Séminaires : Structures algébriques ordonnées

In this talk we discuss Positivstellensätze in the general context of a unital, commutative, not necessarily finitely generated, real algebra. We focus on the dual problem, and give an introduction to (real) infinite dimensional moment problems (i.e. when measures are supported on real infinite dimensional spaces). We will focus on the following problem: when can a linear functional on a unital commutative real algebra A be represented as an integral w.r.t. a Radon measure on the real character space X(A) equipped with the Borel σ-algebra generated by the weak topology? Our main idea is to construct X(A) as a projective limit of the character spaces of all finitely generated subalgebras of A, to be able to exploit the classical finite dimensional moment theory in the infinite dimensional case. We thus obtain existence results for representing measures defined on the cylinder σ-algebra on X(A), carried by the projective limit construction. If in addition the well-known Prokhorov (ε-K) condition is fulfilled, then we can solve our problem by extending such representing measures from the cylinder to the Borel σ-algebra on X(A). These results allow us to establish e.g. infinite dimensional analogues of the classical Riesz-Haviland.

The purpose is to begin to generalize the theory developed in [DM] (see below) to diagonal quadratic forms with non zero-divisor coefficients in reduced unitary pre-ordered commutative rings. We present:
1. Horn-geometric axioms entailing the equivalence between ring-theoretic isometry and representation by this class of forms is faithfully coded by the corresponding concepts in an associated reduced special group.
A preordered ring, <A, T>, satisying these axioms is called NT-faithfully quadratic (N for "multiplicative set of non zero-divisors in A").
2. A natural correspondence between rings that satisfy the axioms in (1) above and faithfully quadratic rings in the sense of [DM].
3. We prove that any reduced f-ring satisfies the axioms in (1) and, in fact, any preordered f-ring, <A,T>, where T is a preorder satisfying a certain weak cancelation property, verifies the axioms mentioned in item (1). In particular, this applies to real closed rings and rings of continuous functions (bounded or not) over any completely regular topological space.
Joint work with M. Dickmann (IMJ-PRG) and Hugo Ribeiro (USP).
[DM] M. Dickmann, F. Miraglia "Faithfully Quadratic Rings(Memoirs of the AMS 1128, november 2015).

On introduira les relations de divisibilité locale ainsi que ces propriétés élémentaires.  Par la suite on démontrera que la théorie des f-anneaux réels clos, sc-reguliers, projetables, divisible-projetables ayant la premiere propriété de convexité, sans idempotents minimaux est modèle complète en utilisant la relation de divisibilité locale ainsi que la relation radicale associée au spectre des idéaux premiers minimaux. 

In formally real fields K sums of powers with arbitrary even exponents can be
studied by invoking the compact space M = M (K) of all real places, i.e. certain
maps λ : K → P 1 := R ∪ ∞, and the natural representation K → C(M, P 1 ), a 7→ â
based on the evaluation maps â : λ 7→ λ(a). The real holomorphy H = H(K) is
defined as the subring of K consisting of all elements a admitting a finite evaluation
map â : M → R. One is further led to study the series of representations
S n (H) → C(M, S n ), n ∈ N.
In the case of a real (=formally real) function field F over R this general approach
will be substantiated by relating M and H to the family of all smooth models of
F with a compact real locus X which is a real algebraic set and a C ∞ -manifold.
The investigation of line bundles on those manifolds leads to information on the
invertible ideals of H, recent results on continuous rational maps help to study
the image of S n (H) in C(M, S n ). All these informations can be used to derive
qualitative and quantitative results on the representation of an element as a sum of
powers with a given even exponent. Purely algebraic proofs for results of this type
have not been found so far.