Universal property of Kähler differentials
= Universal property of Kähler differentials
{title2=$\operatorname{Hom}_A(\Omega_{A/k},M)\cong\operatorname{Der}_k(A,M)$}
Composing an $A$-linear map with $d:A\to\Omega_{A/k}$ gives a $k$-<derivation>. Every derivation factors uniquely this way because its linearity and product rule are exactly the defining relations of the <Module of Kähler differentials>.