Zeeman's comparison theorem

In homological algebra, Zeeman's comparison theorem, introduced by Christopher Zeeman,^[1] gives conditions for a morphism of spectral sequences to be an isomorphism.

Statement

Comparison theorem—Let ${\displaystyle E_{p,q}^{r},{}^{\prime }E_{p,q}^{r}}$ be first quadrant spectral sequences of flat modules over a commutative ring and ${\displaystyle f:E^{r}\to {}^{\prime }E^{r}}$ a morphism between them. Then any two of the following statements implies the third:

1. ${\displaystyle f:E_{2}^{p,0}\to {}^{\prime }E_{2}^{p,0}}$ is an isomorphism for every p.
2. ${\displaystyle f:E_{2}^{0,q}\to {}^{\prime }E_{2}^{0,q}}$ is an isomorphism for every q.
3. ${\displaystyle f:E_{\infty }^{p,q}\to {}^{\prime }E_{\infty }^{p,q}}$ is an isomorphism for every p, q.

Illustrative example

As an illustration, we sketch the proof of Borel's theorem, which says the cohomology ring of a classifying space is a polynomial ring.^{[citation needed]}

First of all, with G as a Lie group and with ${\displaystyle \mathbb {Q} }$ as coefficient ring, we have the Serre spectral sequence ${\displaystyle E_{2}^{p,q}}$ for the fibration ${\displaystyle G\to EG\to BG}$. We have: ${\displaystyle E_{\infty }\simeq \mathbb {Q} }$ since EG is contractible. We also have a theorem of Hopf stating that ${\displaystyle H^{*}(G;\mathbb {Q} )\simeq \Lambda (u_{1},\dots ,u_{n})}$, an exterior algebra generated by finitely many homogeneous elements.

Next, we let ${\displaystyle E(i)}$ be the spectral sequence whose second page is ${\displaystyle E(i)_{2}=\Lambda (x_{i})\otimes \mathbb {Q} [y_{i}]}$ and whose nontrivial differentials on the r-th page are given by ${\displaystyle d(x_{i})=y_{i}}$ and the graded Leibniz rule. Let ${\displaystyle {}^{\prime }E_{r}=\otimes _{i}E_{r}(i)}$. Since the cohomology commutes with tensor products as we are working over a field, ${\displaystyle {}^{\prime }E_{r}}$ is again a spectral sequence such that ${\displaystyle {}^{\prime }E_{\infty }\simeq \mathbb {Q} \otimes \dots \otimes \mathbb {Q} \simeq \mathbb {Q} }$. Then we let

${\displaystyle f:{}^{\prime }E_{r}\to E_{r},\,x_{i}\mapsto u_{i}.}$

Note, by definition, f gives the isomorphism ${\displaystyle {}^{\prime }E_{r}^{0,q}\simeq E_{r}^{0,q}=H^{q}(G;\mathbb {Q} ).}$ A crucial point is that f is a "ring homomorphism"; this rests on the technical conditions that ${\displaystyle u_{i}}$ are "transgressive" (cf. Hatcher for detailed discussion on this matter.) After this technical point is taken care, we conclude: ${\displaystyle E_{2}^{p,0}\simeq {}^{\prime }E_{2}^{p,0}}$ as ring by the comparison theorem; that is, ${\displaystyle E_{2}^{p,0}=H^{p}(BG;\mathbb {Q} )\simeq \mathbb {Q} [y_{1},\dots ,y_{n}].}$