# nLab closed midpoint algebra

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Idea

The idea of a closed midpoint algebra comes from Peter Freyd.

## Definition

A closed midpoint algebra is a cancellative midpoint algebra $(M,\vert)$ equipped with two chosen elements $\bot:M$ and $\top:M$.

## Examples

The unit interval with $a \vert b \coloneqq \frac{a + b}{2}$, $\bot = 0$, and $\top = 1$ is an example of a closed midpoint algebra.

The set of truth values in Girard’s linear logic is a closed midpoint algebra.

## Properties

Every closed midpoint algebra with $\bot = \top$ is trivial.

### Partial order

Every closed midpoint algebra has a natural partial order defined as $a \leq b$ for elements $a$ and $b$ in $M$ if there exists $c$ in $M$ such that $a \vert \top = b \vert c$. Dually, $a \leq b$ for elements $a$ and $b$ in $M$ if there exists $c$ in $M$ such that $c \vert a = \bot \vert b$.

Every closed midpoint algebra homomorphism is a monotone.

### Function algebras

For any set $S$ and any closed midpoint algebra $M$, the set of functions from $S$ to $M$ is a closed midpoint algebra $(S\to M,\vert_{S\to M},\bot_{S\to M},\top_{S\to M})$, defined by

• $(f\vert_{S\to M} g)(x) = f(x)\vert_M g(x)$ for all $f,g:S\to M$ and $x:M$

• $(\bot_{S\to M})(x) = \bot_M$ for all $x:M$

• $(\top_{S\to M})(x) = \top_M$ for all $x:M$

### Definite integration

Let $M$ and $N$ be continuous closed midpoint algebras and let $M\to_C N$ be the space of continuous functions from $M$ to $N$. Then there exists an operator $\int (-)(x) \mathrm{d}x: (M\to_C N)\to (M\to_C N)$ called definite integration such that

$\int \top_{M\to N}(x) \mathrm{d}x = \top_N$
$\int \bot_{M\to N}(x) \mathrm{d}x = \bot_N$
$\int f(x) \vert_N g(x) \mathrm{d}x = \int f(x) \mathrm{d}x \vert_N \int g(x) \mathrm{d}x$
$\int f(x) \mathrm{d}x = \int f(\bot_M \vert_M x) \mathrm{d}x \vert_N \int f(x \vert_M \top_M) \mathrm{d}x$

for all $x$ in $M$.

When $M$ is the unit interval on the real numbers $[0,1]$, and $N$ is an interval $[a,b]$ on the real numbers, written in conventional notation the axioms become:

$\int_{0}^{1} a(x) \mathrm{d}x = a$
$\int_{0}^{1} b(x) \mathrm{d}x = b$
$\int_{0}^{1} \frac{f(x) + g(x)}{2} \mathrm{d}x = \frac{\int_{0}^{1} f(x) \mathrm{d}x + \int_{0}^{1} g(x) \mathrm{d}x}{2}$
$\int_{0}^{1} f(x) \mathrm{d}x = \frac{\int_{0}^{1} f(\frac{0 + x}{2}) \mathrm{d}x + \int_{0}^{1} f(\frac{x + 1}{2}) \mathrm{d}x}{2}$

## References

• Peter Freyd, Algebraic real analysis, Theory and Applications of Categories, Vol. 20, 2008, No. 10, pp 215-306 (tac:20-10)