nLab circle type



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

logiccategory theorytype theory
trueterminal object/(-2)-truncated objecth-level 0-type/unit type
falseinitial objectempty type
proposition(-1)-truncated objecth-proposition, mere proposition
proofgeneralized elementprogram
cut rulecomposition of classifying morphisms / pullback of display mapssubstitution
cut elimination for implicationcounit for hom-tensor adjunctionbeta reduction
introduction rule for implicationunit for hom-tensor adjunctioneta conversion
logical conjunctionproductproduct type
disjunctioncoproduct ((-1)-truncation of)sum type (bracket type of)
implicationinternal homfunction type
negationinternal hom into initial objectfunction type into empty type
universal quantificationdependent productdependent product type
existential quantificationdependent sum ((-1)-truncation of)dependent sum type (bracket type of)
equivalencepath space objectidentity type/path type
equivalence classquotientquotient type
inductioncolimitinductive type, W-type, M-type
higher inductionhigher colimithigher inductive type
-0-truncated higher colimitquotient inductive type
coinductionlimitcoinductive type
presettype without identity types
completely presented setdiscrete object/0-truncated objecth-level 2-type/set/h-set
setinternal 0-groupoidBishop set/setoid
universeobject classifiertype of types
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




The circle type is an axiomatization of the homotopy type of the (shape of) the circle in the context of homotopy type theory.


As a higher inductive type, the circle is given by

Inductive Circle : Type
  | base : Circle
  | loop : Id Circle base base

This says that the type is inductively constructed from

  1. a term of circle type whose interpretation is as the base point of the circle,

  2. a term of the identity type of paths between these two terms, which interprets as the 1-cell of the circle

    baseloopbase base \stackrel{loop}{\to} base \,

    Hence a non-constant path from the base point to itself.

As a suspension

The circle type could also be defined as the suspension type Σ2\Sigma \mathbf{2} of the type of booleans 2\mathbf{2}.

Using torsors

The circle can also be defined without HITs using only univalence, as the type of \mathbb{Z}-torsors. One can then prove that this type satisfies the same induction principle (propositionally). This is due to Dan Grayson.


Its induction principle says that for any P:S 1TypeP:S^1\to Type equipped with a point base:P(base)base' : P(base) and a dependent path? loop:base=baseloop':base'= base', there is f: (x:S 1)P(x)f:\prod_{(x:S^1)} P(x) such that:

f(base)=baseapd f(loop)=loopf(base)=base'\qquad apd_f(loop) = loop'

As a special case, its recursion principle says that given any type XX with a point x:Xx:X and a loop l:x=xl:x=x, there is f:S 1Xf:S^1 \to X with

f(base)=xap f(loop)=lf(base)=x\qquad ap_f(loop)=l

Ω(S 1)=\Omega(S^1)=\mathbb{Z}

There are several proofs in the HoTT book that the loop space Ω(S 1)\Omega(S^1) of the circle is the integers \mathbb{Z}. (Any such proof requires the univalence axiom, since without that it is consistent that S 1S^1 is contractible. Indeed, S 1S^1 is contractible if and only if UIP holds.)

See also


A formalization of the shape homotopy type ʃS 1Bʃ S^1 \simeq \mathbf{B}\mathbb{Z} of the circle as a higher inductive type in homotopy type theory, along with a proof that ΩʃS 1\Omega ʃS^1\simeq {\mathbb{Z}} (and hence π 1(ʃS 1)\pi_1(ʃS^1) \simeq \mathbb{Z}):

The proof is formalized therein using the Agda proof assistant. See also

Last revised on June 8, 2022 at 01:36:27. See the history of this page for a list of all contributions to it.