category with duals (list of them)
dualizable object (what they have)
abstract duality: opposite category,
Being dualizable may often be thought of as a category-theoretic notion of finiteness for objects in a monoidal category. For instance, a vector space is dualizable in Vect with its standard tensor product precisly if it is a finite-dimensional vector space; and a spectrum is dualizable in the stable homotopy category with its smash product precisely if it is a finite spectrum.
A more precise intuition is that an object is dualizable if its “size” is no larger than the “additivity” of the monoidal category. Since Vect and the stable homotopy category are finitely additive, but not infinitely so, dualizability there is a notion of finiteness. This is the case for many monoidal categories in which one considers dualizability. However, in a monoidal category which is not additive at all, such as Set (or any cartesian monoidal category), only the terminal object is dualizable—whereas in an “infinitely additive” monoidal category such as Rel or SupLat, many “infinite” objects are dualizable. (In , all objects are dualizable.)
An object in a monoidal category is dualizable if it has an adjoint when regarded as a morphism in the one-object delooping bicategory corresponding to . Its adjoint in is called its dual in and often written as .
If is braided then left and right adjoints in are equivalent; otherwise one speaks of being left dualizable or right dualizable.
Explicitly this means the following:
A right duality between objects
a morphism of the form
called the counit of the duality, or the evaluation map;
a morphism of the form
called the unit or coevaluation map
and counit (or evaluation)
satisfying the ‘triangle identities’ familiar from the concept of adjunction. With this convention, if in is interpreted as composition in in diagrammatic order, then right duals in are the same as right adjoints in — whereas if in is interpreted as composition in in classical ‘Leibnizian’ order, then right duals in are the same as left adjoints in .
Of course, in a symmetric monoidal category, there is no difference between left and right duals.
See category with duals for more discussion.
is the one obtained by by composing the duality unit, the counit and the braiding…
Let be a finite-dimensional vector space over a field , and let be its usual dual vector space. We can define to be the obvious pairing. If we also choose a finite basis of , and let be the dual basis of , then we can define by sending to . It is easy to check the triangle identities, so is a dual of in .
Let be a finite-dimensional manifold, choose an embedding for some , and let be the Thom spectrum of the normal bundle of this embedding. Then the Thom collapse map defines an which exhibits as a dual of in the stable homotopy category. This is a version of Spanier-Whitehead duality.
See at KK-theory – Poincare duality.
See at (∞,1)-category of (∞,1)-modules – Compact generation for more.
Dualizable objects support a good abstract notion of trace. (…)
Dualizable objects in an symmetric monoidal (∞,1)-category are already fully dualizable objects. The cobordism hypothesis implies that there is a canonical -action on the ∞-groupoid of dualizable objects, and this is just the dualizing operation. See at cobordism hypothesis – Framed version – Implications: Canonical O(n)-action.
In a closed monoidal category is also called the weak dual of (e.g. Becker-Gottlieb, p. 5), to contrast with the monoidal dual as above, which would then be called the strong dual . If the induced morphism is an equivalence this weak dual is called a reflexive weak dual.
If is a compact closed category, def. 3, then the weak dual is also the strong dual object to in the above monoidal sense. Here dualization exhibits as a star-autonomous category ( is the star-operation)
This appears as (Lurie, def. 2.3.5).
This means that an object in is dualizable if there exists unit and counit 1-morphism that satisfy the triangle identity up to homotopy. The definition does not demand that this homotopy is coherent (that it satisfies itself higher order relations up to higher order k-morphisms).
If the structure morphisms of the adjunction of a dualizable object have themselves all adjoints, then the object is called a fully dualizable object.
As before, we may equivalently state this after delooping the monoidal structure and passing to the -category . Then has duals for objects precisely if has all adjoints.
|geometry||monoidal category theory||category theory|
|perfect module||(fully-)dualizable object||compact object|
A large number of further examples can be found in
The notion of duals in a symmetric monoidal -category is due to section 2.3 of