\begin{definition}[Definition:Differential Module]
Let $R$ be a commutative ring with unity.
Let $M$ be an $R$-module.
A '''differential''' on $M$ is a homomorphism $d:M\to M$ of $R$-modules such that $d^2 = d\circ d = 0$.
The pair $(M,d)$ is called a '''differential module'''.
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\end{definition}