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as far as I know, there are two main ways to have a relative version of De Rham Cohomology for a pair (M,N), where M and N are smooth manifolds and N is a closed (as a topological subspace) submanifold of M:

1) Godbillon, Elements de topologie algébrique: $\Omega^p(M,N)$ is the space of all forms on $M$ whose restriction to $N$ is zero. This is a subalgebra of $\Omega^p(M)$ so it defines a cohomological space $H^p(M,N)$.

2) Bott-Tu, Differential forms in algebraic topology: this times, $\Omega^p(M,N)=\Omega^p(M)\oplus \Omega^{p-1}(N)$ with differential $d(\omega,\theta)=(d\omega,i^*(\omega)-d\theta)$, where $i:M\to N$ is the inclusion.

Does these two cohomologies give the same results? Otherwise, are they related and how are they related?

Bott and Tu's paragraph on the relative De Rham cohomology is very short. Does someone know a good reference on this subject?

Thank you in advance.

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the otimes should be an oplus... –  Xandi Tuni Oct 30 '12 at 8:52
Yes, thanks Johannes for editing. And for your answer. I just seen it right now (because of Sandy), so I need time to check it and ask details. –  Taladris Nov 1 '12 at 0:17

1 Answer 1

up vote 9 down vote accepted

A chain map $\Theta$ from the Godbillon theory to the Bott-Tu version is given by $\omega \mapsto (\omega,0)$ (note that is a chain map only on $\Omega^{p} (M;N)_{G}$). I claim that this induces an isomorphism on cohomology. A couple of special cases is obvious: if $N=\emptyset$, then both theories agree with absolute de Rham theory. If $N \to M$ is a homotopy equivalence, both theories are trivial by long exact sequences and homotopy invariance of the absolute theory.

For the general case, pick a tubular neighborhood $U$ of $N$. You get short exact sequences of chain complexes (in both cases)

$$ 0\to \Omega (M;N) \to \Omega(U;N) \oplus \Omega (M-N) \to \Omega (U-N) \to 0 $$

(exactness is checked by means of a partition of unity), and $\Theta$ compares the both short exact sequences. The associated (Mayer-Vietoris) exact sequence and the $5$-lemma concludes the proof.

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What is $G$ here, and what does it mean for $\Theta$ to "compare" two short exact sequences? –  Vidit Nanda Oct 30 '12 at 10:08
I assume $G$=Godbillon. That is $\Omega(M;N)$ with a $G$ subscript is the Godbillon definition and without a $G$ is the Bott-Tu definition. –  Michael Murray Oct 30 '12 at 10:27
@Michael: absence of the G subscript means that the above exact sequence of chain complexes can be defined for the two kinds of relative cohomology (with different maps in the sequences of course). –  Taladris Nov 1 '12 at 3:10
@Vel: "Compare" means that we have a commutative diagram connecting the exact sequences $ 0\to \Omega(M;N)_{G} \to \Omega(U;N)_{G}\oplus\Omega(M\setminus N)\to \Omega(U\setminus N)\to 0$ and $ 0\to \Omega(M;N)_{BT} \to \Omega(U;N)_{BT}\oplus\Omega(M\setminus N)\to \Omega(U\setminus N)\to 0$ (where the $G$ and $BT$ respectively mean Godbillon and Bott-Tu) and the vertical maps are $\Theta$, $\Theta\times id$ and $id$. These maps induce isomorphisms in cohomology (as Johannes claimed). @Johannes: thank you! I considered the chain map $\Theta$ but unsuccessfully. –  Taladris Nov 1 '12 at 3:12

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