Let $E_1$ and $E_2$ be objects in an abelian category $\mathcal{C}$. How can I see the natural action of $\mathrm{Hom}(E_2,E_1)$ on $\mathrm{Ext}^1(E_2,E_1)$?
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5$\begingroup$ I don't believe that there is such a natural action unless $E_1 = E_2$. What did you have in mind? $\endgroup$– Qiaochu YuanCommented Jun 11, 2016 at 0:15
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$\begingroup$ See here: mathoverflow.net/questions/223996/… $\endgroup$– Thomas PoguntkeCommented Jun 12, 2016 at 16:31
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