View source for Stability theory (in logic)

A branch of [[Model theory|model theory]] that provides a structure theory for models of first-order theories. The collection of all first-order sentences true in a given structure forms a complete first-order theory <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870801.png" />. For example, the models of the theory of the complex field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870802.png" /> are exactly the algebraically closed fields of characteristic zero. Complete theories in a fixed countable language <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870803.png" /> that have an infinite model are considered below. These theories are classified as unstable, stable, superstable, or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870804.png" />-stable, according to the number of complete types (see below) over each model of the theory (cf. also [[Stable and unstable theories|Stable and unstable theories]]).

Let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870805.png" /> denote the collection of complete types over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870806.png" />. The cardinality of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870807.png" /> is always at most <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870808.png" />, and for unstable <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s0870809.png" /> this maximum is usually attained. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708010.png" /> is stable in a cardinal number <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708011.png" /> if for every set of cardinality <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708012.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708013.png" />. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708014.png" /> is unstable if it is stable in no <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708015.png" />. <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708016.png" /> is stable if it is stable in some <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708017.png" />, superstable if it is stable in all cardinalities beyond the continuum and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708019.png" />-stable if it is stable in every infinite cardinality. The increasing degrees of stability are reflected in an increasing ability to assign invariants to the models of a theory. Unstable theories include any linear ordering. Every theory of modules is stable. A separably closed field is stable, but not superstable. A superstable group does not admit an infinite descending chain of definable subgroups (with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708020.png" /> infinite). For example, the additive group of integers is superstable. An algebraically closed field or an algebraic group over an algebraically closed field is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708021.png" />-stable.

Without loss of generality all models of a first-order theory <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708022.png" /> can be considered imbedded in a fixed universal domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708023.png" />. For any element <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708024.png" /> and a subset <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708025.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708026.png" />, the type of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708027.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708028.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708029.png" />, is the collection of all formulas with one free variable, and with parameters from <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708030.png" />, that are true in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708031.png" /> if the free parameters are replaced by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708032.png" />. Two elements of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708033.png" /> realize the same type over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708034.png" /> precisely if they are in the same orbit under the group of automorphisms of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708035.png" /> that fix <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708036.png" />.

The Löwenheim–Skolem theorem (cf. [[Gödel completeness theorem|Gödel completeness theorem]]) guarantees that each theory has a model in each infinite cardinality. One of the principal accomplishments of stability theory has been to classify the functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708037.png" /> that assign to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708038.png" /> the number of isomorphism types of models <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708039.png" /> with cardinality <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708040.png" />. M.D. Morley began this process with his 1963 generalization of a theorem of E. Steinitz by showing that for any first-order theory <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708041.png" /> (as for the theory of algebraically closed fields), <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708042.png" /> (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708043.png" /> is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708044.png" /> categorical) if and only if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708045.png" /> is categorical in every uncountable cardinality (cf. [[Categoricity in cardinality|Categoricity in cardinality]]). The 1971 Baldwin–Lachlan theorem continues this analogy by showing that an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708046.png" /> categorical theory has either <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708047.png" /> or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708048.png" /> models.

In roughest form, each model is determined by a  "skeleton"  and by a  "closure"  of that skeleton to the entire model. The notion of a skeleton is elaborated below. The simplest type of closure is closure under functions. A first-order language can be expanded so that each subset of the universal domain has an Ehrenfeucht–Mostowski hull (generated by the functions in the language) that is a model. In general, this operation destroys stability of the theory. A weaker notion of closure assigns to each <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708049.png" /> a prime model over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708050.png" /> that can be imbedded in every model containing <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708051.png" />. In general, no such assignment is possible, but it is if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708052.png" /> is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708053.png" />-stable.

S. Shelah began around 1970 the program of determining those properties of a theory <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708054.png" /> that imply <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708055.png" /> for all uncountable <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708056.png" /> and whose negations aid in the development of a structure theory for models of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708057.png" />. He established that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708058.png" /> is unstable, or even not superstable, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708059.png" /> has the maximum possible value, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708060.png" />, for all uncountable <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708061.png" />. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708062.png" /> is unstable, there is a formula <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708063.png" /> that defines a linear order on an infinite set of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708064.png" />-tuples from <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708065.png" />. The compactness theorem allows one to construct for each linear order type <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708066.png" /> of cardinality <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708067.png" /> a subset of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708068.png" /> that is linearly ordered by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708069.png" /> in type <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708070.png" />. By a formidable combinatorial argument, Shelah showed that for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708071.png" /> of these orderings the Ehrenfeucht–Mostowski hulls remain non-isomorphic, thus establishing that each unstable theory has the maximal number of models in each uncountable cardinality. The argument in the non-superstable case is similar, but the linear orderings are replaced by trees of height <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708072.png" />.

In any stable theory one can find a dependence relation, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708073.png" /> forks over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708074.png" />, analogous to algebraic or linear dependence. In particular, the analogue of the exchange axiom holds. In general, however, the associated closure relation (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708075.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708076.png" /> if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708077.png" /> forks over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708078.png" />) is not transitive. A type <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708079.png" /> is called regular if this closure relation is transitive when restricted to the realizations of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708080.png" />. Thus, a dimension (as in linear algebra) can be assigned to each regular type <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708081.png" />. Much of the structure theory depends on reducing the structure of a model to the structure of the regular types realized in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708082.png" />. The collection of regular types, their dimensions and certain relations among them constitute the skeleton of a model. It may be that several regular types carry the same information regarding the structure of a model; if so, they are said to be non-orthogonal.

If each independent pair of models has a minimal closure and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708083.png" /> is superstable, then each model of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708084.png" /> can be analyzed as prime over a tree of countable height. If this tree is well-founded, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708085.png" /> can be bounded as a function of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708086.png" />. If this closure does not exist ( "notop" ) or is not minimal ( "dop" ) or the tree is not well-founded ( "deep" ), <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708087.png" /> has the maximal number of models in almost-all cardinalities. This structure/non-structure dichotomy is known as the main gap. The analysis discussed so far (1990) suffices to establish (Shelah, late 1970's) the Morley conjecture that the spectrum function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708088.png" /> is increasing, except possibly from <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708089.png" /> to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708090.png" />.

The exact calculation of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708091.png" /> in the well-behaved case requires an understanding of the geometric structure of regular types. It is possible to define a [[Combinatorial geometry|combinatorial geometry]] on the set of realizations of a regular type. B.I. Zil'ber initiated the use of this geometric structure to obtain global information about the models of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708092.png" />. In the simplest case (an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708093.png" />-categorical strictly-minimal set) this geometry must be either trivial, an affine space over a finite field or a projective space over a finite field. With this analysis it is possible to show that no totally categorical theory is finitely axiomatizable, but every such is axiomatizable by a single sentence and an  "axiom of infinity" -scheme.

R.L. Vaught conjectured in the early 1960's that a complete first-order theory has either countable many or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708094.png" /> countable models. Shelah has established the result for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708095.png" />-stable theories. S. Buechler and L. Newelski have extended the result to certain superstable theories. This extension depends heavily on the geometric analysis.

Stability theory interacts with classical algebra in several ways. The effect of imposing stability-theoretic constraints on algebraic structures has been extensively investigated: Every stable semi-simple ring is a matrix ring over an algebraically closed field; every superstable division ring is an algebraically closed field. G.L. Cherlin (middle of the 1970's) conjectured that every simple <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708096.png" />-stable group (of finite rank) is an algebraic group over an algebraically closed field.

Recently, strong algebraic results have been used to obtain model-theoretic information. In 1985, U. Hrushovski proved that if a pair of types is weakly orthogonal but not orthogonal, then either a pure Abelian group or the projective special linear group over the complex numbers is interpretable in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708097.png" />. Using this result he derived the purely model-theoretic result that a stable uni-dimensional theory (any pair of non-algebraic types are non-orthogonal) must be superstable.

In the 1990's, research in stability theory continues in several directions: further analysis of stable theories, examination of stable groups and fields, study of countable homogeneous structures, analysis of unstable theories (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/s/s087/s087080/s08708098.png" />-minimality), and the generalization of stability-theoretic techniques beyond the first-order context.

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  J.T. Baldwin,   "Classification theory: 1985"  J.T. Baldwin (ed.) , ''Classification Theory (Chicago 1985). Proc. US-Israel Binational Workshop on Model Theory in mathematical logic'' , ''Lect. notes in math.'' , '''1292''' , Springer  (1988)  pp. 1–23</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  J.T. Baldwin,   "Fundamentals of stability theory" , Springer  (1988)</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  W. Hodges,   "What is a structure theory?"  ''Bull. London Math. Soc.'' , '''19'''  (1987)  pp. 209–237</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  D. Lascar,   "Stability in model theory" , Longman  (1987)  (Translated from French)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  A. Pilay,   "An introduction to stability theory" , Clarendon Press  (1983)</TD></TR><TR><TD valign="top">[a6]</TD> <TD valign="top">  B. Poizat,   "Groupes stables" , Nur Al-mantiq Wal-ma'rifah , Villeurbanne, France  (1987)</TD></TR><TR><TD valign="top">[a7]</TD> <TD valign="top">  S. Shelah,   "Classification theory and the number of nonisomorphic models" , North-Holland  (1978)</TD></TR><TR><TD valign="top">[a8]</TD> <TD valign="top">  S. Shelah,   "Classification of first-order theories which have a structure theory"  ''Bull. Amer. Math. Soc.'' , '''12'''  (1985)  pp. 227–232</TD></TR></table>