2
$\begingroup$

Before stating my question I would like to provide afew motivating examples:

Examples:

  1. In the category of Finitely-generated-projective $R$-modules, we have that: $M^{\vee}:=Hom_R(M,R)$ satisfies: $Hom_R(M^{\vee},R)\cong M$.
  2. If $G$ is a locally compact abelian topological group over the circle group $T$, then the set: $G^{\wedge}:=Hom_{CGrp}(G,T)$ of continuous group homomorphisms into the circle group can be given the structure of a locally compact abelian topological group. Moreover the Pontryagin duality theorem states that: $Hom_{CGrp}(G^{\wedge},T)\cong G$ as topological groups (note: here $T$ is the terminal object since all groups are considered over $E$).
  3. In the category of Banach spaces over ${\mathbb{R}}$the above construction fails, however fails for $Hom_{\mathbb{R}}(B,{\mathbb{R}})$ however when restricting our attention to the subcategory of continous linear functionals (CLFs) the $B^{\prime}:=Hom_{CLFs}(B,{\mathbb{R}})$ satisfies the duality relationship: $Hom_{CLFs}(B^{\prime},\mathbb{R})\cong B$ for many Banach spaces $B$.

My goal is to understand when this idea can be extended: Question: When does there exist a faithful subcategory $\mathfrak{C}$ of a category $\mathfrak{D}$ with terminal object $0$ such that, the Hom functor of $\mathfrak{C}$ is internal and for every object $D$ in $\mathfrak{D}$, when considered in $\mathfrak{C}$ there exists some object $D'\in \mathfrak{D}$ satisfying: $Hom_{\mathfrak{C}}(D',0)\cong D$ such that the association $D \mapsto D'$ is functorial?

$\endgroup$
1

1 Answer 1

5
$\begingroup$

Here is a construction that covers the first example but not, I think, the other two. Suppose $C$ is a closed symmetric monoidal category with unit object $1$ and that $c$ is a dualizable object in $C$. Then the dual $c^{\ast}$ can functorially identified with the internal hom $[c, 1]$, where $1$ is the monoidal unit, and in particular we always have the reflexivity condition

$$c \cong [[c, 1], 1]$$

because this always holds for the monoidal dual in a symmetric monoidal category. Examples of such $C$ include the symmetric monoidal categories $\text{Mod}(k)$ of modules over a commutative ring $k$, where the dualizable objects are precisely the finitely presented projective $k$-modules.

$\endgroup$
2
  • $\begingroup$ I think there is a notion of topological tensor making the third one work, not sure though $\endgroup$
    – ABIM
    Oct 16, 2015 at 4:04
  • 5
    $\begingroup$ It's true that the category of Banach spaces and maps of norm at most $1$ is closed monoidal, with monoidal product given by the projective tensor product. Unfortunately I think the dualizable Banach spaces are the finite-dimensional ones, so they don't exhaust the reflexive Banach spaces. $\endgroup$ Oct 16, 2015 at 4:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.