47 research outputs found
Randomizations of models as metric structures
The notion of a randomization of a first order structure was introduced by
Keisler in the paper Randomizing a Model, Advances in Math. 1999. The idea was
to form a new structure whose elements are random elements of the original
first order structure. In this paper we treat randomizations as continuous
structures in the sense of Ben Yaacov and Usvyatsov. In this setting, the
earlier results show that the randomization of a complete first order theory is
a complete theory in continuous logic that admits elimination of quantifiers
and has a natural set of axioms. We show that the randomization operation
preserves the properties of being omega-categorical, omega-stable, and stable
On perturbations of Hilbert spaces and probability algebras with a generic automorphism
International audienceWe prove that , the theory of infinite dimensional Hilbert spaces equipped with a generic automorphism, is -stable up to perturbation of the automorphism, and admits prime models up to perturbation over any set. Similarly, , the theory of atomless probability algebras equipped with a generic automorphism is -stable up to perturbation. However, not allowing perturbation it is not even superstable
Toward classifying unstable theories
The paper deals with two issues: the existence of universal models of a
theory T and related properties when cardinal arithmetic does not give this
existence offhand. In the first section we prove that simple theories (e.g.,
theories without the tree property, a class properly containing the stable
theories) behaves ``better'' than theories with the strict order property, by
criterion from [Sh:457]. In the second section we introduce properties SOP_n
such that the strict order property implies SOP_{n+1}, which implies SOP_n,
which in turn implies the tree property. Now SOP_4 already implies
non-existence of universal models in cases where earlier the strict order
property was needed, and SOP_3 implies maximality in the Keisler order, again
improving an earlier result which had used the strict order property