How much do universes matter in topos theory? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-25T17:07:00Z http://mathoverflow.net/feeds/question/108578 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/108578/how-much-do-universes-matter-in-topos-theory How much do universes matter in topos theory? David Carchedi 2012-10-01T22:58:54Z 2012-10-02T03:46:44Z <p>Suppose we fix two Grothendieck universes $\mathcal{U} \in \mathcal{V}.$ Then one has that $\mathcal{U}$-$\mathbf{Set},$ the category of $\mathcal{U}$-small sets, is a locally $\mathcal{U}$-small, $\mathcal{V}$-small category. Grothendieck universes were used often by, well, Grothendieck, in his work with topoi. Part of the reason was to work with "large sites". One important example is that for a given topos $\mathcal{E},$ it carries its <em>canonical</em> topology. Grothendieck universes can be used to make sense of the statement that $\mathcal{E}$ is equivalent to sheaves over itself with respect to this topology, but lets look at another idea. If $\mathcal{E}=Sh\left(C,J\right)$ is a sheaf topos, we will instead of considering sheaves with values in the large category of all sets, consider only sheaves with values in $\mathcal{U}$-$\mathbf{Set}.$ And this is a <em>$\mathcal{U}$-topos.</em> Lets take this idea and run with it:</p> <p>There is a functor from $\mathcal{U}$-topoi to $\mathcal{V}$-topoi which sends a $\mathcal{U}$-topos $\mathcal{E}$ to $$Sh\left(\mathcal{E},\mathcal{V}\mbox{-}\mathbf{Set}\right),$$ the $\mathcal{V}$-topos of sheaves on $\mathcal{E}$ with its canonical topology, considered as a $\mathcal{V}$-small site.</p> <p>This easily generalizes for $n$-topoi for any $n$. Are these functors full and faithful?</p> <p>For $n=0$ the answer is yes, and remains yes for $n=1$ if we restrict to localic topoi, for topological reasons- but is it true for higher $n$?</p> <p>(I foresee a possible complication for $\infty$-topoi which are not equivalent to sheaves on a site.)</p> http://mathoverflow.net/questions/108578/how-much-do-universes-matter-in-topos-theory/108595#108595 Answer by Jacob Lurie for How much do universes matter in topos theory? Jacob Lurie 2012-10-02T03:46:44Z 2012-10-02T03:46:44Z <p>The change-of-universe construction is faithful but not full. For example, let X be the topos of sets and let Y be the classifying topos for abelian groups. The category of geometric morphisms from X to Y is equivalent to the category of abelian groups. If you pass to a larger universe, you get more abelian groups.</p>