infinite sets
E.g. take the theory of \(\mathbb{N}\) in the empty language.
This theory has quantifier elimination, so definable sets (in one variable) are finite or cofinite.
See .
ACF - algebraically closed fields
Axiomatized by finitely many field axioms (in \(\mathcal{L}=\{+,\cdot,0,1\}\)), and the infinite axiom scheme saying every polynomial has a root.
The theory can be completed by specifying the characteristic. By quantifier elimination, definable sets (in one variable) are Boolean combinations of polynomial zerosets and so finite or cofinite.
See .
\(\mathbb{Q}\)-vector spaces
Axiomatized by the axioms for torsion-free divisible abelian groups in \(\mathcal{L}=\{+,0\}\).
By quantifier elimination, definable sets (in one variable) are given as Boolean combinations of formulas of the form nx = a, where n is an integer and a is an arbitrary element. Since the group is torsion free, such sets are finite or cofinite.
See .
DCF_{0} - differentially closed fields of characteristic 0
Shown to be ω-stable by Blum.
See .
everywhere infinite forest
Fraïssé limit of finite trees.
Models consist of infinitely many infinitely branching trees. Not strongly minimal as the set of neighbors of a specific vertex is definable, infinite and coinfinite.
Morley rank ω. Also referred to as the free pseudoplane.
See , .
\((\mathbb{Z},+,-,0,1)\)
Quantifier elimination up to the definable predicates P_{n}, which distinguish the elements divisible by n, for n > 0. This shows superstability. Any set of prime numbers gives a complete type over \(\emptyset\) of an element divisible by exactly the prime numbers in the set. Thus \(S_1(\emptyset)\) is uncountable and the theory is not ω-stable.
See .
finitely refining equivalence relations
Infinitely many equivalence relations \(\{E_0,E_1,\ldots\}\) such that \(E_0\) has two infinite classes, and each \(E_i\)-class is partitioned into two infinite \(E_{i+1}\) classes.
See , .
infinitely refining equivalence relations
Infinitely many equivalence relations \(\{E_0,E_1,\ldots\}\) such that \(E_0\) has infinitely many infinite classes, and each \(E_i\)-class is partitioned into infinitely many infinite \(E_{i+1}\) classes.
See , .
DCF_{p} - differentially closed fields of characteristic p
Shown to be stable, but not superstable, by Shelah (1973).
See .
free group on n > 1 generators
Shown to be stable, but not superstable, by Sela (2006).
See .
RCF - real closed fields
Complete theory with quantifier elimination in \(\mathcal{L}=\{+,\cdot,0,1,<\}\).
See .
\((\mathbb{Z},<)\) - discrete linear order
See .
\((\mathbb{Q},<)\) - dense linear order
See .
\((\mathbb{Z},+,<)\) - Presburger Arithmetic
Quantifier elimination up to predicates P_{n} naming the elements divisible by n, for n > 0. Not o-minimal since, e.g., 2\(\mathbb{Z}\) is a definable infinite discrete set.
See .
\((\mathbb{Z},~x\mapsto x+1)\)
By quantifier elimination, definable sets (in one variable) are of Boolean combinations of formulas of the form x = a + n, where a is an arbitrary element and n is an integer. Since successors are unique, such sets are finite or cofinite.
See .
ACVF - algebraically closed valued fields
See .
\((\mathbb{Q},+,\cdot,v(x)\geq v(y))\) - field of p-adics with valuation
See .
random graph
Axiomatized in graph language by sentences saying that if A and B are finite disjoint sets of vertices then there is a vertex connected to everything in A and nothing in B.
Forking independence is the same as algebraic independence and algebraic closure is trivial, so the theory is supersimple. Not stable since xRy has the order property.
See , .
pseudo-finite fields
Defined as ultraproducts of finite fields.
See .
ACFA - algebraically closed fields with a generic automorphism
See .
QACFA - quasi-algebraically closed fields with a generic automorphism
See .
ultraproduct of \(\mathbb{Q}_p\) (p-adic field)
See .
generic K_{n}-free graph
Given n ≥ 3, K_{n} denotes the complete graph on n vertices. Axiomatized in the graph language by sentences stating that if A and B are finite disjoint sets of vertices, with A K_{n-1}-free, then there is a vertex connected to everything in A and nothing in B.
See , .
\((\mathbb{Q},\textrm{cyc})\) - cyclic order on the rationals
The cyclic order on the rationals is cyc(a,b,c) if and only if a < b < c or b < c < a or c < a < b.
Can be interpreted in a real closed field, so NIP. The unique 1-type over \(\emptyset\) proves "x = 0, or x = 1, or cyc(0,x,1), or cyc(1,x,0)". Each of these disjuncts divides over \(\emptyset\), so the type forks over \(\emptyset\), which shows that nonforking fails existence.
See .
SCF\(_p^n\) - separably closed fields of characteristic p and Eršov invariant \(n\leq\infty\)
Given a separably closed field F of characteristic p, the Eršov invariant is \[n=[F:F^p]\in\mathbb{Z}^+\cup\{\infty\}.\]
Shown to be stable by Macintyre, Shelah, and Wood (1975).
See .
\((\mathbb{R},+,\cdot,0,1,\textrm{exp})\) - real exponential field
Shown to be o-minimal by Wilkie (1996).
See .
\(((\mathbb{Z}/4\mathbb{Z})^\omega,+)\)
Totally categorical of Morley rank 2.
See , .
finitely cross-cutting equivalence relations
Infinitely many equivalence relations \(\{E_0,E_1,\ldots\}\) such that each \(E_i\) has two infinite classes, and for all \(n<\omega\) and \(I\subseteq n\),
\[\forall x\exists y\left(\bigwedge_{i\in I}E_i(x,y)\wedge\bigwedge_{i\in n\backslash I}\neg E_i(x,y)\right)\]
See .
universal graph omitting a bowtie
Complete theory of the universal and existentially closed countable graph omitting a 'bowtie' (sum of two triangles sharing a single vertex).
See , .
universal graph omitting a family of odd cycles
Given odd n ≥ 3, define the complete theory of the universal and existentially closed countable graph omitting all odd cycles of length at most n.
See , .
universal directed graph omitting directed cycles of length ≤ n
Complete theory of the universal and existentially closed countable directed graph omitting directed cycles of length ≤ n.
See .
ZFC - set theory
In the language \(\mathcal{L}=\{\in\}\).
\((\mathbb{Z},+,\cdot,0,1)\)
Complete theory of the ring of integers (a completion of Peano Arithmetic).
\((\mathbb{Q}^n,<_1,\ldots,<_n)\)
Complete theory of \(\mathbb{Q}^n\), for n >1, with coordinate orderings. In particular, given \(x,y\in\mathbb{Q}^n\), set \[x<_i y ~\Leftrightarrow~ x_i < y_i. \]
When n=1, this theory is the usual DLO. In general, the theory has dp-rank n.
See .
Urysohn sphere
The unique universal and ultrahomogeneous separable metric space (with distances bounded by 1).
There are few options for what language to use for this structure. One is to use distance relations \(d(x,y)\leq r\), for \(r\in\mathbb{Q}\). Alternatively, one consider the Urysohn sphere as a metric structure in continuous logic.
See , .
free n^{th} root of the complete graph
The unique universal and ω-homogeneous metric space with distances in {0,1,...,n}.
This theory was originally defined with graph relations
R^{i} for 0 ≤
i ≤
n, where
R^{i}(x,y) holds if and only if
x and
y can be connected by a path of length
i.
The graph structure and metric structure can be thought of as the same via
d(x,y) = min{i : R^{i}(x,y)}
See
,
.
Hrushovski's new strongly minimal set
Hrushovski constructed an example of a strongly minimal theory, which is not locally modular and does not interpret an infinite group. This disproved Zilber's conjecture
that a strongly minimal theory must either be locally modular or interpret an infinite field.
See , .
infinitely cross-cutting equivalence relations
Infinitely many equivalence relations \(\{E_0,E_1,\ldots\}\) such that each \(E_i\) has infinitely many infinite classes, and for all \(i<\omega\), each \(E_{i+1}\) class splits
each \(E_i\) class into infinitely many pieces.
See .
non-simple generic limit of \((\mathcal{K}_f,\leq)\) for good \(f\)
\((\mathcal{K}_f,\leq)\) is a class of finite structures (in a finite relational language), which is closed under free amalgamation and is equipped with a predimension and a control function \(f\).
\(f\) is good if \(\mathcal{K}_f\) is closed under free \(\leq\)-amalgamation.
We consider the theory of the generic structure \(\mathcal{M}_f\). These theories are always NSOP_{4}, and the non-simple case is always SOP_{3} and TP2.
The simple case can also be characterized by the closure of \(\mathcal{K}_f\) under independence theorem diagrams.
See .
VFA_{0}
The limit theory (as p→∞) of the Frobenius automorphism acting on an algebraically closed valued field of characteristic p.
See .
atomless Boolean algebras
The theory of the Fraissé limit of Boolean algebras in the language \(\{0,1,\neg,\wedge,\vee\}\).
See .
generic K_{n,r}-free r-graph
Given n > r > 2, K_{n,r} denotes the complete r-graph on n vertices.
See .
a strictly stable superflat graph
Given integers m and n, let Km
n be the class of graphs obtained
from the complete graph on n vertices by replacing each edge with a path containing at most m new vertices.
A graph G is superflat if for all m there is some n such that G omits Km
n.
G is ultraflat if there is some n such that for all m, G omits Km
n.
Define the following strictly stable superflat graph. Begin with vertex set ω^{<ω}∪ω^{ω}. For each σ in ω^{ω}
and m < ω, add a path from σ to σ|_{m} containing m new vertices.
Any superflat graph is stable; any ultraflat graph (and so any planar graph) is superstable.
The graph above interprets infinitely refining equivalence relations, and so is strictly stable.
See , , .
imperfect bounded PAC fields
A variety defined over a field K is absolutely irreducible if it is not the union of two algebraic sets defined over some algebraically closed extension of K
A field K is pseudo-algebraically closed (PAC) if every absolutely irreducible variety defined over K has a K-rational point.
K is perfect if either it has characteristic 0, or if K has characteristic p then K^{p} = K.
K is bounded if for all n > 1, K has finitely many separably algebraic extensions of degree n.
If K is a PAC field then Th(K) is simple if and only if K is bounded. The theory of a perfect bounded PAC field is supersimple.
See , , .
T_{feq} - parameterized equivalence relations
Consider the language \(\mathcal{L}=\{P,Q,E\}\), where \(P,Q\) are unary relations and \(E\) is a ternary relation.
T_{feq} is the model completion of the following theory:
- \(P\) and \(Q\) form a partition.
- For all \(a\) in \(P\), \(E(a,x,y)\) is an equivalence relation on \(Q\).
See , . In this theory is shown to be NSOP_{3} and not simple.
In it is shown to be NTP1. In it is claimed to be NSOP1, but the proof is currently under scrutiny. See also .
list of open examples
ω-free PAC fields
A variety defined over a field K is absolutely irreducible if it is not the union of two algebraic sets defined over some algebraically closed extension of K
A field K is pseudo-algebraically closed (PAC) if every absolutely irreducible variety defined over K has a K-rational point.
A PAC field K is ω-free if there is an elementary substructure L of K, whose absolute Galois group Aut(L^{sep}/L) is isomorphic to \(\hat{F}_\omega\), which is defined below.
Let \(F_\omega\) be the free group on countably many generators. Let \(\mathcal{N}\) be the family of normal, finite-index subgroups of \(F_\omega\) containing cofinitely many generators. Then
\[\hat{F}_\omega=\varprojlim_{N\in\mathcal{N}}F_\omega/N\]
See , . In this theory is shown to be NSOP_{3} and not simple;
and remarked to probably be NSOP1. See also .
list of open examples
infinite-dimensional vector space with a bilinear form
Defined in the 2-sorted language of vector spaces over an algebraically closed field.
See , .
list of open examples
strongly minimal
o-minimal
ω-stable, dp-minimal, and not strongly minimal
superstable, dp-minimal, and not ω-stable
stable, dp-minimal, and not superstable
dp-minimal, SOP, and not o-minimal
IP, SOP, NTP2
supersimple and unstable
simple and unstable, but not supersimple
NTP2, TP1, and NSOP_{3}
It is unknown whether or not the implication SOP3 \(\Rightarrow\) TP1 is strict.
NTP2, SOP_{3}, and NSOP_{4}
It is unknown whether or not the strong order property hierarchy is strict inside NTP2.
NTP2, SOP_{n}, and NSOP_{n+1} for some
n ≥ 4,
It is unknown whether or not the strong order property hierarchy is strict inside NTP2.
NTP2 and NSOP_{∞}, and SOP_{n} for all n ≥ 1
It is unknown whether or not the strong order property hierarchy is strict inside NTP2.
NTP2, NSOP, and SOP_{∞}
It is unknown whether or not the strong order property hierarchy is strict inside NTP2.
NSOP1 and TP2
NTP1 and SOP1
It is unknown whether or not the implication TP1 \(\Rightarrow\) SOP1 is strict.
TP2, TP1, and NSOP_{3}
It is unknown whether or not the implication SOP3 \(\Rightarrow\) TP1 is strict.
TP2, SOP_{3}, and NSOP_{4}
TP2, SOP_{n}, and NSOP_{n+1} for some
n ≥ 3
To keep this section distinct from the others, we would technically want to say n ≥ 4.
However every known example of a theory that is SOP_{n} and NSOP_{n+1} for some n ≥ 4, has some analog for any n ≥ 3.
TP2, NSOP_{∞}, and SOP_{n} for all n ≥ 1
TP2, SOP_{∞}, and NSOP
TP2 and SOP
NIP, and not dp-minimal
ω-stable, and not dp-minimal
superstable, not ω-stable, and not dp-minimal
stable, not superstable, and not dp-minimal