categorical trace

The term *categorical trace* usually refers to the concept introduced in

Let $C$ be a 2-category and $F : B \to B$ a 1-endomorphism on one of its objects. Then the **categorical trace** $Tr(F)$ of $F$ is the collection of 2-morphisms from the identity $Id_B$ on $B$ into $B$

$Tr(F) = Hom_C(Id_B, F)
\,.$

- For $C = KV2Vect$, the 2-category of Kapranov-Voevodsky 2-vector spaces, a 1-endomorphism $Vect^n \stackrel{F}{\to} Vect^n$ is an $n \times n$-matrix $(V_{i j})_{i,j}$ of vector spaces $V_{i j} \in Vect$. The categorical trace on that is the direct sum of the diagonal entries, $Tr(F) = \oplu_{i} V_{i i}$. Hence under the decategorification functor from $KV2Vect$ to vectors and matrices with entries in $\mathbb{N}$, this becomes the ordinary trace of matrices.

- The categorical trace is closely related to the span trace.

Created on January 29, 2009 21:53:38
by Urs Schreiber
(134.100.222.156)