Characterization of Metacategory via Equations

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Theorem

Let $\mathbf C_0$ and $\mathbf C_1$ be collections of objects.

Let $\operatorname{Cdm}$ and $\operatorname{Dom}$ assign to every element of $\mathbf C_1$ an element of $\mathbf C_0$.

Let $\operatorname{id}$ assign to every element of $\mathbf C_0$ an element of $\mathbf C_1$.


Denote with $\mathbf C_2$ the collection of pairs $\tuple {f, g}$ of elements of $\mathbf C_1$ satisfying:

$\Dom g = \Cdm f$

Let $\circ$ assign to every such pair an element of $\mathbf C_1$.


Then $\mathbf C_0, \mathbf C_1, \operatorname{Cdm}, \operatorname{Dom}, \operatorname{id}$ and $\circ$ together determine a metacategory $\mathbf C$ if and only if the following seven axioms are satisfied:

\(\ds \Dom {\operatorname{id}_A} = A\) \(\qquad\) \(\ds \Cdm {\operatorname{id}_A} = A\)
\(\ds f \circ \operatorname{id}_{\Dom f} = f\) \(\) \(\ds \operatorname{id}_{\Cdm f} \circ f = f\)
\(\ds \Dom {g \circ f} = \Dom f\) \(\) \(\ds \Cdm {g \circ f} = \Cdm g\)
\(\ds h \circ \paren {g \circ f}\) \(=\) \(\ds \paren {h \circ g} \circ f\)

with $A$ and $f,g,h$ arbitrary elements of $\mathbf C_0$ and $\mathbf C_1$, respectively.

Further, in the last two lines it is presumed that all compositions are defined.


Hence it follows that:


Proof




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