I'm currently in need an explicit formula in classical cohomology which I'm pretty sure is well known, but which I've been unable to find in the references I am aware of. 


Let $X$ be a smooth manifold and let $\mathcal{U}=\{U\_\alpha\}$ be a fixed open cover of $X$ such that all the finite intersections $U\_{\alpha\_1}\cap\cdots U\_{\alpha\_n}$ are contractible. Consider the following two cochain complexes:


the de Rham complex $\Omega^\bullet(X)$ of $X$ 


the total complex of the Cech-de Rham bicomplex $\Omega^\bullet(\mathcal{U}\_\bullet)$.


The restriction of a global form on $X$ to the open sets $U\_\alpha$ gives a linear map


$j: \Omega^\bullet(X) \to Tot^\bullet(\Omega^\bullet(\mathcal{U}\_\bullet))$


which, if I'm not wrong here, is a injective quasi-isomorphism of cochain complexes. I've been able to prove this (if I've not made mistakes), by brute force: i.e. by showing that $j$
is bijective in cohomology. But I'd like to have a fancier proof by writing an explicit "globalization" morphism


$\pi : Tot^\bullet(\Omega^\bullet(\mathcal{U}\_\bullet)) \to \Omega^\bullet(X)$


such that


$\pi j= id\_{\Omega^\bullet(X)}$


$j \pi = id\_{Tot^\bullet(\Omega^\bullet(\mathcal{U}\_\bullet))} + [d\_{tot},K]$


with $K$ some explicit morphism of graded vector spaces $Tot^\bullet(\Omega^\bullet(\mathcal{U}\_\bullet)) \to Tot^\bullet(\Omega^\bullet(\mathcal{U}\_\bullet))[-1]$.


I guess one should be able to build $K$ by using a partition of the unit subordinate to the
cover $\mathcal{U}$, but somehow I got lost in the computation. Since I feel this should be a well known fact, I'm asking here for direct references before attempting back to write $K$ myself. 



An explicit homotopy equivalence between the de Rham complex and the Cech-de Rham total complex
Thanks to an email by Chris Rogers, I now know that my question above is precisely the subject of Proposition 9.5 in Bott-Tu, Differential Forms in Algebraic Topology., where an explicit formula for the homotopy operator in terms of a partition of unit subordinate to the given open cover is given.


They also write "The not very intuitive formulas below were obtained, after repeated tries, by a careful bookkeeping of the inductive steps in the proof of Proposition 8.8." (where Proposition 8.8 is the statement that $j$ is a quasiisomorphism), which comforts me a lot in view of my failed attemps. Yet, since Bott-Tu is a basic reference on the subject I should have checked it, too, before asking on MO. Sorry for having not done.


