DIRECT LIMIT

In mathematics, the 'direct limit' (also called the 'inductive limit') is a general method of taking limits of "directed families of objects". We will first give the definition for algebraic structures like groups and modules, and then the general definition which can be used in any category.

Contents

Formal definition

Algebraic objects

General definition

Examples

Related constructions and generalizations

Formal definition

Algebraic objects

In this section we will understand ''objects'' to be sets with a given algebraic structure such as groups, rings, modules (over a fixed ring), algebras (over a fixed field), etc. We will also understand ''homomorphisms'' in the corresponding setting (group homomorphisms, etc.).
We start with the definition of a 'direct system' of objects and homomorphisms. Let (''I'', ≤) be a directed poset. Let {''A''_''i'' | ''i'' ∈ ''I''} be a family of objects indexed by ''I'' and suppose we have a family of homomorphisms ''f''_''ij'': ''A''_''i'' → ''A''_''j'' for all ''i'' ≤ ''j'' with the following properties:
# ''f''_''ii'' is the identity in ''A''_''i'',
# ''f''_''ik'' = ''f''_''jk'' o ''f''_''ij'' for all ''i'' ≤ ''j'' ≤ ''k''.
Then the pair (''A''_''i'', ''f''_''ij'') is called a direct system over ''I''.
The underlying set of the 'direct limit', ''A'', of the direct system (''A''_''i'', ''f''_''ij'') is defined as the disjoint union of the ''A''_''i'''s modulo a certain equivalence relation:
:
ight)igg/{x_i sim x_j mid mbox{there exists } kin I mbox{ such that } f_{ik}(x_i) = f_{jk}(x_j)}
Heuristically, two elements in the disjoint union are equivalent if and only if they "eventually become equal" in the direct system. One naturally obtains from this definition ''canonical morphisms'' φ_''i'': ''A''_''i'' → ''A'' sending each element to its equivalence class. The algebraic operations on ''A'' are defined via these maps in the obvious manner.

General definition

The direct limit can be defined abstractly in an arbitrary category by means of a universal property. Let (''X''_''i'', ''f''_''ij'') be a direct system of objects and morphisms in a category ''C'' (same definition as above). The direct limit of this system is an object ''X'' in ''C'' together with morphisms φ_''i'': ''X''_''i'' → ''X'' satisfying φ_''i'' = φ_''j'' O ''f''_''ij''. The pair (''X'', φ_''i'') must be universal in the sense that for any other such pair (''Y'', ψ_''i'') there exists a unique morphism ''u'': ''X'' → ''Y'' making all the "obvious" identities true; i.e., the diagram

must commute for all ''i'', ''j''. The direct limit is often denoted
:
with the direct system (''X''_''i'', ''f''_''ij'') being understood.
Unlike for algebraic objects, the direct limit may not exist in an arbitrary category. If it does, however, it is unique in a strong sense: given another direct limit ''X''′ there exists a ''unique'' isomorphism ''X''′ → ''X'' commuting with the canonical morphisms.
We note that a direct system in a category ''C'' admits an alternative description in terms of functors. Any directed poset ''I'' can be considered as a small category where the morphisms consist of arrows ''i'' → ''j'' if and only if ''i'' ≤ ''j''. A direct system is then just a covariant functor ''I'' → ''C''.

Examples

★ Let ''I'' be any directed poset with a greatest element ''m''. The direct limit of any corresponding direct system is isomorphic to ''X''_''m'' and the canonical morphism φ_''m'': ''X''_''m'' → ''X'' is an isomorphism.

★ Let ''p'' be a prime number. Consider the direct system composed of the groups 'Z'/''p''^''n'''Z' and the homomorphisms 'Z'/''p''^''n'''Z' → 'Z'/''p''^''n''+1'Z' which are induced by multiplication by ''p''. The direct limit of this system consists of all the ''p''^''n''th roots of unity, and is called the ''p''^∞-group.

★ Let ''F'' be a ''C''-valued sheaf on a topological space ''X''. Fix a point ''x'' in ''X''. The open neighborhoods of ''x'' form a directed poset ordered by inclusion (''U'' ≤ ''V'' if and only if ''U'' contains ''V''). The corresponding direct system is (''F''(''U''), ''r''_''U'',''V'') where ''r'' is the restriction map. The direct limit of this system is called the ''stalk'' of ''F'' at ''x'', denoted ''F''_''x''. The canonical morphisms ''F''(''U'') → ''F''_''x'' are called ''germs''.

★ Direct limits in the category of topological spaces are given by placing the final topology on the underlying set-theoretic direct limit.

★ A collection of subsets $M\_i$ of a set $M$ can be partially ordered by inclusion. Its limit is the union .

Related constructions and generalizations

The categorical dual of the direct limit is called the inverse limit (or projective limit). More general concepts are the limits and colimits of category theory. The terminology is somewhat confusing: direct limits are colimits while inverse limits are limits.