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For questions about groups whose elements commute.

4 votes

constructive Serre classes

Any subclass $\mathcal{C}$ of an abelian category determines a smallest Serre class containing it, by iteratively adding (the zero object and) the object $Y$ for any exact sequence $X \to Y \to Z$ whe …
Ingo Blechschmidt's user avatar
6 votes

Constructively, is the unit of the “free abelian group” monad on sets injective?

Yes! Here is a proof which is slightly different from both your proof and the proof in Mines–Richman–Ruitenberg. First define the similarity relation on $\mathrm{List}(R \times X)$ as in Mines–Richma …
Ingo Blechschmidt's user avatar