An initial object? in a category is an object with exactly one morphism from to any object of . An obvious generalization is that of a weighted initial object?, i.e. an object with exactly morphisms to any object ; the ‘function’ which assigns to an object the number of morphisms from to is the weight.
This is nicely formalized in terms of functors: let be a functor. Then a weighted initial object with weight is, if it exists, an object representing the functor , i.e., such that
naturally in .
The trivial weight corresponds to the usual notion of initial object.
Mike Shulman: This is an interesting notion, but it is not a special case of the general notion of weighted limit. The weight for a weighted limit of a diagram is a functor , not . Since is the empty category for initial objects, no nontrivial weights are possible in that case. Of course, what you call a “-weighted initial object” is the same a representation of .
Domenico Fiorenza: I see the choice for the category was a bad one.. I had written everithing in terms of here since when there’s just one category in sight it’s customary to call it , but if one is reading at this entry with weighted limits in mind, this should rather be . Now I’ll edit the page using instead; I hope this will make clear what I had in mind.
Mike Shulman: Changing the name of the category doesn’t change the meaning of the definition, does it? I don’t understand how this is supposed to make it clearer.
Domenico Fiorenza: My fault: when you said that the weight for a weighted limit of a diagram is a functor , not , I guessed that in the way I had written things I had made you get confused, since in the previous version of this page was a functor from to . Thus I tried to clarify thing changing the notation accordingly.
So let me write here what I am suggesting the weighted limit could be, for a functor , to see if I’m able to make the aim of this entry clearer: should be a terminal object in the category of functors from to exteding ; here is the category obtatined by adding a -weighted initial object to .
Mike Shulman: Ah, yes! It is quite right that a -weighted cone (or cylinder) over a diagram is the same as a functor from the cograph of (considered as a profunctor ) to which restricts to along the inclusion of . In fact, this is the basis for a natural generalization of weighted limits, where we consider profunctors for arbitrary , rather than merely ; there are some remarks about this generalization at proarrow equipment.
I would suggest, however, that you not use the phrase “-weighted initial object” since that construction isn’t actually itself a weighted (co)limit; it’s just something you can use to help define the weighted limit.
If is any quasi-category, is a functor, and denotes the terminal quasi-category, the weighted join is the quasi-category obtained from by ‘adding’ a -weighted initial object to .
More in general, if is another quasi-category, the weighted join is the quasi-category intuitively obtained by making arrows stem from any object of to any object of .
(details on the formal construction of to be added)
Mike Shulman: It seems as though possibly you would want to start from a functor , rather than simply . In that case, perhaps your “weighted join” is the same as the cograph of a profunctor (in the quasicategory context)?
Fosco: I think Mike Shulman is right, as I was motivated to open this question on MO in order to clarify the role of profunctors in the definition of limits. It seems pretty obvious to me that the generalization which leads to consider representing object for the functor sending to can be easily carried on to consider
This led me to wonder if, in the same vein that regards the good old limit as the terminal object in the category , where is the collage along the terminal profunctor, then the limit of weighted by can be seen as the terminal object in the category , where is merely the cograph of (I’m certain you see the pattern!).