Differential Topology – Can One Glue De Rham Cohomology Classes on a Differential Manifold?

derham-cohomologydifferential-topologysheaf-theory

Let $M$ be a differential manifold and $\mathcal H^k$ the presheaf of real vector spaces associating to the open subset $U\subset M$ the $k$-th de Rham cohomology vector space: $\mathcal H^k(U)=H^k_{DR}(U)$. Is this presheaf a sheaf?
Of course not! Indeed, given any non-zero cohomology class $0\neq[\omega]\in \mathcal H^k(U)$ represented by the closed $k$-form $\omega\in \Omega^k_M(U)$ there exists (by Poincaré's Lemma) a covering $(U_i)_{i\in I}$ of $U$ by open subsets $U_i\subset U$ such that $[\omega]\vert U_i=[\omega\vert U_i]=0\in \mathcal H^k(U_i)$, and thus the first axiom for a presheaf to be a sheaf is violated.
But what about the second axiom?
My question:
Suppose we are given a differential manifold M, a covering $(U_\lambda)_{\lambda \in \Lambda}$of $M$ by open subsets $U_\lambda \subset M$, closed differential $k-$forms $\omega_\lambda \in \Omega^k_M(U_\lambda)$ satisfying $[\omega_\lambda]\vert U_\lambda \cap U_\mu=[\omega_\mu]\vert U_\lambda \cap U_\mu\in \mathcal H^k(U_\lambda\cap U\mu)$ for all $\lambda,\mu \in \Lambda$.
Does there then exist a closed differential form $\omega\in \Omega^k(M)$ such that we have for the restrictions in cohomology: $[\omega]\vert U_\lambda=[\omega _\lambda]\in \mathcal H^k(U_\lambda)$ for all $\lambda\in \Lambda$ ?

Remarks

  1. This is an extremely naïve question which, to my embarrassment, I cannot solve.
    I have extensively browsed the literature and consulted some of my friends, all brilliant geometers (albeit not differential topologists), but they didn't know the answer offhand.
    For what it's worth, I would guess (but not conjecture!) that such glueing is impossible.
  2. If the covering of $X$ has only two opens then we can glue.
    This follows immediately from Mayer-Vietoris's long exact sequence
    $$\cdots \to \mathcal H^k(M) \to \mathcal H^k(U_1) \oplus \mathcal H^k(U_2) \to \mathcal H^k(U_1\cap U_2)\to \cdots$$

Update
My brilliant friends didn't answer offhand but a few hours later, unsurprisingly, they came back to me with splendid counterexamples! See below.

Best Answer

No.

Make $M$ by gluing three strips to two discs to form a thrice-punctured sphere. Take three open sets $U_\lambda$, each made by both discs and two of the strips. Then each $U_\lambda$ is homeomorphic to annulus and thus has $1$-dimensional $H^1$.

The pairwise intersections, made from one strip connecting two discs, are contractible and so their $H^1$ vanishes. Thus, for $k=1$, the agreement condition on the pairwise intersections is vacuous.

If your claim held, then we could choose a $1$-form on $M$ that restricts to an arbitrary cohomology class on each of the three $U_\lambda$, making the first de Rham cohomology of $M$ at least three-dimensional. But in fact it is only two-dimensional. Instead, there is a relation where the integrals around three clockwise loops around the three punctures sum to $0$, because these loops form the boundary of a particular subset of $M$.

It's true if the intersections $U_\lambda \cap U_\kappa \cap U_\mu$ are empty for all distinct $\lambda,\kappa,\mu$, by iteratively applying the Mayer-Vietoris sequence or applying a single exact sequence in sheaf cohomology.

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