93 research outputs found
Definability of groups in -stable metric structures
We prove that in a continuous -stable theory every type-definable
group is definable. The two main ingredients in the proof are:
\begin{enumerate} \item Results concerning Morley ranks (i.e., Cantor-Bendixson
ranks) from \cite{BenYaacov:TopometricSpacesAndPerturbations}, allowing us to
prove the theorem in case the metric is invariant under the group action; and
\item Results concerning the existence of translation-invariant definable
metrics on type-definable groups and the extension of partial definable metrics
to total ones. \end{enumerate
The linear isometry group of the Gurarij space is universal
We give a construction of the Gurarij space, analogous to Katetov's
construction of the Urysohn space. The adaptation of Katetov's technique uses a
generalisation of the Arens-Eells enveloping space to metric space with a
distinguished normed subspace. This allows us to give a positive answer to a
question of Uspenskij, whether the linear isometry group of the Gurarij space
is a universal Polish group
On uniform canonical bases in lattices and other metric structures
We discuss the notion of \emph{uniform canonical bases}, both in an abstract
manner and specifically for the theory of atomless lattices. We also
discuss the connection between the definability of the set of uniform canonical
bases and the existence of the theory of beautiful pairs (i.e., with the finite
cover property), and prove in particular that the set of uniform canonical
bases is definable in algebraically closed metric valued fields
Modular functionals and perturbations of Nakano spaces
We settle several questions regarding the model theory of Nakano spaces left
open by the PhD thesis of Pedro Poitevin \cite{Poitevin:PhD}. We start by
studying isometric Banach lattice embeddings of Nakano spaces, showing that in
dimension two and above such embeddings have a particularly simple and rigid
form. We use this to show show that in the Banach lattice language the modular
functional is definable and that complete theories of atomless Nakano spaces
are model complete. We also show that up to arbitrarily small perturbations of
the exponent Nakano spaces are -categorical and -stable. In
particular they are stable
On Roeckle-precompact Polish group which cannot act transitively on a complete metric space
We study when a continuous isometric action of a Polish group on a complete
metric space is, or can be, transitive. Our main results consist of showing
that certain Polish groups, namely and
, such an action can never be transitive (unless the
space acted upon is a singleton). We also point out "circumstantial evidence"
that this pathology could be related to that of Polish groups which are not
closed permutation groups and yet have discrete uniform distance, and give a
general characterisation of continuous isometric action of a Roeckle-precompact
Polish group on a complete metric space is transitive. It follows that the
morphism from a Roeckle-precompact Polish group to its Bohr compactification is
surjective
Continuous and Random Vapnik-Chervonenkis Classes
We show that if is a dependent theory then so is its Keisler
randomisation . In order to do this we generalise the notion of a
Vapnik-Chervonenkis class to families of -valued functions (a
\emph{continuous} Vapnik-Chervonenkis class), and we characterise families of
functions having this property via the growth rate of the mean width of an
associated family of convex compacts
Model theoretic stability and definability of types, after A. Grothendieck
We point out how the "Fundamental Theorem of Stability Theory", namely the
equivalence between the "non order property" and definability of types, proved
by Shelah in the 1970s, is in fact an immediate consequence of Grothendieck's
"Crit{\`e}res de compacit{\'e}" from 1952. The familiar forms for the defining
formulae then follow using Mazur's Lemma regarding weak convergence in Banach
spaces
Lipschitz functions on topometric spaces
We study functions on topometric spaces which are both (metrically) Lipschitz
and (topologically) continuous, using them in contexts where, in classical
topology, ordinary continuous functions are used. We study the relations of
such functions with topometric versions of classical separation axioms, namely,
normality and complete regularity, as well as with completions of topometric
spaces. We also recover a compact topometric space from the lattice of
continuous -Lipschitz functions on , in analogy with the recovery of a
compact topological space from the structure of (real or complex) functions
on
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