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Stream: deprecated: id my structure

Topic: Quotients of (monoidal) functors


view this post on Zulip Martti Karvonen (May 01 2023 at 17:41):

I'm in a situation where I have a (lax monoidal) functor FF into Set and equivalence relations A\approx_A on each F(A)F(A). I can write down conditions under which F/F/\approx becomes a lax monoidal functor (so that the quotient maps F(A)F(A)/AF(A)\to F(A)/\approx_A form a monoidal natural transformation), but I'm wondering if there's some standard terminology for this kind of thing. Has anyone seen this before? What would you call it?

view this post on Zulip John Baez (May 01 2023 at 18:09):

I don't know, but it sounds like your functor might actually be a functor into [[setoids]], where a setoid is a set with an equivalence relation.

view this post on Zulip John Baez (May 01 2023 at 18:10):

(As you can see from the link, people are still fighting a bit about what setoids actually are.)

view this post on Zulip Martti Karvonen (May 01 2023 at 21:58):

That's a good way of looking at the situation!

view this post on Zulip Morgan Rogers (he/him) (May 02 2023 at 09:39):

An alternative take is that you can view FF as a (co)presheaf and A{\approx}_A as an equivalence relation on it in the category of (co)presheaves. I don't know what the monoidal product is doing, though (which monoidal structure on Set\mathrm{Set} is it hitting?) but Day convolution might be useful to handle the monoidal structure.

view this post on Zulip Martti Karvonen (May 02 2023 at 14:24):

I'm using the cartesian monoidal structure on Set. I guess in this take FF becomes an internal monoid (wrt Day convolution), and \approx is then a congruence on this monoid.