# nLab circle object

Contents

### Context

#### 2-Category Theory

higher category theory

# Contents

## Idea

The semantics of the circle type in (2,1)-category theory.

## Definition

In a weak (2,1)-category $C$ with terminal object $*$, a circle object is an object $S^1$ in $C$ with a global element $\beta:* \to S^1$ and an equivalence $\lambda:\beta \cong \beta$ such that for every other object $A$ in $C$ with a global element $\beta_A:* \to A$ and an equivalence $\lambda_A:\beta_A \cong \beta_A$, there is a functor $f:S^1 \cong A$ and a functor $f^{'}:(\beta \cong \beta) \to (\beta_A \cong \beta_A)$ and equivalences $p:f \circ \beta \cong \beta_A$ and $q:f^{'} \circ \lambda \cong \lambda_A \circ f^{'}$ satisfying coherence laws.

## Properties

The loop space object of the object $S^1$ with global element $\beta$ the circle object is equivalent to an integers object $Z$.