# Are there two non-isomorphic modules such that all the Hom-sets are isomorphic? [closed]

Prove or disprove:

If $M, N$ are R-module and for all $P$ R-module $Hom(M,P) \simeq Hom(N,P)$ then $M\simeq N$

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## closed as too localized by Fernando Muro, Vladimir Dotsenko, Martin Brandenburg, Dan Petersen, Mark SapirJul 6 '12 at 20:50

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This follows from Yoneda lemma if you assume this isomorphisms are natural. – Piotr Pstrągowski Jul 5 '12 at 20:04
And if you do not assume the isomorphisms between Hom-sets to be natural, then for example over a field the question boils down to whether it is possible for two non-isomorphic vector spaces to have isomorphic duals. Over the field with two elements this is simply a question about the cardinality of power sets, which might very well be independant of ZFC. – Piotr Pstrągowski Jul 5 '12 at 20:14
I suggest "Are modules isomorphic if their Hom-sets are all isomorphic?" (or something like that). – Mark Grant Jul 6 '12 at 11:36
@Piotr: could you please explain the "boils down" a bit further? The implication "isomorphic duals" $\implies$ "all hom spaces isomorphic" seems to require some implication of the sort $2^\kappa=2^\lambda\implies \alpha^\kappa=\alpha^\lambda$ for all cardinals $\alpha$. Is this true? – user2035 Jul 6 '12 at 11:52
I've cleaned up this comment thread; the removed comments are copied at tea.mathoverflow.net/discussion/1403/some-cleaned-up-comments. – Anton Geraschenko Jul 6 '12 at 17:11