Pregeometry (model theory)
Pregeometry, and in full combinatorial pregeometry, are essentially synonyms for "matroid". They were introduced by G.-C. Rota with the intention of providing a less "ineffably cacophonous" alternative term. Also, the term combinatorial geometry, sometimes abbreviated to geometry, was intended to replace "simple matroid". These terms are now infrequently used in the study of matroids.
In the branch of mathematical logic called model theory, infinite finitary matroids, there called "pregeometries" (and "geometries" if they are simple matroids), are used in the discussion of independence phenomena.
It turns out that many fundamental concepts of linear algebra – closure, independence, subspace, basis, dimension – are preserved in the framework of abstract geometries.
The study of how pregeometries, geometries, and abstract closure operators influence the structure of first-order models is called geometric stability theory.
Contents
Definitions
Pregeometries and geometries
A combinatorial pregeometry (also known as a finitary matroid), is a second-order structure: Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (X,\text{cl})\, , where Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}:\mathcal{P}(X)\to\mathcal{P}(X)\,
(called the closure map) satisfies the following axioms. For all and :
- Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}:(\mathcal{P}(X),\subseteq)\to(\mathcal{P}(X),\subseteq)\,
is an homomorphism in the category of partial orders (monotone increasing), and dominates Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{id}\, (I.e. implies Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): A\subseteq\text{cl}(A)\subseteq\text{cl}(B)\,
.) and is idempotent.
- Finite character: For each Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\in\text{cl}(A)\,
there is some finite with Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\in\text{cl}(F)\,
.
- Exchange principle: If Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): b\in\text{cl}(C\cup\{a\})\smallsetminus\text{cl}(C)\,
, then Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\in\text{cl}(C\cup\{b\})
(and hence by monotonicity and idempotence in fact Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\in\text{cl}(C\cup\{b\})\smallsetminus\text{cl}(C)\,
).
A geometry is a pregeometry in which The closure of singletons are singletons and the closure of the empty set is the empty set.
Independence, bases and dimension
Given sets , is independent over if Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\notin \text{cl}((A\setminus\{a\})\cup B)
for any .
A set is a basis for over if it is independent over and Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): A\subset \text{cl}(A_0\cup B) .
Since a pregeometry satisfies the Steinitz exchange property all bases are of the same cardinality, hence the definition of the dimension of over as Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{dim}_B A = |A_0|
has no ambiguity.
The sets are independent over if Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{dim}_{B\cup C} = \dim_C A'
whenever is a finite subset of . Note that this relation is symmetric.
In minimal sets over stable theories the independence relation coincides with the notion of forking independence.
Geometry automorphism
A geometry automorphism of a geometry is a bijection such that Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \sigma\text{cl}(X)=\text{cl}(X)\sigma
for any .
A pregeometry is said to be homogeneous if for any closed and any two elements there is an automorphism of which maps to and fixes pointwise.
The associated geometry and localizations
Given a pregeometry Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (S,\text{cl})
its associated geometry (sometimes referred in the literature as the canonical geometry) is the geometry Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (S',\text{cl}') where
- Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): S'=\{\text{cl}(a)\mid a\in S\setminus \text{cl} (\emptyset)\}
, and
- For any , Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}'(\{\text{cl}(a)\mid a\in X = \{\text{cl}'(b)\mid b\in\text{cl}X\}
Its easy to see that the associated geometry of a homogeneous pregeometry is homogeneous.
Given the localization of is the geometry Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (S,\text{cl}_A)
where Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}_A(X)=\text{cl}(X\cup A)
.
Types of pregeometries
Let Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (S,\text{cl})
be a pregeometry, then it is said to be:
- trivial (or degenerate) if Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}(X)=\bigcup\{\text{cl}(a)\mid a\in X\}
,
- modular if any two closed finite dimensional sets satisfy the equation Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{dim}(X\cup Y) = \text{dim}(X) + \text{dim}(Y) - \text{dim}(X\cap Y)
(or equivalently that is independent of over .
- locally modular if it has a localization at a singleton which is modular
4. (locally) projective if it is non-trivial and (locally) modular 5. locally finite if closures of finite sets are finite
Triviality, modularity and local modularity pass to the associated geometry and are preserved under localization.
If is a locally modular homogeneous pregeometry and Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): a\in S\setminus\text{cl}\emptyset
then the localization of in is modular.
The geometry is modular if and only if whnever , , Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{dim}\{a,b\}=2
and Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{dim}_A\{a,b\} \le 1 then Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): (\text{cl}\{a,b\}\cap\text{cl}(A))\setminus\text{cl}\emptyset\ne\emptyset
.
Examples
The trivial example
If is any set we may define Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{cl}(A)=A . This pregeometry is a trivial, homogeneous, locally finite geometry.
Vector spaces and projective spaces
Let be a field (a division ring actually suffices) and let be a -dimensional vector space over . Then is a pregeometry where closures of sets are defined to be their span.
This pregeometry is homogeneous and modular. Vector spaces are considered to be the prototypical example of modularity.
is locally finite if and only if is finite.
is not a geometry, as the closure of any nontrivial vector is a subspace of size at least .
The associated geometry of a -dimensional vector space over is the -dimensional projective space over . It is easy to see that this pregeometry is a projective geometry.
Affine spaces
Let be a -dimensional affine space over a field . Given a set define its closure to be its affine hull (i.e. the smallest affine subspace containing it).
This forms a homogeneous -dimensional geometry.
An affine space is not modular (for example, if and be parallel lines then the formula in the definition of modularity fails). However, it is easy to check that all localizations are modular.
Algebraically closed fields
Let be an algebraically closed field with Failed to parse (Missing <code>texvc</code> executable. Please see math/README to configure.): \text{tr.deg}(k)\ge\omega , and define the closure of a set to be its algebraic closure.
While vector spaces are modular and affine spaces are "almost" modular (i.e. everywhere locally modulare), algebraically closed fields are examples of the other extremity, not being even locally modular (i.e. none of the localizations is modular).
References
H.H. Crapo and G.-C. Rota (1970), On the Foundations of Combinatorial Theory: Combinatorial Geometries. M.I.T. Press, Cambridge, Mass.
Pillay, Anand (1996), Geometric Stability Theory. Oxford Logic Guides. Oxford University Press.