Contents

# Contents

## Introduction

This article is about structure on a closed interval of real numbers, generally taken to be $I = [0, 1]$, that is derivable from a coalgebraic perspective. This topic was introduced by Peter Freyd.

## Coalgebraic description of $I$

For the moment we work classically, over the category of sets. A bipointed set is a set equipped with two elements, that is a cospan of the form

$\array{ 1 & & & & 1 \\ & ^\mathllap{x_0} \searrow & & \swarrow^\mathrlap{x_1} & \\ & & X & & }$

where $x_0$ and $x_1$ might coincide. There is a monoidal product on $Cospan(1, 1)$ given by cospan composition (formed by taking pushouts); this monoidal product is denoted $\vee$. (Explicitly, $(X,x_0,x_1) \vee (Y,y_0,y_1)$ is $((X \uplus Y)/(x_1 \sim y_0), x_0, y_1)$.) The monoidal unit is a $1$-element set with its unique bipointed structure. The category of such cospans or bipointed sets is denoted $Cos$.

Inside $Cos$ is the full subcategory of two-pointed sets, where $x_0$ and $x_1$ are distinct. Let $Twop$ be the category of two-pointed sets. The monoidal product $\vee$ restricts to a functor

$\vee \colon Twop \times Twop \to Twop$

and one can define the square

$sq = (Twop \stackrel{\Delta}{\to} Twop \times Twop \stackrel{\vee}{\to} Twop)$

A $sq$-coalgebra is a two-pointed set $X$ together with a map $\xi: X \to X \vee X$. An example is given by $I = [0, 1]$, where $I \vee I$ is identified with the interval $[0, 2]$ and the coalgebra structure $I \to I \vee I$ is identified with multiplication by $2$, $[0, 1] \to [0, 2]$. This map will be denoted $\alpha$.

###### Theorem (Freyd)

$(I, \alpha)$ is terminal in the category of $sq$-coalgebras.

## Corecursively defined operations on $I$

We now define a number of operations on $I$. For $0 \leq x \leq 1$, define ${x \uparrow} \coloneqq \min(2 x, 1)$ and ${x \downarrow} \coloneqq \max(2 x - 1, 0)$. These give unary operations on $I$ which can also be defined as maps in $Cos$ using the coalgebra structure $\alpha$:

$I \stackrel{{(-) \uparrow}}{\to} I = (I \stackrel{\alpha}{\to} I \vee I \stackrel{I \vee !}{\to} I \vee 1 \cong I)$
$\,$
$I \stackrel{{(-) \downarrow}}{\to} I = (I \stackrel{\alpha}{\to} I \vee I \stackrel{! \vee I}{\to} 1 \vee I \cong I)$

We similarly define unary operations ${(-)\uparrow}$, ${(-) \downarrow}$ for any $sq$-coalgebra $(X, \xi)$. For any coalgebra $X$ and $x \in X$, either ${x \downarrow} = x_0$ or ${x \uparrow} = x_1$. Moreover, if $i_0 \colon X \to X \vee X$ and $i_1 \colon X \to X \vee X$ are the evident pushout inclusions, we have $\xi(x) = i_0({x \uparrow})$ if ${x \downarrow} = x_0$, and $\xi(x) = i_1({x \downarrow})$ if ${x \uparrow} = x_1$. This means that coalgebra structures can be recovered from algebraic structures consisting of two constants $x_0, x_1$ and two unary operations $\uparrow$, $\downarrow$, although we must consider a coherent but non-algebraic axiom

$\vdash {x \uparrow} = x_1 or {x \downarrow} = x_0$

### Order-theoretic operations

Next, we define meet and join operations on $I$, making it a lattice, by exploiting corecursion. A slick corecursive definition of the order $\leq$ is that $x \leq y$

• if ${x \downarrow} = 0$ and ${x \uparrow} \leq {y \uparrow}$, or

• if ${y \uparrow} = 1$ and ${x \downarrow} \leq {y \downarrow}$.

If one prefers to work with operations, one could define the meet operation $\wedge \colon I \times I \to I$ by putting a suitable coalgebra structure on $I \times I$ and using terminality of the coalgebra $I$ to define $\wedge$ as a coalgebra map. A coalgebra structure

$\xi \colon I \times I \to (I \times I) \vee (I \times I)$

which works is

• $\xi(x, y) = i_0({x \uparrow}, {y \uparrow})$ if ${x \downarrow} = 0$ or ${y \downarrow} = 0$;

• $\xi(x, y) = i_1({x \downarrow}, {y \downarrow})$ if ${x \uparrow} = 1 = {y \uparrow}$.

### Midpoint operations

The general midpoint operation is not as easy to construct as one might think, but to start with we do have operations which take the midpoint between a given point and an endpoint. Namely, the left midpoint operation is the unary operation defined by

$l = (I \stackrel{i_0}{\to} I \vee I \stackrel{\alpha^{-1}}{\to} I)$

and the right midpoint operation is defined by

$r = (I \stackrel{i_1}{\to} I \vee I \stackrel{\alpha^{-1}}{\to} I).$

## Constructive aspects

There are two versions of this construction, classically equivalent, in constructive mathematics, one of which produces the unit interval of MacNeille real numbers, the other of which produces the unit interval of located Dedekind real numbers. Both start with $Cos$ (which is straightforward) but define $Twop$ differently, and the monoidal product $\vee$ is subtler.

For the MacNeille real numbers, define $Twop_M$ in the most obvious way,

$\neg\; x_0 = x_1 .$

For the Dedekind real numbers, define $Two_D$ by requiring

$\forall x,\; \neg\; x = x_0 \;\vee\; \neg\; x = x_1 .$

(The two statements are equivalent assuming the classical de Morgan law $\neg(p \wedge q) \Rightarrow \neg{p} \vee \neg{q}$, but the latter is stronger in intuitionistic logic.)

In $Cos$, let $X \vee Y$ be a subset of $X \times Y$, with $(x,y) \in X \vee Y$ iff

$(\neg\; x = x_1 \;\Rightarrow\; y = y_0) \;\wedge\; (\neg\; y = y_0 \;\Rightarrow\; x = x_1) .$

(A pushout can be constructed using $x = x_1 \;\vee\; y = y_0$ instead; this is stronger in intuitionistic logic but equivalent assuming excluded middle. If equality happens to be stable in $X$ and $Y$, then only the classical de Morgan law is needed to make this a pushout.)

As in the classical case, this makes $Twop_M$ and $Twop_D$ into “semigroupal” categories (monoidal categories but without the monoidal unit), since the unit object (which exists in $Cos$ as the singleton) does not exist in $Twop_M$ or in $Twop_D$, but we can still define $sq_M$ and $sq_D$ and consider their coalgebras. As stated above, $[0,1]_M$ is the final coalgebra of $sq_M$, and $[0,1] = [0,1]_D$ is the final coalgebra of $sq_D$; both are equipped with $0$ and $1$.

Another approach to the Dedekind real interval looks simpler from the perspective of constructive analysis. Start with $Set_\#$, the category of $T_0$ point–point apartness spaces and strongly extensional functions between them. (A point–point apartness space is a set equipped with an apartness relation, that is a binary relation $\#$ satisfying properties dual to those of an equivalence relation; it is $T_0$ iff the apartness relation is tight; and a function between such spaces is strongly extensional iff it reflects $#$.) On $Cos_\#$, let $X \vee Y$ be the subset of $X \times Y$ satisfying

$(x \# x_1 \;\Rightarrow\; y = y_0) \;\wedge\; (y \# y_0 \;\Rightarrow\; x = x_1) .$

Define $Twop_\#$ by

$x \# x_0 \;\vee\; x \# x_1 .$

Now defining $sq_\#$ on $Twop_\#$, its final coalgebra is again the Dedekind unit interval (equipped with its endpoints).

## Computational aspects

… explain how the classical version corresponds to computation with (unsigned) binary digits … the constructive (Dedekind) version corresponds to computation with signed binary digits …

The original development by Freyd is

• Peter Freyd, Real coalgebra, post to the categories list, December 22, 1999 (web)

The constructive version is also due to Freyd:

• Peter Freyd, Reality Check, post to the categories list, July 31, 2000 (web)

Freyd published 8 years later:

• Peter Freyd, Real Algebraic Analysis, TAC 20 no.10 (2008) pp. 215–306. (web)

There is also a complete development in the Elephant: