For some cardinal, say a morphism in is relatively -compact if for all (∞,1)-pullbacks along to -compact objects, , the pulled back object is itself a -compact object.
A presentable (∞,1)-category is an (∞,1)-topos precisely if
it has universal colimits;
for sufficiently large regular cardinals , has a classifying object for relatively -compact morphisms.
In ∞Grpd the relatively -compact morphisms, , def. are precisely those all whose homotopy fibers
over all objects are -small infinity-groupoids.
We may write as an (∞,1)-colimit over itself (see there)
and then use the fact that ∞Grpd – being an (∞,1)-topos – has universal colimits, to obtain the (∞,1)-pullback diagram
exhibiting as an -colimit of -small objects over . By stability of -compact objects under -small colimits (see here) it follows that is -compact if is.
This is due to Charles Rezk. The statement appears as HTT, theorem 6.1.6.8.
Last revised on March 2, 2019 at 05:07:08. See the history of this page for a list of all contributions to it.