Subobject classifier: Difference between revisions

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In mathematics, especially in [[category theory]], a '''subobject classifier''' is a special object Ω of a category such that, intuitively, the [[subobject]]s of any object ''X'' in the category correspond to the morphisms from ''X'' to Ω. In typical examples, that morphism assigns "true" to the elements of the subobject and "false" to the other elements of ''X.'' Therefore, a subobject classifier is also known as a "truth value object" and the concept is widely used in the categorical description of logic. Note however that subobject classifiers are often much more complicated than the simple binary logic truth values {true, false}.

== Introductory example ==

As an example, the set Ω = {0,1} is a subobject classifier in the [[category of sets]] and functions: to every subset ''A'' of ''S'' defined by the inclusion function '' j '' : ''A'' → ''S'' we can assign the function ''χ_A'' from ''S'' to Ω that maps precisely the elements of ''A'' to 1, and the elements outside ''A'' to 0 (seein other words, ''χ_A'' is the [[indicator function|characteristic function]] of ''A''). EveryConversely, every function from ''S'' to Ω arises in this fashion from precisely one subset ''A''.

To be clearer, consider a [[subset]] ''A'' of ''S'' (''A'' ⊆ ''S''), where ''S'' is a set. The notion of being a subset can be expressed mathematically using the so-called characteristic function χ_''A'' : S → {0,1}, which is defined as follows:

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In this way, the collection of all subsets of ''S'' and the collection of all maps from ''S'' to Ω = {0,1} are [[isomorphic]].

To categorize this notion, recall that, in category theory, a subobject is actually represented by a pair consisting of an object ''A'' and a [[monomorphism|monic arrow]] ''A → S'' (interpreted as the inclusion into another object ''S''). Accordingly, '''true''' refers to the element 1, which is selected by the arrow: '''true''': {0} → {0, 1} that maps 0 to 1. The subset ''A'' of ''S'' can now be defined as the [[pullback (category theory)|pullback]] of '''true''' along the characteristic function χ_''A'', shown on the following diagram:

[[Image:SubobjectClassifier-01.pngsvg|center|frameless|class=skin-invert]]

Defined that way, χ is a morphism ''Sub''_C(''S'') → Hom_C(S, Ω). By definition, Ω is a '''subobject classifier''' if this morphism χ is an isomorphism.

== Definition ==

For the general definition, we start with a category '''C''' that has a [[terminal object]], which we denote by 1. The object Ω of '''C''' is a ''subobject classifier'' for '''C''' if there exists a morphism

:1 → Ω

with the following property:

:For each [[monomorphism]] ''j'': ''U'' → ''X'' there is a unique morphism ''χ_j'': ''X'' → Ω such that the following [[commutative diagram]]

[[Image:SubobjectClassifier-02.pngsvg|center|frameless|class=skin-invert]]

:is a [[pullback diagram]]—that is, ''U'' is the [[limit (category theory)|limit]] of the diagram:

[[Image:SubobjectClassifier-03.pngsvg|center|frameless|class=skin-invert]]

The morphism ''χ_j'' is then called the '''classifying morphism''' for the subobject represented by ''j''.

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