nLab tuple extensionality


Set theory

Type theory

natural deduction metalanguage, practical foundations

  1. type formation rule
  2. term introduction rule
  3. term elimination rule
  4. computation rule

type theory (dependent, intensional, observational type theory, homotopy type theory)

syntax object language

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

logicset theory (internal logic of)category theorytype theory
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

homotopy levels


Category theory



In set theory

Let \mathbb{N} be the set of natural numbers, let n:n:\mathbb{N} be an arbitrary natural number, let Fin(n)\mathrm{Fin}(n) be the finite set with nn elements, and let AA be an arbitrary set. Then nn-tuple extensionality states that for every tuple a:Fin(n)Aa:\mathrm{Fin}(n) \to A and b:Fin(n)Ab:\mathrm{Fin}(n) \to A, a= Fin(n)Aba =_{\mathrm{Fin}(n) \to A} b if and only if a(i)= Ab(i)a(i) =_A b(i) for all elements iFin(n)i \in \mathrm{Fin}(n).

In dependent type theory

In dependent type theory, nn-tuple extensionality states that given two nn-tuples a:Fin(n)Aa:\mathrm{Fin}(n) \to A and b:Fin(n)Ab:\mathrm{Fin}(n) \to A there is an equivalence of types between the identity type a= Fin(n)Aba =_{\mathrm{Fin}(n) \to A} b and the dependent tuple type (i:Fin(n))(a(i)= Ab(i))(i:\mathrm{Fin}(n)) \to (a(i) =_{A} b(i)):

Γn:ΓAtypeΓ,a:Fin(n)A,b:Fin(n)Atupext(a,b):(a= Fin(n)Ab)(i:Fin(n))(a(i)= Ab(i))\frac{\Gamma \vdash n:\mathbb{N} \quad \Gamma \vdash A \; \mathrm{type}}{\Gamma,a:\mathrm{Fin}(n) \to A, b:\mathrm{Fin}(n) \to A \vdash \mathrm{tupext}(a, b):(a =_{\mathrm{Fin}(n) \to A} b) \simeq (i:\mathrm{Fin}(n)) \to (a(i) =_{A} b(i))}

There is also a version of nn-tuple extensionality for dependent tuple types called dependent nn-tuple extensionality, which states that given two dependent nn-tuples a:(i:Fin(n))A(i)a:(i:\mathrm{Fin}(n)) \to A(i) and b:(i:Fin(n))A(i)b:(i:\mathrm{Fin}(n)) \to A(i) there is an equivalence of types between the identity type a= (i:Fin(n))A(i)ba =_{(i:\mathrm{Fin}(n)) \to A(i)} b and the dependent tuple type (i:Fin(n))(a(i)= A(i)b(i))(i:\mathrm{Fin}(n)) \to (a(i) =_{A(i)} b(i)):

Γn:Γ,i:Fin(n)A(i)typeΓ,a:(i:Fin(n))A(i),b:(i:Fin(n))A(i)dtupext(a,b):(a= (i:Fin(n))A(i)b)(i:Fin(n))(a(i)= A(i)b(i))\frac{\Gamma \vdash n:\mathbb{N} \quad \Gamma, i:\mathrm{Fin}(n) \vdash A(i) \; \mathrm{type}}{\Gamma,a:(i:\mathrm{Fin}(n)) \to A(i), b:(i:\mathrm{Fin}(n)) \to A(i) \vdash \mathrm{dtupext}(a, b):(a =_{(i:\mathrm{Fin}(n)) \to A(i)} b) \simeq (i:\mathrm{Fin}(n)) \to (a(i) =_{A(i)} b(i))}


Tuple extensionality is always true in set theory and dependent type theory, for the same reason that product extensionality is true.

Indeed, one could express tuple extensionality as two separate cases representing nullary tuples and binary tuples:

In dependent type theory, this becomes:

See also

Created on January 10, 2023 at 00:54:54. See the history of this page for a list of all contributions to it.