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Kähler differentials
From Citizendium, the Citizens' Compendium
Definition
Let
be an algebra. An A differential of B into an A-module M is a map
such that
- D(a) = 0 for all
- D(b + b') = D(b) + D(b') for
- D(bb') = b'D(b) + bD(b')
Observe that the set of all such maps DerA(B,M) is a B-module. Moreover, DerA(B, − ) is a representable functor; we call the representative ΩB / A the module of Kähler differentials. That is, ΩB / A satisfies the following universal property:

