Day’s reflection theorem [Day (1972)] gives conditions under which a reflective subcategory of a symmetric monoidal closed category is closed under internal homs.
One memorable consequence of the theorem is that the subcategory is closed under internal homs if the reflector is symmetric strong monoidal.
(Day) Let be a fully faithful functor with left adjoint , and suppose given a symmetric monoidal closed structure on with tensor and internal hom . Then for objects of and of , the following are equivalent:
is an isomorphism;
is an isomorphism;
is an isomorphism;
is an isomorphism.
Some remarks before the proof:
(a) Since is fully faithful, the monad is idempotent. Consequently, is monadic, and algebra structures on an object are unique when they exist. This makes algebra structure on a property, that for some in , unique up to isomorphism. Call this property “being in ”. Thus condition says that is in for a particular in . (If is cartesian closed and whenever , we say is an exponential ideal in .) Notice that all -maps between objects in are algebra maps, by full faithfulness of .
(b) If is in , then for any the unit induces an isomorphism , since every is an algebra map and is the free algebra on .
We prove , and then . Some parts are merely sketched; refer to Day for full details.
is obvious from the symmetric structure and
is proven by giving an algebra structure , obtained by currying the following composite:
(by idempotence of , to prove this map is an algebra structure, it suffices to show it is left inverse to the unit).
is proven by considering the diagram
which commutes by functoriality and naturality, and where the maps labeled are isomorphisms by assuming , and the bottom map labeled is an isomorphism by invoking remark (b). It follows that the top horizontal map is also an isomorphism. Since this holds for all objects of , the map is an isomorphism by the Yoneda lemma, so that holds.
Finally, is proven by considering the diagram
and again applying a Yoneda lemma argument.
If is a strong symmetric monoidal functor, then for any in and in , the internal hom is in .
Full faithfulness of is equivalent to the counit being an isomorphism. By a triangle identity, this forces to be an isomorphism for any unit . This in turn forces the arrow in the diagram
to be an isomorphism, where the vertical arrows are invertible symmetry constraints and the maps are isomorphisms. Now apply the equivalence between and from the reflection theorem.
If is cartesian closed, and the reflector preserves products, then is cartesian closed. Conversely, if is cartesian closed and is a dense functor, then the reflector preserves products.
We have already seen under the hypotheses that if are in , then the exponential as calculated in is in . Furthermore, inherits products from , because is monadic and monadic functors reflect limits. The adjunction holds because it holds when interpreted in and is fully embedded in .
For the converse, you can see Theorem 3.10 of Street 1980.
Brian Day, A reflection theorem for closed categories, Journal of Pure and Applied Algebra 2 1 (1972), 1-11. [doi:10.1016/0022-4049(72)90021-7]
Ross Street, Cosmoi of internal categories, Transactions of the American Mathematical Society 258 2 (1980) 271-318 [doi:10.2307/1998059]
A version for skew monoidal categories is given in:
Last revised on October 15, 2024 at 19:17:36. See the history of this page for a list of all contributions to it.