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In Rel, both products and coproducts amount to taking a disjoint union of sets. Is there a category whose object class can be interpreted as the class of all sets, in which both products and coproducts amount to taking Cartesian products, in the classical sense?

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How trivial of a solution would you accept? For example, the category with a single object (any set of your choice) and one arrow. –  Zev Chonoles Apr 25 '13 at 0:45
    
@ZevChonoles, I edited the question, it now says "Is there a category whose object class can be interpreted as the class of all sets, in which both products and coproducts amount to taking Cartesian products, in the classical sense?" –  goblin Apr 25 '13 at 0:47
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Easy: let $\mathcal{C}$ be the category whose objects "are" sets, and whose morphisms $X \to Y$ are the group homomorphisms from the free abelian group generated by $X$ to the free abelian group generated by $Y$. This has the required property. –  Zhen Lin Apr 25 '13 at 7:12
    
@MartinBrandenburg, the idea is that not only products, but also coproducts amount to Cartesian products. –  goblin Apr 25 '13 at 8:01
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So you claim that $\hom(F(X \times Y),F(Z)) \cong \hom(F(X),F(Z)) \times \hom(F(Y),F(Z))$ and $\hom(F(X),F(Y \times Z)) \cong \hom(F(X),F(Z)) \times \hom(F(Y),F(Z))$? Neither is clear to me. –  Martin Brandenburg Apr 25 '13 at 8:03

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