Multiplicative graph

For comparison, this diagram shows a typical arrow composition in an ordinary category. (Without arrow composition, it is simply a directed graph.) The arrows $f$ and $g$ in the diagram are consecutive; they connect in $B=\text{cod}(f)=\text{dom}(g).$ In an ordinary category, the composition of any pair of consecutive arrows exists, whereas in a multiplicative graph, a pair of consecutive arrows is not necessarily composable.

In mathematics, a multiplicative graph (in French: graphe multiplicatif or neocategoryTemplate:R in some English-language papers) is an algebraic structure in category theory. It is a generalization of an ordinary category in the sense that neither the associativity of arrow composition nor the composibility of a pair of connected arrows are assumed. While an ordinary category is a notion combining a directed graph and a monoidal structure, a multiplicative graph is a partial magma-like structure. Namely, it is a structure in one‑to‑one correspondence with the nodes of a directed graph, and each object has left and right identity morphisms, but composability is partial, and, moreover, associativity is not required.

Arrow composition in an ordinary category satisfies the following property: if arrows $f$ and $g$ connect in the sense that $\text{dom}(g)=\text{cod}(f),$ their composition $g\circ f$ is defined, and, moreover, $\text{dom}(g\circ f)=\text{dom}(f)$ and $\text{cod}(g\circ f)=\text{cod}(g).$ In a multiplicative graph, however, condition $\text{dom}(g)=\text{cod}(f)$ does not guarantee the existence of $g\circ f$ within that structure without further assumptions – but if this composition exists, it also satisfies $\text{dom}(g\circ f)=\text{dom}(f)$ and $\text{cod}(g\circ f)=\text{cod}(g).$ When drawing a diagram for a multiplicative graph, it is almost always necessary to explicitly draw all existing arrows that play a role in the argument. For example, as shown in Coppey (1980), square diagrams in a multiplicative graph can take one of five types depending on which potential compositions in the diagram are actually defined.

This notion first appears in Ehresmann's book Catégories et structures. The French school bases its definition of sketch on the notion of a multiplicative graph,Template:R because this definition required a category-like structure that avoided redundant axioms as much as possible. This structure is the multiplicative graph, and this is a type of relaxed notion of category, such as a semicategory.Template:R

Cury studied enriched multiplicative graph. As a more general notion, there is the compositional graph, and multiplicative graphs can be seen as strongly identitive compositional graphs.

Definition

Template:Group-like structures A multiplicative graph $\mathrm{\Sigma }$ is couple formed by a set denoted by $\underset{\_}{\mathrm{\Sigma }}$, and a partial law of composition $\kappa$ on $\underset{\_}{\mathrm{\Sigma }}$ satisfying the following axioms:Template:R

1. $\kappa$ is a mapping from a subset of $\underset{\_}{\mathrm{\Sigma }}\times \underset{\_}{\mathrm{\Sigma }}$ (denoted by $\mathrm{\Sigma }\ast \mathrm{\Sigma }$ and called the set of composable couples) into $\underset{\_}{\mathrm{\Sigma }}$; instead of $\kappa (y,x)$, we write $y\circ x$ and we call $y\circ x$ the composite of $(y,x)$.
2. There exists a reflexive graph $(\underset{\_}{\mathrm{\Sigma }},\beta ,\alpha )$ (i.e. $\alpha$ and $\beta$ are retractions from $\underset{\_}{\mathrm{\Sigma }}$ onto a subset of $\underset{\_}{\mathrm{\Sigma }}$, denoted by ${\mathrm{\Sigma }}_0$), such that:

(existence of units): For each element $x$ of $\underset{\_}{\mathrm{\Sigma }}$, the composites $x\circ \alpha (x)$ and $\beta (x)\circ x$ are defined, and we have

$x\circ \alpha (x)=x=\beta (x)\circ x.$

Here, $\alpha (x)$ is the right identity of $x$ and is called the source of $x$, while $\beta (x)$ is the left identity of $x$ and is called the target of $x$;

(coherence of dom/codTemplate:R): If the composite $y\circ x$ is defined, then:

$\alpha (y\circ x)=\alpha (x),\beta (y\circ x)=\beta (y),\text{and}\alpha (y)=\beta (x).$

From the condition 2, the reflexive graph $(\mathrm{\Sigma },\beta ,\alpha )$ is uniquely defined.

Example

• An ordinary category is a multiplicative graph if it satisfies the following two axioms:Template:RTemplate:R

(composibility):all the couples $(y,x)$ where $\alpha (y)=\beta (x)$ are composable (so that $\mathrm{\Sigma }\ast \mathrm{\Sigma }$ is the pullback of $(\alpha ,\beta )$);

(strong associativity):the law of composition being furthermore associative.

• For the two axioms above, a notion got by adding only the associativity axiom to a multiplicative graph, that is, a notion that an ordinary category without the composibility axiom, this is called a precategory.Template:R But, this is not standard terminology, a precategory is usually synonymous with a semigroupoid and does not require each object to have an identity morphism.

See also

• Partial groupoid
• Non-associative algebra

References

• Template:Cite journal
• Template:Cite journal
• Template:Cite journal
• Template:Cite journal
• Template:Cite thesis
  • Template:Cite journal
  • Template:Cite journal
  • Template:Cite journal
  • Template:Cite journal
• Template:Cite journal
• Template:Cite book
• Template:Cite book
• Template:Cite journal

External link

• Template:Cite arXiv
• Template:Cite web