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• CommentRowNumber1.
• CommentAuthorPeter Heinig
• CommentTimeJul 10th 2017

This is just to suggest another “test problem”, concerning a question implicity asked in the thread

Higher model theory

namely how to professionally/routinely/usually handle the fact that according to the usual conventions, one may arbitrarily modify diagrammatically-given axioms by introducing identity arrows into them, and then the resulting diagram, while of course dismissable by “nothing happened, you did the category-theoretic equivalent of a Reidemeister move”, can “trigger” different category-theoretic concepts/ideas: I could point to at least one literature reference which treats the axioms for

• a vertically-strict, horizontally-weak double-category

by displaying the parallel pair of functors

$\array{ & & & & & \\ & & \overset{cod}{\longrightarrow} & & & \\ \mathsf{D}_1 & & & & & \mathsf{D}_0 \\ & & \overset{dom}{\longrightarrow}& & & \\ & & & & & }$

as the span of functors

$\array{ & && & & \\ & & & & & \\ \mathsf{D}_0 & & & \overset{\text{----------------------}}{\text{------------------}} && & \mathsf{D}_0 \\ & \overset{cod}{\nwarrow} & & \overset{dom}{\nearrow} & & \\ & & \mathsf{D}_1 & & & }$

which then may “trigger” the concept of pushouts-in-CAT, and another literature reference which arbitrarily”blows-up” the parallel pair of functors into a cospan of functors

$\array{ & && & & \\ & & & & & \\ \mathsf{D}_1 & & & \overset{\text{----------------------}}{\text{------------------}} && & \mathsf{D}_1 \\ & \overset{cod}{\searrow} & & \overset{dom}{\swarrow} & & \\ & & \mathsf{D}_0 & & & }$

which may “trigger” the concept of pullbacks-in-CAT.

One of the references actually uses pullbacks-in-CAT in the sequel .

I do not expect there to be much to be said, in particular since pushouts and pullbacks are dual concepts (which in turn is reflected in how small the “edit-distance” from the above code for the above span into the above code of the above cospan is) so that here in a sense even less happens than in the case of the example of monoidal bicategories axiomatized with, alternatively, a triangular or a quadrangular $\mu$iddle unitor. Except for some evident things, such as that a reasonable definition of Mod() should judge

Mod(axiomatization of double-categories with a parallel pair)

“equal” to

Mod(axiomatization of double-categories with a span)

“equal” to

Mod(axiomatization of double-categories with a cospan)

and therefore must somehow always simultaneously be “aware” of both sides of any two dual cat-concepts.

Again, there probably is not much to say here, I am just floating this particularly “trivial” and symmetric and self-dual example. But who knows?

• CommentRowNumber2.
• CommentAuthorMike Shulman
• CommentTimeJul 10th 2017

2-categorical algebra is very well-studied. 2-monads are one aspect of it, also there are 2-algebraic theories and so on. I’m not sure what you mean by “trigger”, but if you just mean that drawing things in a different way may make the reader think of different ideas, then that has nothing to do with categorification.