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For trying to understand how general a certain theorem is, I'm looking for an example of an essentially small abelian category which has enough projectives and enough injectives, but whose category of projectives is not equivalent to the category of injectives.

By a result of Auslander, each such category can be written as the category $\operatorname{mod} \operatorname{proj} \mathcal{A}$ of coherent/finitely presented functors (likewise as $\operatorname{mod}\operatorname{inj}\mathcal{A}$). Typical examples up to this point include the category of finite dimensional modules over a finite dimensional algebra, or generalisations of this, e.g. what is sometimes called a dualising $k$-variety, where the category is a $k$-category for a ground field and the $k$-duality provides an equivalence.

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  • $\begingroup$ For fin.dim. modules for a fin.dim. algebra, the categories of projectives and injectives are equivalent, not dual. Is that what you meant? $\endgroup$
    – Jeremy Rickard
    Commented Feb 9, 2018 at 16:18
  • $\begingroup$ Yes. But your example was precisely what I was looking for. $\endgroup$
    – Julian Kuelshammer
    Commented Feb 9, 2018 at 17:38

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The category of countable abelian groups is an essentially small abelian category, and has enough projectives and injectives (the countable free abelian groups and the countable divisible groups respectively). However, there is an injective with endomorphism ring $\mathbb{Q}$, but no such projective, so the categories of projectives and injectives can't be equivalent or dual.

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