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Freyd was the first to formalize a striking comparison between abelian categories and topoi, showing that their exactness properties can be jointly captured by the axioms of AT categories, and the difference between the two is simply modulated by the initial objects: in every AT category, there is an initial object $0$. An object $X$ is of type A if $\pi_X : X\times 0 \to X$ is an isomorphism, and of type T if $\pi_0 : X\times 0 \to 0 $ is an isomorphism. It turns out that an AT category with all objects type A is an abelian category, and one with all objects type T is a topos. In this sense, if the initial object thinks its in a commutative ring, you have a topos, and if it thinks its in an abelian group, you have an abelian category.

On the homotopic side of things, topoi correspond to $\infty$-topoi and abelian categories to stable $\infty$-categories. I was wondering if there was any known work on whether Freyd's observations for the $1$-categorical case lift to the homotopic case? Specifically, is there a class of $\infty$-categories with some homotopy-coherent exactness properties that subsumes stable $\infty$-categories and $\infty$-topoi, and such that categories with all objects "type T" are $\infty$-topoi, and likewise for "type A" objects and stable $\infty$-categories, analogous to the above result?

To give some context, I am investigating the Goodwillie calculus, and I think the process of stabilizing an $\infty$-category corresponds to something like taking the K-theory of a commutative ring. I think $\mathbb{F}_{1}$ plays a special role in it all, as decategorifying AT categories yields some sort of "AT algebras," and the unique abelian group which is also a ring is $\mathbb{F}_{1}$, just as the unique abelian topos is $\ast$. Finding out just how it fits into the picture will help me generalize the Goodwillie calculus further, and allow for the correspondence between analysis and homotopy to be fleshed out a bit more.

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    $\begingroup$ FWIW: when I wrote that nLab article some years ago, Urs Schreiber asked a similar question: nforum.ncatlab.org/discussion/1235/at-category/#Item_4, and David Corfield might have asked something similar somewhere. (Aside: I'm not happy with the way it reads in the opening; in earlier versions I had been over-enthusiastic, and then later over-compensated for that in subsequent revisions so by now it sounds like AT categories are being put down a little. I should correct that sometime.) $\endgroup$ Commented Nov 11, 2016 at 18:37
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    $\begingroup$ We carried on the conversation here: nforum.ncatlab.org/discussion/5538/at-category . $\endgroup$ Commented Nov 11, 2016 at 20:01
  • $\begingroup$ The exactness properties that characterize AT categories are pretty ugly and inelegant, which reminds me of something John Baez has said in the past: decategorifying pretty things gives you ugly things. Maybe there is something similar going on here to that which happens with triangulated categories, where the "nonexotic" examples of AT categories arise as model categories of some class of nice $\infty$-categories. I'm not sure how to go about investigating further in this direction, though. Does anything immediately suggest that this might be the case? $\endgroup$ Commented Nov 12, 2016 at 18:39

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