Stable theory

In the mathematical field of model theory, a complete theory is called stable if it does not have too many types. One goal of classification theory is to divide all complete theories into those whose models can be classified and those whose models are too complicated to classify, and to classify all models in the cases where this can be done. Roughly speaking, if a theory is not stable then its models are too complicated and numerous to classify, while if a theory is stable there might be some hope of classifying its models, especially if the theory is superstable or totally transcendental.

Stability theory was started by , who introduced several of the fundamental concepts, such as totally transcendental theories and the Morley rank.
Stable and superstable theories were first introduced by , who is responsible for much of the development of stability theory. The definitive reference for stability theory is , though it is notoriously hard even for experts to read, as mentioned, e.g., in .

Definitions

''T'' will be a complete theory in some language.
*''T'' is called ''κ''-stable (for an infinite cardinal ''κ'') if for every set ''A'' of cardinality ''κ'' the set of complete types over ''A'' has cardinality ''κ''.
*ω-stable is an alternative name for ℵ₀-stable.
*''T'' is called stable if it is ''κ''-stable for some infinite cardinal ''κ''.
*''T'' is called unstable if it is not ''κ''-stable for any infinite cardinal ''κ''.
*''T'' is called superstable if it is ''κ''-stable for all sufficiently large cardinals ''κ''.
*Totally transcendental theories are those such that every formula has Morley rank less than ∞.

As usual, a model of some language is said to have one of these properties if the complete theory of the model has that property.

An incomplete theory is defined to have one of these properties if every completion, or equivalently every model, has this property.

Unstable theories

Roughly speaking, a theory is unstable if one can use it to encode the ordered set of natural numbers. More precisely, Saharon Shelah's ''unstable formula theorem'' in model theory characterizes the unstable theories by the nonexistence of countably infinite half graphs. Shelah defines a complete theory as having the ''order property'' if there exist a model $M$ of the theory, a formula $\phi(\bar x, \bar y)$ on two finite tuples of free variables $\bar x$ and $\bar y$, and, a system of countably many values $\bar x_i$ and $\bar y_i$ for these variables such that the pairs $\bigl\$ form the edges of a countable half graph on vertices $\bar x_i$ and $\bar y_i$. Intuitively, the existence of these half graphs allows one to construct the comparison operation of an infinite ordered set within the model, via the equivalence $(i\le j) \Leftrightarrow \bigl(M\models\phi(\bar x_i,\bar y_j)\bigr)$. The unstable formula theorem of states that a complete theory is unstable if and only if it has the order property.

The number of models of an unstable theory ''T'' of any uncountable cardinality ''κ'' ≥ , ''T'', is the maximum possible number 2^''κ''.

Examples:
*Most sufficiently complicated theories, such as set theories and Peano arithmetic, are unstable.
*The theory of the rational numbers, considered as an ordered set, is unstable. Its theory is the theory of dense total orders without endpoints. More generally, the theory of every infinite total order is unstable.
*The theory of addition of the natural numbers is unstable.
*Any infinite Boolean algebra is unstable.
*Any monoid with cancellation that is not a group is unstable, because if ''a'' is an element that is not a unit then the powers of ''a'' form an infinite totally ordered set under the relation of divisibility. For a similar reason any integral domain that is not a field is unstable.
*There are many unstable nilpotent groups. One example is the infinite dimensional Heisenberg group over the integers: this is generated by elements ''x''_''i'', ''y''_''i'', ''z'' for all natural numbers ''i'', with the relations that any of these two generators commute except that ''x''_''i'' and ''y''_''i'' have commutator ''z'' for any ''i''. If ''a''_''i'' is the element ''x''₀''x''₁...''x''_''i''−1''y''_''i'' then ''a''_''i'' and ''a''_''j'' have commutator ''z'' exactly when ''i'' < ''j'', so they form an infinite total order under a definable relation, so the group is unstable.
*Real closed fields are unstable, as they are infinite and have a definable total order.

Stable theories

''T'' is called stable if it is ''κ''-stable for some cardinal ''κ''.
Examples:
*The theory of any module (mathematics), module over a ring (mathematics), ring is stable.
*The theory of a countable number of equivalence relations, (''E''_''n'')_''n''∈N, such that each equivalence relation has an infinite number of equivalence classes and each equivalence class of ''E''_''n'' is the union of an infinite number of different classes of ''E''_''n''+1 is stable but not superstable.
* showed that free groups, and more generally torsion-free group, torsion-free hyperbolic groups, are stable. Free groups on more than one generator are not superstable.
*A differentially closed field is stable. If it has non-zero characteristic of a field, characteristic it is not superstable, and if it has zero characteristic it is totally transcendental.

Superstable theories

''T'' is called superstable if it is stable for all sufficiently large cardinals, so all superstable theories are stable. For countable ''T'', superstability is equivalent to stability for all ''κ'' ≥ 2^ω.
The following conditions on a theory ''T'' are equivalent:
*''T'' is superstable.
*All types of ''T'' are ranked by at least one notion of rank.
*''T'' is ''κ''-stable for all sufficiently large cardinals ''κ''
*''T'' is ''κ''-stable for all cardinals ''κ'' that are at least 2^{, ''T'',} .

If a theory is superstable but not totally transcendental it is called strictly superstable.

The number of countable models of a countable superstable theory must be 1, ℵ₀, ℵ₁, or 2^ω. If the number of models is 1 the theory is totally transcendental. There are examples with 1, ℵ₀ or 2^ω models, and it is not known if there are examples with ℵ₁ models if the continuum hypothesis does not hold. If a theory ''T'' is not superstable then the number of models of cardinality ''κ'' > , ''T'', is 2^''κ''.

Examples:
*The additive group of integers is superstable, but not totally transcendental. It has 2^ω countable models.
*The theory with a countable number of unary relations ''P''_''i'' with model the positive integers where ''P''_''i''(''n'') is interpreted as saying ''n'' is divisible by the ''i''th prime is superstable but not totally transcendental.
*An abelian group ''A'' is superstable if and only if there are only finitely many pairs (''p'',''n'') with ''p'' prime, ''n'' a natural number, with ''p''^''n''''A''/''p''^''n''+1''A'' infinite.

Totally transcendental theories and ω-stable

*Totally transcendental theories are those such that every formula has Morley rank less than ∞. Totally transcendental theories are stable in ''λ'' whenever λ ≥ , ''T'', , so they are always superstable. ω-stable is an alternative name for ℵ₀-stable. The ω-stable theories in a countable language are ''κ''-stable for all infinite cardinals ''κ''. If , ''T'', is countable then ''T'' is totally transcendental if and only if it is ω-stable. More generally, ''T'' is totally transcendental if and only if every restriction of ''T'' to a countable language is ω-stable.

Examples:
*Any ω-stable theory is totally transcendental.
*Any finite model is totally transcendental.
*An infinite field is totally transcendental if and only if it is algebraically closed. (Angus Macintyre, Macintyre's theorem.)
*A differentially closed field in characteristic 0 is totally transcendental.
*Any theory with a countable language that is categorical theory, categorical for some uncountable cardinal is totally transcendental.
*An abelian group is totally transcendental if and only if it is the direct sum of a divisible group and a group of bounded torsion group, exponent.
*Any linear algebraic group over an algebraically closed field is totally transcendental.
*Any group of finite Morley rank is totally transcendental.

See also

*Spectrum of a theory
*Morley's categoricity theorem
*List of first-order theories
*Stability spectrum

References

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*T. G. Mustafin, Stable Theories [in Russian], Karaganda (1981).
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* (Translated from the 1987 French original.)
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*{{Citation , last1=Shelah , first1=Saharon , title=Classification theory and the number of nonisomorphic models , orig-year=1978 , publisher=Elsevier , edition=2nd , series=Studies in Logic and the Foundations of Mathematics , isbn=978-0-444-70260-9 , year=1990 , url-access=registration , url=https://archive.org/details/classificationth0092shel

External links

*A. Pillay
Lecture notes on model theory
*A. Pillay
Lecture notes on stability theory
*A. Pillay
Lecture notes on applied stability theory

Model theory