Tag Archives: stability

Model theoretic stability and definability of types, after A. Grothendieck

It had happened more than once that combinatorial model theoretic dividing lines introduced by Shelah were invented independently in different fields of mathematics. This curious note by Itai Ben Yaacov gives another example of this phenomenon:

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 “Critères de compacité” from 1952. The familiar forms for the defining formulae then follow using Mazur’s Lemma regarding weak convergence in Banach spaces.

In a meeting in Kolkata in January 2013, the author asked the audience who had first defined the notion
of a stable formula and when, and to the expected answer replied that, no, it had been Grothendieck, in
the fifties.

MALOA “final” conference in Luminy

I had just attended the final MALOA conference “Logic and interactions“.  MALOA was a European network, which is now essentially finished (though the “final” meeting was not the last one, there will be a workshop in Manchester soon). The meeting took place at CIRM in Luminy, a wonderful place not only for mathematical reasons:

Luminy(you can see some more photos here).

As MALOA was about logic, rather than model theory, the topics of the talks were quite diverse, ranging from purely algebraic model theory (definable valuations and height bound in arithmetic nullstellensatz) to proof theory and even “logical description for behaviour analysis in aerospace systems”. Not sure how productive this diversity is, but at least it is entertaining.

Some talks that I found of particular interest are:

  • Talks by Deirdre Haskell  and Chris Laskowski on NIP, VC-density and connections to probability and combinatorics (in general one could safely add to this list some more subjects including set theory). These all are quite fascinating topics which deserve some postings in the future. There are already some examples of importing ideas from combinatorics (e.g. the beautiful (p,q)-theorem of Alon, Kleitman and Matousek) to prove model-theoretic results (e.g. UDTFS for NIP theories), but I believe that many more connections remain to be discovered.
  • Todor Tsankov spoke about generalizations of de Finetti’s theorem. Classical de Finetti’s theorem from probability theory says that a sequence of random variables is exchangeable if and only if it is independent and identically distributed over its tail sigma-algebra. Various multi-dimensional generalizations of this characterization form the so-called exchangeability theory. This theorem can viewed as providing a classification of all probability measures on 2^{\mathbb{N}} invariant under the action of S_{\infty}. Now, in a general situation, given a permutation group G acting on a countable set M, one can’t really hope to give any kind of “classification” of G-invariant measures on 2^{\mathbb{N}} as we are in the context of the general ergodic theory. However, it appears that if the group G is sufficiently large compared to the index set, one can arrive at stronger results. Todor’s approach is to consider oligomorphic groups, i.e. such that the action of G on M^n has only finitely many orbits for each n. These groups are familiar to model theorists as automorphism groups of \omega-categorical structures. Todor provides a classification in the case when the underlying structure has trivial algebraic closure, and gives some promising partial results in the general case. In fact, this subject appears to have a lot to do with model theory. I am involved in a project together with Itai Ben Yaacov, of an abstract model theoretic approach to de Finetti’s theory in terms of the forking calculus, canonical bases and Morley sequences in the context of an arbitrary stable first-order theory, in the sense of continuous logic (which specializes to the classical case considering the theory of [0, 1]-valued random variables equipped with the L^1 metric).

Also I gave what was probably my last talk as a “student”. I spoke about some new results with Pierre Simon and Anand Pillay concerning definable topological dynamics in NIP theories. The slides are available here. We show that notions like definable (extreme) amenability of a definable group, as well as various model theoretic components, are not affected by adding externally definable sets to the picture (that is, passing to a Shelah’s expansion of a model). These facts appear to have some applications to the questions of Newelski on describing G/G^{00} in terms of the so-called Ellis group.