I am back again, with two more questions about enriched category theory. As usual, what I am really searching for are references so I can avoid writing any category-theoretic proofs myself. The good news is that Max Kelly's book on enriched category theory is actually making sense to me now (thank you, Max!). I think these two questions are not answered in there, but I could be wrong. The first is a generalization of the enriched Yoneda lemma. Recall that this says that [A,V](X^*,F) is isomorphic to FX. Here [A,V] denotes the V-category of V-functors from A to V, X is an object of A, and X^* is the V-functor that takes Y to A(X,Y). (and A is small). What I want is [A,D](Y tensor X^*,F) is isomorphic to D(Y,FX). Here D is a V-category that is tensored over V, X is an object of A, and Y is an object of D. The functor Y tensor X^* takes Z to Y tensor A(X,Z). If you take D = V and Y = the unit of V, you recover the Yoneda lemma above. I don't remember if any completeness or cocompleteness assumptions on D are necessary here because I always assume D is bicomplete. Has this stonger Yoneda lemma appeared in print anywhere? I think the proof is the same as the usual Yoneda lemma. My second question has to do with Brian Day's old work. He shows that if A is a small symmetric monoidal V-category, then [A,V] is a closed symmetric monoidal V-category. What I want to say is that if D is a (bicomplete) V-category that is tensored and cotensored over V, then [A,D] is an [A,V]-category that is tensored and cotensored over [A,V] (A is as above, a small symmetric monoidal V-category). This is obviously a generalization of Day's work, and is obviously proved by following along in Day and changing a few V's to D's. Has it ever appeared in print? What I am actually doing (with Manos Lydakis), in case anyone is wondering why I am asking all these questions, is examining the homotopy theory of [A,D]. So I assume V and D are themselves model categories and put different model structures on [A,D]. For certain A you recover what algebraic topologists call spectra and symmetric spectra. The main goal is to explain and generalize the Goodwillie calculus of functors. Mark Hovey 12-Jan-2002 09:09:20 -0400,2877;000000000000-00000000
This is in reply to the queries in Mark Hovey's letter of 12 Jan, of which I shall try to include a copy below; I am limited by the inadequacies of this home computer. The first isomorphism he asks about, namely (using o for tensor product) [A,D](Y o A(X,-),F] =~ D(Y,FX), may be seen as a simple consequence of the fact that Y o A(X,-) is the left Kan extension of Y : I --> D along X : I --> A, where I is the unit V-category; see formula (4.18) or (4.25) of my book. The second question concerns an easy extension of Brian Day's convolution monoidal structure on a presheaf category. An easy way of seeing the truth of Hovey's observation is to recall that a tensored V-category structure on a (mere) category A corresponds to an _action_ of V on A having an appropriate right adjoint; see [Janelidze and Kelly, TAC 9 (2001), 61 - 91], foot of page 66; but the idea itself is quite old. Now V --> [D,D] easily gives [A,V] --> [[A,D],[A,D]]. Of course there is checking to do. Max Kelly. 15-Jan-2002 08:46:06 -0400,1185;000000000000-00000000
participants (2)
-
Mark Hovey -
maxk@maths.usyd.edu.au