# Contents

## Idea

In dependent type theory, the 0-truncation modality of a type $A$ can be defined by localization of a type $A$ at the circle type $S^1$. This means that a type $A$ is 0-truncated, i.e. a set, if it is $S^1$-local, which means that, in addition to axiom K and uniqueness of identity proofs, there is another way to make the types of a universe into sets: by stipulating that every type is $S^1$-local, or that the canonical function

$\mathrm{const}_{A, S^1} \equiv \lambda x:A.\lambda i:S^1.x:A \to (S^1 \to A)$

which takes elements of $A$ to constant functions in $S^1 \to A$, is an equivalence of types. This is (tentatively) called the axiom of circle type localization or the axiom of $S^1$-localization.

Assuming that one has the function $\mathrm{const}_{A, S^1}:A \to (S^1 \to A)$ defined in the dependent type theory, the syntactic rules for the axiom of $S^1$-localization in a universe $\mathcal{U}$ is given by:

$\frac{\Gamma \vdash A:\mathcal{U} \quad \Gamma \vdash f:S^1 \to A}{\Gamma \vdash \mathrm{circlocal}_{A}:\mathrm{isEquiv}(\mathrm{const}_{A, S^1})}$

There is also a definitional version of $S^1$-localization which says that $\mathrm{const}_{A, S^1}$ is a definitional isomorphism:

$\frac{\Gamma \vdash A \; \mathrm{type}}{\Gamma, p:S^1 \to A \vdash \mathrm{const}_{A, S^1}^{-1}(p):A}$
$\frac{\Gamma \vdash A \; \mathrm{type}}{\Gamma, x:A \vdash \mathrm{const}_{A, S^1}^{-1}(\lambda i:S^1.x) \equiv x:A}$
$\frac{\Gamma \vdash A \; \mathrm{type}}{\Gamma, p:S^1 \to A \vdash \lambda i:S^1.\mathrm{const}_{A, S^1}^{-1}(p) \equiv p:S^1 \to A}$

### Relation to axiom K

One can prove axiom K (positive copy induction rules) from the axiom of circle type localization (negative copy inference rules):

###### Theorem

Suppose that every type $A$ is definitionally $S^1$-local.

Then definitional axiom K holds: given any type $A$, and any type family $C(p)$ indexed by loops $p:S^1 \to A$ in $A$, and given any dependent function $t:\prod_{x:A} C(\lambda i:S^1.x)$ which says that for all elements $x:A$, there is an element of the type defined by substituting the constant loop of $x:A$ into $C$, $C(\lambda i:S^1.x)$, one can construct a dependent function $K_A(t):\prod_{z:S^1 \to A} C(z)$ such that for all $x:A$, $K_A(t, \lambda i:S^1.x) \equiv t(x):C(\lambda i:S^1.x)$.

###### Proof

$K_A(t)$ is defined to be

$K_A(t) \equiv \lambda p:S^1 \to A.t(\mathrm{const}_{A, S^1}^{-1}(p))$

and by the computation rules of loop types as negative copies, one has that for all $x:A$,

$\mathrm{const}_{A, S^1}^{-1}(\lambda i:\mathbb{I}.x) \equiv x$

and so by definition of $\mathrm{ind}_{S^1 \to A}(t)$ and the judgmental congruence rules for substitution, one has

$K_A(t, \lambda i:S^1.x) \equiv t(\mathrm{const}_{A, S^1}^{-1}(\lambda i:S^1.x)) \equiv t(x)$

One has the following analogies between localization at a specific type and the type theoretic letter rule that it proves:

localization ruletype theoretic letter rule
$\mathbb{I}$-localizationJ-rule
$S^1$-localizationK-rule

## Consequences

Since the boolean domain $\mathbb{2}$ is $S^1$-local, the axiom of $S^1$-localization implies that $S^1$ is compact connected:

###### Theorem

Assuming propositional truncation, where $\Omega$ is the type of all $\mathcal{U}$-small propositions with type reflector $T$, and the axiom of $S^1$-localization, if the function $\mathrm{const}_{2, S^1}$ is an equivalence of types, then for all functions $P:S^1 \to \Omega$, if for all $x:S^1$, $T(P(x)) \vee \neg T(P(x))$ is contractible, then either for all $x:S^1$, $T(P(x))$ is contractible, or for all $x:S^1$, $\neg T(P(x))$ is contractible.

###### Proof

If $P:S^1 \to \Omega$ is such that for all $x:S^1$, $T(P(x)) \vee \neg T(P(x))$ is contractible, then there is a function $P':S^1 \to \mathbb{2}$ into the booleans type $\mathbb{2}$ with $\delta_{P'}^{1_2}(x):(P'(x) = 1_2)) \simeq T(P(x))$ and $\delta_{P'}^{0_2}(x):(P'(x) = 0_2)) \simeq \neg T(P(x))$. Since $\mathbb{2}$ is $S^1$-local, then, by the axiom of $S^1$-localization, $P'$ is constant, which implies that either for all $x:S^1$, $T(P(x))$ is contractible, or for all $x:S^1$, $\neg T(P(x))$ is contractible. Thus, $S^1$ is compact connected

## In spatial type theory

In spatial type theory, a crisp type $\Xi \vert () \vdash A$ is discrete if the function $(-)_\flat:\flat A \to A$ is an equivalence of types. There is a variant of the above axiom called the axiom of circle type cohesion or axiom $S^1 \flat$, which states that given any crisp type $\Xi \vert () \vdash A \; \mathrm{type}$, $A$ is discrete if and only if $A$ is $S^1$-local, or if $\mathrm{const}_{A, S^1}$ is an equivalence of types.

$\frac{\Xi \vert () \vdash A \; \mathrm{type}}{\Xi \vert () \vdash S^1 \flat\mathrm{ax}_A:\mathrm{isEquiv}(\lambda x:\flat A.x_\flat) \simeq \mathrm{isEquiv}(\mathrm{const}_{A, S^1})}$

This allows us to define discreteness for non-crisp types: a type $A$ is discrete if $A$ is $S^1$-local, resulting in a flavor of cohesive homotopy type theory where the shape modality is the 0-truncation modality. This rule is equivalent to the axiom of circle type localization if every type is discrete.