(2,1)-quasitopos?
structures in a cohesive (∞,1)-topos
Fix a Grothendieck universe $\mathcal{U}$ and a smaller universe $\mathcal{V} \in \mathcal{U}$. Let $\mathcal{V}$ be the reference-universe, so that sets in $\mathcal{V}$ are called small sets, sets in $\mathcal{U}$ are called large, and sets not necessarily in $\mathcal{U}$ are called very large.
Write ∞Grpd for the (large) (∞,1)-category of small ∞-groupoids and $\infty GRPD$ for the very-large $(\infty,1)$-category of large $\infty$-groupoids.
Then the general procedures of universe enlargement can be applied to any large $(\infty,1)$-category to produce a very-large one. Specifically, we have the locally presentable enlargement: for $C$ a large (∞,1)-category with small (∞,1)-colimits, write
for the full sub-(∞,1)-category of the (∞,1)-category of (∞,1)-functors that preserves small (∞,1)-limits. As described at universe enlargement, if $C$ is locally presentable, then $\Uparrow C$ can be identified with the naive enlargement, which consists of the large models of the “theory” of which $C$ consists of the small models.
In particular, when $C$ is an (∞,1)-sheaf (∞,1)-topos $Sh(S)$ of small (∞,1)-sheaves, then $\Uparrow C$ can be identified with a (∞,1)-category of large (∞,1)-sheaves on the same site. (That is, with a suitable accessible left-exact-reflective subcategory of $PSH(S)$ rather than $Psh(S)$—it is not yet known how to specify such a reflective subcategory purely in terms of data on $S$.) Thus, we refer to $\Uparrow C$ as the very large $(\infty,1)$-sheaf $(\infty,1)$-topos on $C$.
Note that since every topos can be identified with the category of sheaves on itself for the canonical topology, it is also reasonable to denote $\Uparrow \mathbf{H}$ by $SH(\mathbf{H})$. $\Uparrow C$ can also also be identified with the category of ind-objects of $\mathbf{H}$, for a suitable regular cardinal $\kappa$ (namely, the cardinal of $\mathcal{V}$).
For every $(\infty,1)$-topos $\mathbf{H}$ there is an (∞,1)-functor
that preserves large (∞,1)-colimits and finite (∞,1)-limits. It is defined by sending $F : (\infty,1)Topos^{op} \to \infty GRPD$ to the composite
This is HTT, lemma 6.3.5.21.
This is discussed in section 6.3 of
The definition of $\Uparrow\mathbf{H}$ is in Notation 6.3.5.16 and Remark 6.3.5.17. The relation to ind-objects appears as remark 6.3.6.18.
Last revised on December 11, 2010 at 17:58:50. See the history of this page for a list of all contributions to it.