Remark 60.6.8. Let $A \to B$ be a ring map. Let $\Omega _{B/A} \to \Omega$ be a quotient satisfying the assumptions of Algebra, Lemma 10.132.1. Let $M$ be a $B$-module. A connection is an additive map

$\nabla : M \longrightarrow M \otimes _ B \Omega$

such that $\nabla (bm) = b \nabla (m) + m \otimes \text{d}b$ for $b \in B$ and $m \in M$. In this situation we can define maps

$\nabla : M \otimes _ B \Omega ^ i \longrightarrow M \otimes _ B \Omega ^{i + 1}$

by the rule $\nabla (m \otimes \omega ) = \nabla (m) \wedge \omega + m \otimes \text{d}\omega$. This works because if $b \in B$, then

\begin{align*} \nabla (bm \otimes \omega ) - \nabla (m \otimes b\omega ) & = \nabla (bm) \wedge \omega + bm \otimes \text{d}\omega - \nabla (m) \wedge b\omega - m \otimes \text{d}(b\omega ) \\ & = b\nabla (m) \wedge \omega + m \otimes \text{d}b \wedge \omega + bm \otimes \text{d}\omega \\ & \ \ \ \ \ \ - b\nabla (m) \wedge \omega - bm \otimes \text{d}(\omega ) - m \otimes \text{d}b \wedge \omega = 0 \end{align*}

As is customary we say the connection is integrable if and only if the composition

$M \xrightarrow {\nabla } M \otimes _ B \Omega ^1 \xrightarrow {\nabla } M \otimes _ B \Omega ^2$

is zero. In this case we obtain a complex

$M \xrightarrow {\nabla } M \otimes _ B \Omega ^1 \xrightarrow {\nabla } M \otimes _ B \Omega ^2 \xrightarrow {\nabla } M \otimes _ B \Omega ^3 \xrightarrow {\nabla } M \otimes _ B \Omega ^4 \to \ldots$

which is called the de Rham complex of the connection.

Comment #4223 by Dario Weißmann on

Concerning the definition of the maps $\nabla:M\otimes \Omega^i \to M\otimes \Omega^{i+1}$:

Several $\otimes$ should be $\wedge$, i.e.,

There are also:

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