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Homemonoidal category
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monoidal category
A monoidal category is a category which has the structure of a monoid, that is, among the objects there is a binary operation which is associative and has an unique neutral or unit element. Specifically, a category $\mathcal{C}$ is monoidal if
1. 2. there is an isomorphism $a_{{ABC}}:(A\otimes B)\otimes C\cong A\otimes(B\otimes C)$, for arbitrary objects $A,B,C$ in $\mathcal{C}$, such that $a_{{ABC}}$ is natural in $A,B$ and $C$. In other words,

$a_{{BC}}:(\otimes B)\otimes C\Rightarrow\otimes(B\otimes C)$ is a natural transformation for arbitrary objects $B,C$ in $\mathcal{C}$,

$a_{{AC}}:(A\otimes)\otimes C\Rightarrow A\otimes(\otimes C)$ is a natural transformation for arbitrary objects $A,C$ in $\mathcal{C}$,

$a_{{AB}}:(A\otimes B)\otimes\Rightarrow A\otimes(B\otimes)$ is a natural transformation for arbitrary objects $A,B$ in $\mathcal{C}$,

3. there is an object $I$ in $\mathcal{C}$ called the unit object (or simply the unit),
4. for any object $A$ in $\mathcal{C}$, there are isomorphisms:
$l_{A}:I\otimes A\cong A\qquad\mbox{and}\qquad r_{A}:A\otimes I\cong A,$ such that $l_{A}$ and $r_{A}$ are natural in $A$: both $l:I\otimes\Rightarrow$ and $r:\otimes I\Rightarrow$ are natural transformations
satisfying the following commutative diagrams:

unit coherence law
$\xymatrix@+=2cm{(A\otimes I)\otimes B\ar[rr]^{{a_{{AIB}}}}\ar[dr]_{{r_{A}% \otimes 1_{B}}}&&A\otimes(I\otimes B)\ar[dl]^{{1_{A}\otimes r_{B}}}\\ &A\otimes B&}$ 
$\xymatrix@+=2cm{((A\otimes B)\otimes C)\otimes D\ar[rr]^{{a_{{A\otimes B,C,C}}% }}\ar[d]_{{a_{{ABC}}\otimes 1_{D}}}&&(A\otimes B)\otimes(C\otimes D)\ar[dd]^{{% a_{{A,B,C\otimes D}}}}\\ (A\otimes(B\otimes C))\otimes D\ar[d]_{{a_{{A,B\otimes C,D}}}}&&\\ A\otimes((B\otimes C)\otimes D)\ar[rr]_{{1_{A}\otimes a_{{BCD}}}}&&A\otimes(B% \otimes(C\otimes D))}$
The bifunctor $\otimes$ is called the tensor product on $\mathcal{C}$, and the natural isomorphisms $a,l,r$ are called the associativity isomorphism, the left unit isomorphism, and the right unit isomorphism respectively.
Some examples of monoidal categories are

A prototype is the category of isomorphism classes of vector spaces over a field $\mathbb{K}$, herein the tensor product is the associative operation and the field $\mathbb{K}$ itself is the unit element.

The category of sets is monoidal. The tensor product here is just the settheoretic cartesian product, and any singleton can be used as the unit object.

The category of (left) modules over a ring $R$ is monoidal. The tensor product is the usual tensor product of modules, and $R$ itself is the unit object.

The category of bimodules over a ring $R$ is monoidal. The tensor product and the unit object are the same as in the previous example.
Monoidal categories play an important role in the topological quantum field theories (TQFT).
Mathematics Subject Classification
8100 no label found1800 no label found18D10 no label found Forums
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