a proper field extension K/F is a subgroup of the W-group of F if and only if F is a formally real pythagorean field and K = F(√−1). This theorem can be viewed as an analogue of the classical Artin-Schreier’s theorem describing fields fixed by finite subgroups of absolute Galois groups. We also obtain precise results in the case when a is a double-rigid element in F. Some of these results carry over to the general setting.

Galois Theory of Essential Extensions of Modules

1972 ◽  
Vol 24 (4) ◽  
pp. 573-579 ◽  
Author(s):  
Sylvia Wiegand
Keyword(s):  
Galois Theory ◽  
Algebraic Closure ◽  
Essential Extension ◽  
Algebraic Extension ◽  
Injective Hull ◽  
Algebraic Systems ◽  
Image Position ◽  
Essential Extensions

The purpose of this paper is to exploit an analogy between algebraic extensions of fields and essential extensions of modules, in which the role of the algebraic closure of a field F is played by the injective hull H(M) of a unitary left R-module M. (The notion of * ‘algebraic’ extensions of general algebraic systems has been studied by Shoda; see, for example [5].)In this analogy, the role of a polynomial p(x) is played by a homomorphism of R-modules(1)which will be called an ideal homomorphism into M. The process of solving the equation p(x) = 0 in F, or in an algebraic extension of F, will be replaced by the process of extending an ideal homomorphism (1) to a homomorphism F* from R into M, or into an essential extension of M.

Elementary properties of the Boolean hull and reduced quotient functors

2003 ◽  
Vol 68 (3) ◽  
pp. 946-971 ◽  
Author(s):  
M. A. Dickmann ◽  
F. Miraglia
Keyword(s):  
Quadratic Form ◽  
Algebraic Closure ◽  
Reflection Principle ◽  
Negative Answer ◽  
Real Field ◽  
Special Group ◽  
Natural Embedding ◽  
First Order ◽  
Special Groups

In [12] we proved the following isotropy-reflection principle:Theorem. Let F be a formally real field and let Fp denote its Pythagorean closure. The natural embedding of reduced special groups from Gred(F) into Gred(Fp) = G(FP) induced by the inclusion of fields, reflects isotropy.Here Gred(F) denotes the reduced special group (with underlying group Ḟ/ΣḞ2) associated to the field F, henceforth assumed formally real; cf. [11], Chapter 1, §3, for details.The result proved in [12] is, in fact, more general. For example, the Pythagorean closure Fp can be replaced in the statement above by the intersection of all real closures of F (inside a fixed algebraic closure). Similar statements hold, more generally, for all relative Pythagorean closures of F in the sense of Becker [3], Chapter II, §3.Since the notion of isotropy of a quadratic form can be expressed by a first-order formula in the natural language LSG for special groups (with the coefficients as parameters), this result raises the question whether the embedding ιFFp: Gred(F) ↪ G (Fp) is elementary. Further, since the LSG-formula expressing isotropy is positive-existential, one may also ask whether ιFFp reflects all (closed) formulas ofthat kind with parameters in Gred(F).In this paper we give a negative answer to the first of these questions, for a vast class of formally real (non-Pythagorean) fields F (Prop. 5.1). This follows from rather general preservation results concerning the “Boolean hull” and the “reduced quotient” operations on special groups.

RATIONAL JORDAN DECOMPOSITION OF BILINEAR FORMS

2005 ◽  
Vol 07 (06) ◽  
pp. 769-786 ◽  
Author(s):  
DRAGOMIR Ž. ĐOKOVIĆ ◽  
KAIMING ZHAO
Keyword(s):  
Vector Space ◽  
Algebraic Closure ◽  
Real Field ◽  
Closed Field ◽  
Bilinear Forms ◽  
Dimensional Vector ◽  
Jordan Decomposition ◽  
Dimensional Vector Space ◽  
Closed Fields ◽  
Algebraically Closed Fields

This is a continuation of our previous work on Jordan decomposition of bilinear forms over algebraically closed fields of characteristic 0. In this note, we study Jordan decomposition of bilinear forms over any field K0 of characteristic 0. Let V0 be an n-dimensional vector space over K0. Denote by [Formula: see text] the space of bilinear forms f : V0 × V0 → K0. We say that f = g + h, where f, g, [Formula: see text], is a rational Jordan decomposition of f if, after extending the field K0 to an algebraic closure K, we obtain a Jordan decomposition over K. By using the Galois group of K/K0, we prove the existence of rational Jordan decompositions and describe a method for constructing all such decompositions. Several illustrative examples of rational Jordan decompositions of bilinear forms are included. We also show how to classify the unimodular congruence classes of bilinear forms over an algebraically closed field of characteristic different from 2 and over the real field.

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