Profunctors corresponding to "partial functors" - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T09:40:20Z http://mathoverflow.net/feeds/question/35784 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/35784/profunctors-corresponding-to-partial-functors Profunctors corresponding to "partial functors" Aleks Kissinger 2010-08-16T18:36:40Z 2010-08-18T14:01:28Z <p>Suppose we have a span of categories $C \overset{F}{\hookleftarrow} D \overset{G}{\rightarrow} E$, where $F$ is a subcategory embedding. We can lift these normal functors to profunctors $\hat F$ and $\hat G$ and compose the "formal" adjoint $\hat F^\dagger$ with $\hat G$ to obtain a profunctor $\hat G \hat F^\dagger : C \nrightarrow E$. To what extent could one think of this as a partial functor, and what nice behaviours could it inherit from $F$ and $G$? For instance, if $F$ reflects products and $G$ preserves them, does $\hat G \hat F^\dagger$ preserve them where ever it is defined?</p> http://mathoverflow.net/questions/35784/profunctors-corresponding-to-partial-functors/35969#35969 Answer by Mike Shulman for Profunctors corresponding to "partial functors" Mike Shulman 2010-08-18T14:01:28Z 2010-08-18T14:01:28Z <p>One thing along these lines that you can say is that if D has, and G preserves, finite limits (or more generally is <a href="http://ncatlab.org/nlab/show/flat+functor" rel="nofollow">flat</a>), then so does $\hat{G}$ considered as a cocontinuous functor $[D^{op},Set] \to [E^{op},Set]$. Since $\hat{F}^\dagger : [C^{op},Set] \to [D^{op},Set]$ is just precomposition with F, it preserves all limits and colimits; thus $\hat{G} \hat{F}^\dagger$ preserves finite limits as soon as G does, without any hypothesis on F. I don't know whether this can be extended to other kinds of limits.</p> <p>Regarding the more general question of whether $\hat{G} \hat{F}^\dagger$ could be considered a "partial functor," another way to describe it is as the left Kan extension of the composite $D \overset{G}{\to} E \hookrightarrow [E^{op},Set]$ along F. If F is fully faithful, then such an extension is an honest extension, i.e. it restricts back along F to the original functor. So one could think of it as obtained by extending G to objects not in D in the most universal way possible: it maps an object $c\in C$ to the formal colimit (viewing $[E^{op},Set]$ as the free cocompletion of E) over all approximations to c by objects of D.</p> <p>On the other hand, <em>every</em> profunctor can be obtained as a composite $\hat{G} \hat{F}^\dagger$ for some functors G and F (not necessarily an embedding): let the intermediate category D be the two-sided discrete fibration corresponding to that profunctor. An arbitrary profunctor can be thought of as a "generalized functor," but usually not specifically a "partial functor." However, perhaps faithfulness, or full-and-faithfulness, of F implies some properties of the resulting profunctor which makes it seem more like a "partial functor."</p>