# nLab Turing category

Contents

### Context

#### Constructivism, Realizability, Computability

intuitionistic mathematics

# Contents

## Definition

A Turing category is a certain categorification of partial combinatory algebras (PCAs) based on restriction categories.

###### Definition

(Turing category)

A Turing category is a cartesian restriction category $(\mathcal{C}, \bar{} )$ with a fixed object A and morphism $\bullet \colon A \times A \rightarrow A$ having the following universal property:

For each $\mathcal{C}$-morphism $f \colon X \rightarrow Y$ there is a section $s \colon Y \rightarrow A$ and retract $r \colon A \rightarrow X$, along with a total map $h \colon \mathbf{1} \rightarrow A$ making the following diagram commute:

While restriction structure captures the function partiality one finds in computability theory, the universal property stated above captures the universal coding of $\mathcal{C}$-morphisms into operations on the Turing object $A$, via the application $\bullet$. Indeed one recovers, as an example, the category $\mathbf{Rec}$, of natural numbers $n, m \in \mathbf{N}$ and partial recursive functions $\mathbf{N}^n \rightarrow \mathbf{N}^m$. This has universal applicative structure:

and universal representation $\langle i, n \rangle = f_i(n)$ of the $i$th computable function. In this case the PCA is Kleene's first algebra. More generally, the computable map category of any PCA forms a Turing category, with Turing object the PCA and its Turing morphism being the applicative structure.

And conversely, a Turing object $A$ of $\mathcal{C}$ and its Turing morphism $\bullet : A \times A \rightarrow A$ form a (relative) PCA. This is effectively a PCA-object internal to an appropriate category:

###### Definition

(Relative PCA)

A (relative) PCA $A$ is a combinatory complete partial applicative system in a cartesian restriction category $\mathcal{D}$, i.e. a morphism $\bullet : A \times A \rightarrow A$ in $\mathcal{D}$ such that finite powers $A^n$ and $A$-computable morphisms form a well-defined cartesian restriction subcategory of $\mathcal{D}$.

Note, however, that not every relative PCA in a Turing category $\mathcal{C}$ is a Turing object for $\mathcal{C}$.

### Basic properties

The arithmetical representation of a (partial) function $f : \mathbf{N} \rightarrow \mathbf{N}$ as a member of the recursive enumeration of relations $\varphi_i \subset \mathbf{N} \times \mathbf{N}$ is universal, and this representation satisfies the Kleene recursion theorems?:

$\bullet$ (smn) Let $f: \mathbf{N} \rightarrow \mathbf{N}$ be a partial computable function. Then there is a partial computable function $s : \mathbf{N} \rightarrow \mathbf{N}$ such that $f(\langle i, n \rangle) = f_{s(i)} (n)$.

$\bullet$ (utm) There is a partial computable function $U : \mathbf{N} \rightarrow \mathbf{N}$ such that $U(\langle i, n \rangle ) = f_i (n)$.

As can be nearly read off the definition above, these properties hold with respect to any Turing object. This is part of the overall motivation for Turing categories, since they provide a more model-independent setting both for stating the main theorems of recursion theory, while also allowing the “intrinsic meaning” of different constructions in computability theory to stand out. For example, in Type II computability (or function realizability) it turns out that function computability with respect to the PCA $\mathbf{N}^\mathbf{N}$ (either as Kleene's second algebra or the Baire space of sequences) has the topological meaning of continuity. Hence, despite the universal availability of Gödel-encodings of all “$A$-computations” for an appropriate data type $A$ into $\mathbf{N}$-computations (a form of the Church-Turing thesis), the theory of Turing categories and their applications to categorical realizability models, for example, are intended to give the intrinsic meaning of computational mathematics without resorting to such a universal representation, much like the success story of Type II computability.

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## Weak Limits and Exact Completions

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Turing categories were isolated categorically and named in: