# Mitchell's embedding theorem

**Mitchell's embedding theorem**, also known as the **Freyd–Mitchell theorem** or the **full embedding theorem**, is a result about abelian categories; it essentially states that these categories, while rather abstractly defined, are in fact concrete categories of modules. This allows one to use element-wise diagram chasing proofs in these categories. The theorem is named after Barry Mitchell and Peter Freyd.

## Details

[edit]The precise statement is as follows: if **A** is a small abelian category, then there exists a ring *R* (with 1, not necessarily commutative) and a full, faithful and exact functor *F*: **A** → *R*-Mod (where the latter denotes the category of all left *R*-modules).

The functor *F* yields an equivalence between **A** and a full subcategory of *R*-Mod in such a way that kernels and cokernels computed in **A** correspond to the ordinary kernels and cokernels computed in *R*-Mod. Such an equivalence is necessarily additive.
The theorem thus essentially says that the objects of **A** can be thought of as *R*-modules, and the morphisms as *R*-linear maps, with kernels, cokernels, exact sequences and sums of morphisms being determined as in the case of modules. However, projective and injective objects in **A** do not necessarily correspond to projective and injective *R*-modules.

## Sketch of the proof

[edit]Let be the category of left exact functors from the abelian category to the category of abelian groups . First we construct a contravariant embedding by for all , where is the covariant hom-functor, . The Yoneda Lemma states that is fully faithful and we also get the left exactness of very easily because is already left exact. The proof of the right exactness of is harder and can be read in Swan, *Lecture Notes in Mathematics 76*.

After that we prove that is an abelian category by using localization theory (also Swan). This is the hard part of the proof.

It is easy to check that the abelian category is an AB5 category with a generator . In other words it is a Grothendieck category and therefore has an injective cogenerator .

The endomorphism ring is the ring we need for the category of *R*-modules.

By we get another contravariant, exact and fully faithful embedding The composition is the desired covariant exact and fully faithful embedding.

Note that the proof of the Gabriel–Quillen embedding theorem for exact categories is almost identical.

## References

[edit]- R. G. Swan (1968).
*Algebraic K-theory, Lecture Notes in Mathematics 76*. Springer. doi:10.1007/BFb0080281. ISBN 978-3-540-04245-7. - Peter Freyd (1964).
*Abelian Categories: An Introduction to the Theory of Functors*. Harper and Row. reprinted with a forward as "Abelian Categories".*Reprints in Theory and Applications of Categories*.**3**: 23–164. 2003. - Mitchell, Barry (July 1964). "The Full Imbedding Theorem".
*American Journal of Mathematics*.**86**(3). The Johns Hopkins University Press: 619–637. doi:10.2307/2373027. JSTOR 2373027. - Charles A. Weibel (1993).
*An introduction to homological algebra*. Cambridge Studies in Advanced Mathematics. doi:10.1017/CBO9781139644136. ISBN 9781139644136.