In category theory, a traced monoidal category is a category with some extra structure which gives a reasonable notion of feedback.

A traced symmetric monoidal category is a symmetric monoidal category C together with a family of functions

${\displaystyle \mathrm {Tr} _{X,Y}^{U}:\mathbf {C} (X\otimes U,Y\otimes U)\to \mathbf {C} (X,Y)}$

called a trace, satisfying the following conditions:

• naturality in ${\displaystyle X}$: for every ${\displaystyle f:X\otimes U\to Y\otimes U}$ and ${\displaystyle g:X'\to X}$,

${\displaystyle \mathrm {Tr} _{X',Y}^{U}(f\circ (g\otimes \mathrm {id} _{U}))=\mathrm {Tr} _{X,Y}^{U}(f)\circ g}$

• naturality in ${\displaystyle Y}$: for every ${\displaystyle f:X\otimes U\to Y\otimes U}$ and ${\displaystyle g:Y\to Y'}$,

${\displaystyle \mathrm {Tr} _{X,Y'}^{U}((g\otimes \mathrm {id} _{U})\circ f)=g\circ \mathrm {Tr} _{X,Y}^{U}(f)}$

• dinaturality in ${\displaystyle U}$: for every ${\displaystyle f:X\otimes U\to Y\otimes U'}$ and ${\displaystyle g:U'\to U}$

${\displaystyle \mathrm {Tr} _{X,Y}^{U}((\mathrm {id} _{Y}\otimes g)\circ f)=\mathrm {Tr} _{X,Y}^{U'}(f\circ (\mathrm {id} _{X}\otimes g))}$

• vanishing I: for every ${\displaystyle f:X\otimes I\to Y\otimes I}$, (with ${\displaystyle \rho _{X}\colon X\otimes I\cong X}$ being the right unitor),

${\displaystyle \mathrm {Tr} _{X,Y}^{I}(f)=\rho _{Y}\circ f\circ \rho _{X}^{-1))$

• vanishing II: for every ${\displaystyle f:X\otimes U\otimes V\to Y\otimes U\otimes V}$

${\displaystyle \mathrm {Tr} _{X,Y}^{U}(\mathrm {Tr} _{X\otimes U,Y\otimes U}^{V}(f))=\mathrm {Tr} _{X,Y}^{U\otimes V}(f)}$

• superposing: for every ${\displaystyle f:X\otimes U\to Y\otimes U}$ and ${\displaystyle g:W\to Z}$,

${\displaystyle g\otimes \mathrm {Tr} _{X,Y}^{U}(f)=\mathrm {Tr} _{W\otimes X,Z\otimes Y}^{U}(g\otimes f)}$

• yanking:

${\displaystyle \mathrm {Tr} _{X,X}^{X}(\gamma _{X,X})=\mathrm {id} _{X))$

(where ${\displaystyle \gamma }$ is the symmetry of the monoidal category).

Properties

• Every compact closed category admits a trace.
• Given a traced monoidal category C, the Int construction generates the free (in some bicategorical sense) compact closure Int(C) of C.