nLab support object

Contents

Contents

Idea

In set theory, the support of a set turns the set into a subsingleton. In dependent type theory, propositional truncation or the bracket type is an operation which takes a type and turns it into a h-propositions, which are the type theoretic equivalent of subsingletons. By the relation between type theory and category theory and the relation between set theory and topos theory, it should be possible to do the same process and turn an object of a category into a subterminal object. These are the support objects, bracket objects, or propositional truncation objects in category theory.

Definition

In category theory

The support object [X][X] of an object XX in a regular category CC is the image of the unique morphism into the terminal object X1X \to 1.

X[X]1X \to [X] \hookrightarrow 1

As a result, that all support objects exist is equivalent to the condition that every morphism into the terminal object has an image factorization.

In allegory theory

The support object [X][X] of an object XX in a unitary tabular allegory CC is the tabulation? of the unique map from XX into the allegorical unit 11.

Properties

In any well-pointed pretopos \mathcal{E}, the support is the coequalizer of the product projection morphisms π 1:X×XX\pi_1:X \times X \to X and π 2:X×XX\pi_2:X \times X \to X

X×X π 2π 1 X p [X].\array{ X \times X && \stackrel{\overset{\pi_1}{\longrightarrow}}{\underset{\pi_2}{\longrightarrow}} && X \\ & \searrow && \swarrow_{\mathrlap{p}} \\ && [X] }.

See also

computational trinitarianism =
propositions as types +programs as proofs +relation type theory/category theory

logicset theory (internal logic of)category theorytype theory
propositionsetobjecttype
predicatefamily of setsdisplay morphismdependent type
proofelementgeneralized elementterm/program
cut rulecomposition of classifying morphisms / pullback of display mapssubstitution
introduction rule for implicationcounit for hom-tensor adjunctionlambda
elimination rule for implicationunit for hom-tensor adjunctionapplication
cut elimination for implicationone of the zigzag identities for hom-tensor adjunctionbeta reduction
identity elimination for implicationthe other zigzag identity for hom-tensor adjunctioneta conversion
truesingletonterminal object/(-2)-truncated objecth-level 0-type/unit type
falseempty setinitial objectempty type
proposition, truth valuesubsingletonsubterminal object/(-1)-truncated objecth-proposition, mere proposition
logical conjunctioncartesian productproductproduct type
disjunctiondisjoint union (support of)coproduct ((-1)-truncation of)sum type (bracket type of)
implicationfunction set (into subsingleton)internal hom (into subterminal object)function type (into h-proposition)
negationfunction set into empty setinternal hom into initial objectfunction type into empty type
universal quantificationindexed cartesian product (of family of subsingletons)dependent product (of family of subterminal objects)dependent product type (of family of h-propositions)
existential quantificationindexed disjoint union (support of)dependent sum ((-1)-truncation of)dependent sum type (bracket type of)
logical equivalencebijection setobject of isomorphismsequivalence type
support setsupport object/(-1)-truncationpropositional truncation/bracket type
n-image of morphism into terminal object/n-truncationn-truncation modality
equalitydiagonal function/diagonal subset/diagonal relationpath space objectidentity type/path type
completely presented setsetdiscrete object/0-truncated objecth-level 2-type/set/h-set
setset with equivalence relationinternal 0-groupoidBishop set/setoid with its pseudo-equivalence relation an actual equivalence relation
equivalence class/quotient setquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive type
-0-truncated higher colimitquotient inductive type
coinductionlimitcoinductive type
presettype without identity types
set of truth valuessubobject classifiertype of propositions
domain of discourseuniverseobject classifiertype universe
modalityclosure operator, (idempotent) monadmodal type theory, monad (in computer science)
linear logic(symmetric, closed) monoidal categorylinear type theory/quantum computation
proof netstring diagramquantum circuit
(absence of) contraction rule(absence of) diagonalno-cloning theorem
synthetic mathematicsdomain specific embedded programming language

Last revised on July 28, 2024 at 18:04:09. See the history of this page for a list of all contributions to it.