# Homotopy Type Theory an axiomatization of the real numbers > history (changes)

Showing changes from revision #12 to #13: Added | Removed | Changed

# Contents

## Definition

### Propositions

A proposition is a type $A$ with an identification $p:a =_A b$ for all elements $a:A$ and $b:A$.

### Sets

A set is a type $A$ such that for all elements $a:A$ and $b:A$, the identity type $a =_A b$ is a proposition.

### Pointed abelian groups

A pointed abelian group is a set $A$ with an element $1:A$ called one and a binary operation $(-)-(-):A \times A \to A$ called subtraction such that

• for all elements $a:A$, $a - a = 1 - 1$

• for all elements $a:A$, $(1 - 1) - ((1 - 1) - a) = a$

• for all elements $a:A$ and $b:A$, $a - ((1 - 1) - b) = b - ((1 - 1) - a)$

• for all elements $a:A$, $b:A$, and $c:A$, $a - (b - c) = (a - ((1 - 1) - c)) - b$

### Pointed halving groups

A pointed halving group is a pointed abelian group G with a function $(-)/2:G \to G$ called halving or dividing by two such that for all elements $g:G$, $g/2 = g - g/2$.

### Totally ordered pointed halving groups

A pointed halving group $R$ is a totally ordered pointed halving group if it comes with a function $\max:R \times R \to R$ such that

• for all elements $a:R$, $\max(a, a) = a$

• for all elements $a:R$ and $b:R$, $\max(a, b) = \max(b, a)$

• for all elements $a:R$, $b:R$, and $c:R$, $\max(a, \max(b, c)) = \max(\max(a, b), c)$

• for all elements $a:R$ and $b:R$, $\max(a, b) = a$ or $\max(a, b) = b$

• for all elements $a:R$ and $b:R$, $\max(a, b) = b$ implies that for all elements $c:R$, $\max(a - c, b - c) = b - c$

### Strictly ordered pointed halving groups

A totally ordered pointed halving group $R$ is a strictly ordered pointed abelian group if it comes with a type family $\lt$ such that

• for all elements $a:R$ and $b:R$, $a \lt b$ is a proposition
• for all elements $a:R$, $a \lt a$ is false
• for all elements $a:R$, $b:R$, and $c:R$, if $a \lt c$, then $a \lt b$ or $b \lt c$
• for all elements $a:R$ and $b:R$, if $a \lt b$ is false and $b \lt a$ is false, then $a = b$
• for all elements $a:R$ and $b:R$, if $a \lt b$, then $b \lt a$ is false.
• $1 - 1 \lt 1$
• for all elements $a:R$ and $b:R$, if $1 - 1 \lt a$ and $1 - 1 \lt b$, then $(1 - 1) \lt a - ((1 - 1) - b)$

The homotopy initial strictly ordered pointed halving group is the dyadic rational numbers $\mathbb{D}$.

### Archimedean ordered pointed halving groups

A strictly ordered pointed halving group $A$ is an Archimedean ordered pointed halving group if for all elements $a:A$ and $b:A$, if $a \lt b$, then there merely exists a dyadic rational number $d:\mathbb{D}$ such that $a \lt h(d)$ and $h(d) \lt b$.

### Sequentially Cauchy complete Archimedean ordered pointed halving groups

Let $A$ be an Archimedean ordered pointed halving group and let

$A_{+} \coloneqq \sum_{a:A} 1 - 1 \lt a$

be the positive elements in $A$.

A sequence in $A$ is a function $x:\mathbb{N} \to A$.

A sequence $x:\mathbb{N} \to A$ is a Cauchy sequence if for all positive elements $\epsilon:A_{+}$, there merely exists a natural number $N:\mathbb{N}$ such that for all natural numbers $i:\mathbb{N}$ and $j:\mathbb{N}$ such that $N \leq i$ and $N \leq j$, $\max(x_i - x_j, x_j - x_i) \lt \epsilon$.

An element $l:A$ is said to be a limit of the sequence $x:\mathbb{N} \to A$ if for all positive elements $\epsilon:A_{+}$, there merely exists a natural number $N:\mathbb{N}$ such that for all natural numbers $i:\mathbb{N}$ such that $N \leq i$, $\max(x_i - l, l - x_i) \lt \epsilon$

$A$ is sequentially Cauchy complete if every Cauchy sequence in $A$ merely has a limit.

Last revised on June 19, 2022 at 19:52:18. See the history of this page for a list of all contributions to it.