# Coherence condition

In mathematics, and particularly category theory a coherence condition is a collection of conditions requiring that various compositions of elementary morphisms are equal. Typically the elementary morphisms are part of the data of the category.

## An illustrative example: a monoidal category

Part of the data of a monoidal category is a chosen morphism $\alpha_{A,B,C}$, called the associator:

$\alpha_{A,B,C} \colon (A\otimes B)\otimes C \rightarrow A\otimes(B\otimes C)$

for each triple of objects $A,B,C$ in the category. Using compositions of these $\alpha_{A,B,C}$, one can construct a morphism

$( \cdots ( A_N \otimes A_{N-1} ) \otimes A_{N-2} ) \otimes \cdots \otimes A_1) \rightarrow ( A_N \otimes ( A_{N-1} \otimes \cdots \otimes ( A_2 \otimes A_1) \cdots ).$

Actually, there are many ways to construct a morphism from

$( \cdots ( A_N \otimes A_{N-1} ) \otimes \cdot ) \otimes A_2 ) \otimes A_1 )$

to

$( A_N \otimes ( A_{N-1} \otimes \cdots \otimes ( A_2 \otimes A_1) \cdots )$

as a composition of various $\alpha_{A,B,C}$. One coherence condition that is typically imposed is that these compositions are all equal.

Typically one proves a coherence condition using a coherence theorem, which states that one only needs to check a few equalities of compositions in order to know that the rest also hold. In the above example, one only needs to check that, for all quadruples of objects $A,B,C,D$, the following diagram commutes

## Further examples

Two simple examples that illustrate the definition are as follows. Both are directly from the definition of a category.

## Identity

Let f : AB be a morphism of a category containing two objects A and B. Associated with these objects are the identity morphisms 1A : AA and 1B : BB. By composing these with f, we construct two morphisms:

f o 1A : AB, and
1B o f : AB.

Both are morphisms between the same objects as f. We have, accordingly, the following coherence statement:

f o 1A   = f   = 1B o f.

## Associativity of composition

Let f : AB, g : BC and h : CD be morphisms of a category containing objects A, B, C and D. By repeated composition, we can construct a morphism from A to D in two ways:

(h o g) o f : AD, and
h o (g o f) : AD.

We have now the following coherence statement:

(h o g) o f = h o (g o f).

In these two particular examples, the coherence statements are theorems for the case of an abstract category, since they follow directly from the axioms; in fact, they are axioms. For the case of a concrete mathematical structure, they can be viewed as conditions, namely as requirements for the mathematical structure under consideration to be a concrete category, requirements that such a structure may meet or fail to meet.

## References

• Mac Lane, Saunders (1971). "Categories for the working mathematician". Graduate texts in mathematics Springer-Verlag. Especially Chapter VII Part 2.