Theory Of Real Closed Fields Research Articles

Three techniques for obtaining algebraic circle packings

The main purpose of this article is to demonstrate three techniques for proving algebraicity statements about circle packings. We give proofs of three related theorems: (1) that every finite simple planar graph is the contact graph of a circle packing on the Riemann sphere, equivalently in the complex plane, all of whose tangency points, centers, and radii are algebraic, (2) that every flat conformal torus which admits a circle packing whose contact graph triangulates the torus has algebraic modulus, and (3) that if R is a compact Riemann surface of genus at least 2, having constant curvature -1, which admits a circle packing whose contact graph triangulates R, then R is isomorphic to the quotient of the hyperbolic plane by a subgroup of PSL_2(real algebraic numbers). The statement (1) is original, while (2) and (3) have been previously proved in the Ph.D. thesis of McCaughan. Our first proof technique is to apply Tarski's Theorem, a result from model theory, which says that if an elementary statement in the theory of real-closed fields is true over one real-closed field, then it is true over any real closed field. This technique works to prove (1) and (2). Our second proof technique is via an algebraicity result of Thurston on finite co-volume discrete subgroups of the orientation-preserving-isometry group of hyperbolic 3-space. This technique works to prove (1). Our first and second techniques had not previously been applied in this area. Our third and final technique is via a lemma in real algebraic geometry, and was previously used by McCaughan to prove (2) and (3). We show that in fact it may be used to prove (1) as well.

The Semi-Algebraic Theory of Stochastic Games

The asymptotic behavior of the min-max value of a finite-state zero-sum discounted stochastic game, as the discount rate approaches 0, has been studied in the past using the theory of real-closed fields. We use the theory of semi-algebraic sets and mappings to prove some asymptotic properties of the min-max value, which hold uniformly for all stochastic games in which the number of states and players' actions are predetermined to some fixed values. As a corollary, we prove a uniform polynomial convergence rate of the value of the N-stage game to the value of the nondiscount game, over a bounded set of payoffs.

Axiomatisations simples des theories des corps de rolle

Using Becker's results we obtain here a simple first order axiomatization, looking like those by Artin-Schreier and also written in the language of fields, for the theory of Rolle fields (i.e. fields with the Rolle's property for every order). In fields having a finite number of orders, we characterize Rolle fields as those which are pythagorean at level 4 and do not admit any algebraic extension of odd degree. Then we give an axiomatization for Rolle fields having exactly 2n orders (n≥0); in fact, for n=0 we recover an axiomatization of the theory of real-closed fields and for n=1 we get exactly an axiomatization given for the theory of chain-closed fields by the author in [G1]. Finally we prove that a Rolle field with exactly 2n orders is the intersection of n+1 real closures of the field.