Rigid category


In category theory, a branch of mathematics, a rigid category is a monoidal category where every object is rigid, that is, has a dual X* and a morphism 1 → X ⊗ X* satisfying natural conditions. The category is called right rigid or left rigid according to whether it has right duals or left duals. They were first defined by Neantro Saavedra-Rivano in his thesis on Tannakian categories.

Definition

There are at least two equivalent definitions of a rigidity.


are identities. A right rigid object is defined similarly.
An inverse is an object X−1 such that both X ⊗ X−1 and X−1 ⊗ X are isomorphic to 1, the one object of the monoidal category. If an object X has a left inverse X−1 with respect to the tensor product then it is left rigid, and X* = X−1.
The operation of taking duals gives a contravariant functor on a rigid category.

Uses

One important application of rigidity is in the definition of the trace of an endomorphism of a rigid object. The trace can be defined for any rigid category such that taking the '**, the functor of taking the dual twice repeated, is isomorphic to the identity functor, i.e. a pivotal category. Then for any right rigid object X, and any other object Y, we may define the isomorphism


and its reciprocal isomorphism


Then for any endomorphism, the trace is of f is defined as the composition:


We may continue further and define the dimension of a rigid object to be:


Rigidity is also important because of its relation to internal Hom's. If X is a left rigid object, then every internal Hom of the form ' exists and is isomorphic to Z ⊗ Y. In particular, in a rigid category, all internal Hom's exist.

Alternative terminology

A monoidal category where every object has a left dual is also sometimes called a left autonomous category. A monoidal category where every object has both a left and a right dual is sometimes called an autonomous category. An autonomous category that is also symmetric is called a compact closed category.

Discussion

A monoidal category is a category with a tensor product, precisely the sort of category for which rigidity makes sense.