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Consider a vector bundle $E$ over a manifold $M$ with flat connection, $\nabla$. From this data I can form the associative/unital differential graded algebra $\mathcal{A} = \left(\Omega^{\bullet}(M, End(E)), d\right)$ of forms with values in $End(E)$ where the differential is induced by $\nabla$ and squares to zero.

As $\mathcal{A}$ is an algebra over $\mathbb{R}$, I expect that we would say $\mathcal{A}$ is connected when $H^0(\mathcal{A})= \mathbb{R}$. However, when $M$ is simply connected, if I inspect

$$H^0(\Omega^{\bullet}(M, End(E)))= \{ s \in \Gamma(M, End(E)) \ \vert \ \nabla s = 0 \}\ne \mathbb{R}$$

I seem to be considering the $\nabla$-constant sections of $End(E)$. My thinking is that such an $s$ should be equivalent to* a choice of an endomorphism of the fiber of $E$, which is not necessarily just a choice of a real number.

What am I missing? Should I be thinking of the ring as $End(F)$? If so am I working in non-commutative algebra? Please help!

*My intuition for $\nabla s = 0$ is that $s$ does not vary from parallel transport along any choice of curve in $M$ and so given any choice in the fiber we could parallel transport that choice along $M$ (since $\nabla$ is flat this is well-defined).

Update: Note that iff $M$ is a point and the fiber is $\mathbb{R}^n$, then our complex is:

$$ \Omega^0(pt, End(E)) = End(\mathbb{R}^n) \to \Omega^1(pt, End(E))=0 \to \Omega^2(pt, End(E))=0 \to \dots $$

and so $H^0= End(\mathbb{R}^n)$ . I'm now wondering if I should be defining connectedness for some augmented dga.

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    $\begingroup$ For a generic Ε,∇ the only ∇-flat sections of End(E) will be the ones given by multiplication by a real number. (The map that sends a ∇-flat section to its fiber at a fixed point is injective, but not necessarily surjective.) So the condition H^0≅R is quite natural. $\endgroup$ Commented Jun 14, 2023 at 13:07
  • $\begingroup$ Originally I agreed @DmitriPavlov but then you can see in my updated note that if $M$ is a point then it looks to me we should have $H^0 = End(F)$. $\endgroup$
    – cheyne
    Commented Jun 14, 2023 at 23:09
  • $\begingroup$ A point is as far from the generic case as possible, so there is no contradiction. It is hard to say more here unless a specific purpose for which you want a connectedness assumption is identified first. $\endgroup$ Commented Jun 15, 2023 at 1:11
  • $\begingroup$ OK. My understanding is that in the case of real values forms, saying the dga is connected agrees with the case when the manifold is connected. For context, I am working on bundle valued iterated integrals and bar-like constructions on bundle valued forms. In this setting I want to “mod out” by a sub complex which is traditionally proven to be acyclic under some “connectedness” condition. I can say more if helpful. I appreciate the comments! $\endgroup$
    – cheyne
    Commented Jun 15, 2023 at 1:29
  • $\begingroup$ @DmitriPavlov The OP seems to be interested in the simply connected case, where there is no monodromy, and global sections of a vector bundle with an integrable connection could be identified with any fiber. $\endgroup$
    – Z. M
    Commented Jun 15, 2023 at 5:16

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