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In mathematics and particularly category theory, a coherence theorem is a tool for proving a coherence condition. Typically a coherence condition requires an infinite number of equalities among compositions of structure maps. A coherence theorem states that, in order to be assured that all these equalities hold, it suffices to check a small number of identities.

Examples

Consider the case of a monoidal category. Recall that part of the data of a monoidal category is an associator, which is a choice of morphism

${\alpha }_{A,B,C}:(A\otimes B)\otimes C\to A\otimes (B\otimes C)$

for each triple of objects $A,B,C$. Mac Lane's coherence theorem states that, provided the following diagram commutes for all quadruples of objects $A,B,C,D$,

 File:Monoidal-category-pentagon.png 

any pair of morphisms from $(\cdots (A_N\otimes A_{N-1})\otimes \cdots )\otimes A_2)\otimes A_1)$ to $(A_N\otimes (A_{N-1}\otimes \cdots \otimes (A_2\otimes A_1)\cdots )$ constructed as compositions of various ${\alpha }_{A,B,C}$ are equal.

References

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