# Use of indiscernibles in model theory

What is the main use of indiscernibles in model theory? reading through Chang and Keisler's Model Theory it seems that the main motivation for indicernibles is for getting many non-isomorphic models for a theory (like the theory of dense linear order without endpoint). Also, can you recommend the best source for reading about indiscernibles and their uses?

• I'm not sure if this is the sort of thing you're interested in, but: a more modern usage of indiscernible sequences is in the study of "nice" (simple, or stable, or . . .) theories via Morley sequences, which are indiscernible sequences satisfying additional properties. For example, in a simple theory, Morley sequences witness interdependencies in a very nice way (see Grossberg, Iovino, Lessman's paper "A Primer of Simple Theories"). – Noah Schweber Dec 24 '10 at 21:28

Eran,

As far as I know, indiscernibility is used in two ways in model theory. One, as you say, is to obtain many non-isomorphic models. This is for sure the classical use of indiscernibility.

Another, more modern one, is to have access to tools such as Ramsey's theorem and its uncountable version, the Erdős-Rado theorem. This is useful in some formulations of stability theory or (more recently, as in the work of Byunghan Kim) of simplicity. The point is that the notions of forking and dividing are cleaner to formulate in the presence of sufficiently indiscernible sequences. (So one typically works in large saturated structures in this context.) There are several modern references for stability, etc, where the use of indiscernibility is apparent, see for example Frank Wagner's "Simple theories", Mathematics and its applications, Kluwer Academic Publishers, 2000.

A third use of indiscernibility is fairly common in set theory, where it is the most common approach to defining the large cardinal notions known as sharps. A good reference for this use is Kanamori's "The higher infinite".

• As Pagnol says, « Ça fait quatre tiers »... :-) – ACL Oct 22 '14 at 18:14
• Well, no, I am presenting a third use of indiscernibles, but not in model theory proper. (Anyway, the other answers illustrate further applications, so one could argue your comment applies after all.) :-) – Andrés E. Caicedo Oct 22 '14 at 19:04
• Andres, your statement that the classical use of inidscernibles in model theory is incorrect. The first theorem of that kind was obtained by Shelah around 1971 (unstable tehory has many models), however indiscernibles were introduced in 1956 by Ehernfeucht & Mostowski to construct models with many automorphisms. Morley in 1962 used them in two crucial steps in his proof of categoricity: To get \aleph_0-stability from uncountable categoricity and as a skeleton for arbitrary large not \aleph_1 saturated models (by taking primary models over indiscernible sequences). – Rami Grossberg Mar 11 '15 at 18:27

Some other classical uses of indisceribles due to Morley:

• In the proof that $\kappa$-categorical theories are $\omega$-stable (for $\kappa\ge\aleph_1$), he constructs a model of size $\kappa$ realizing only countably many types over each countable set by taking a model generated by well ordered indiscernibles.

• If for all $\alpha<\omega_1$ there is a model of size $\beth_\alpha$ omitting a type $p$, then there are arbitrarily large models omitting $p$, or, more generally, if an $L_{\omega_1,\omega}$ sentence has models of size $\beth_\alpha$ for all $\alpha<\omega_1$,then it has arbitrarily large models. These results need the Erd\"os-Rado partition theorem.

A further application of indiscernibles is to show that a consistent first-order theory with infinite models has models with many automorphisms. In particular, every first-order theory $T$ (in a countable vocabulary) possessing an infinite model has a model whose automorphism group has an undecidable first-order theory (in the vocabulary of groups). This result is due to Bludov, Giraudoux, Glass, and Sabbagh.

I believe this result can be extended to to show that every abstract elementary class $A$ which has members of unboundedly large cardinality also has members in every large enough power which possess undecidable automorphism groups. It follows that $A$ has non-rigid models. If $A$ also has rigid models in large enough cardinality, then $A$ is not categorical. This will hold for for classes defined by sentences or theories in some infinitary logics, e.g. $L_{\omega_{1} \omega}$, a most interesting case.

It may be very hard to construct rigid models in an AEC, and these are frequently not absolutely rigid, e.g. above the first $\omega$-Erdos cardinal, their rigidity can be destroyed by forcing. Under $GCH$ or $V=L$, one can neverthless attempt to build rigid models using diamonds $\lozenge_{\kappa}$ to eliminate automorphisms; in this way, the relative consistency of results refuting categoricity conjectures can be approached.

Another important use of indiscernible sequences and trees one can find in chapter VIII of Shelah's book on classification theory. There are many theorems of the form If T is unstable (not super stable) then T has the maximal number of models. Such theorems are necessary to prove "The Main Gap", this technology has also several applications to group theory.

Another family of theorems that uses indiscernible sets and trees are two Cardinal theorems of Vaught, Morley, Shelah and others.

Another use of indiscernibles was made in 1977 by Paris & Harrington to establish that a ceetain version of the finite Ramsey theorem RT (which is true in the stadard model of arithmetic) can be used to construct a model of Peano's Arithmetic, hence by Godel's incompletness theorem RT is not provable from PA.

Indiscernibles are bounds for Skolem functions in all model-theoretic proofs of unprovability.

• Could you say a bit more? – Noah Schweber Sep 9 '19 at 21:13
• It is the same story in set theory and in arithmetic: whatever inputs a Skolem function gets from below one indiscernible --- the value will come before the next indiscernible. You can see it in many proofs from large cardinals, and the same happens in models of arithmetic (indiscernibles carve out a model of arithmetic by making all quantifiers bounded, thus building a model of full PA from an instance of Paris-Harrington). Yes, there are other uses of indiscernibles (e.g. in Shelah's non-structure theory) but in metamathematics and Ramsey theory they always are upper bounds for witnesses. – andrey bovykin Sep 9 '19 at 21:57
• Also sharps, determinacy, self-embeddings of L, Kunen's indiscernibles, Martin-Harrington ---all the same beautiful story. – andrey bovykin Sep 9 '19 at 22:03