---- > [!definition] Definition. ([[flat connection]]) > We call a [[connection on a vector bundle|connection]] $A$ on a [[vector bundle]] $E$ **flat** if its [[curvature form]] vanishes: $F(A)=0$. A **flat vector bundle** is a [[vector bundle]] together with a choice of flat connection. ^definition > [!basicexample] The **trivial connection** (or **product connection**). > Consider the [[product bundle|trivial product bundle]] $E=B \times \mathbb{R}^{m}$. [[section|Sections]] $s \in \Gamma(E)$ are smooth maps $B \to \mathbb{R}^{m}$, that is, $\Gamma(E)=\Omega_{B}^{0}(E)=C^{\infty}(B; \mathbb{R}^{m})$. > > In this case, the 'elementwise [[exterior derivative]] $d:C^{\infty}(B; \mathbb{R}^{m})=\Omega^{0}(B) \otimes \mathbb{R}^{m} \to \Omega^{1}(B) \otimes \mathbb{R}^{m} applied to $s=(s ^{1},\dots,s ^{m})$, $ds$, satisfies the requirements to be a [[covariant derivative on a vector bundle|covariant derivative]], because [[exterior derivative wedge product rule]] gives, for each $i \in [m]$, $d(fs ^{i})=(df)s ^{i} + f(ds ^{i})$. Hence $d_{A}d_{A}=d^{2}=0$, meaning that $F(A)=0$ and so the connection specified by $d_{A}$, called the **trivial connection**, is flat. > > Partially conversely, any flat connection can *locally* be represented by a zero connection $1$-form $A=0$. This is the content of the [[integrability theorem for flat connections]]. > > > [!intuition] Remark. > > Thus one has, at least locally, that $d_{A}=ds \cancel{+ As}$ if and only if the curvature form $F(A)$ vanishes. We can therefore interpret curvature as an *obstruction to the covariant derivative just being a (component-wise) exterior derivative*. > ^proposition > > > > [!basicnonexample] Warning. > > A trivial product bundle may certainly admit other connections, which are not trivial. > ^nonexample > ^basicexample ---- #### ---- #### References > [!backlink] > ```dataview > TABLE rows.file.link as "Further Reading" > FROM [[]] > FLATTEN file.tags as Tag > WHERE Tag = "#definition" OR Tag = "#theorem" OR Tag = "#MOC" OR Tag = "#proposition" OR Tag = "#axiom" > GROUP BY Tag > ``` > [!frontlink] > ```dataview > TABLE rows.file.link as "Further Reading" > FROM outgoing([[]]) > FLATTEN file.tags as Tag > WHERE Tag = "#definition" OR Tag = "#theorem" OR Tag = "#MOC" OR Tag = "#proposition" OR Tag = "#axiom" > GROUP BY Tag > ```