# Serre spectral sequence

In mathematics, the **Serre spectral sequence** (sometimes **Leray-Serre spectral sequence** to acknowledge earlier work of Jean Leray in the Leray spectral sequence) is an important tool in algebraic topology. It expresses, in the language of homological algebra, the singular (co)homology of the total space *X* of a (Serre) fibration in terms of the (co)homology of the base space *B* and the fiber *F*. The result is due to Jean-Pierre Serre in his doctoral dissertation.

## Cohomology spectral sequence

Let be a Serre fibration of topological spaces, and let *F* be the fiber. The Serre cohomology spectral sequence is the following:

Here, at least under standard simplifying conditions, the coefficient group in the -term is the *q*-th integral cohomology group of *F*, and the outer group is the singular cohomology of *B* with coefficients in that group.

Strictly speaking, what is meant is cohomology with respect to the local coefficient system on *B* given by the cohomology of the various fibers. Assuming for example, that *B* is simply connected, this collapses to the usual cohomology. For a path connected base, all the different fibers are homotopy equivalent. In particular, their cohomology is isomorphic, so the choice of "the" fiber does not give any ambiguity.

The abutment means integral cohomology of the total space *X*.

This spectral sequence can be derived from an exact couple built out of the long exact sequences of the cohomology of the pair , where is the restriction of the fibration over the *p*-skeleton of *B*. More precisely, using this notation,

- ,

*f* is defined by restricting each piece on to , *g* is defined using the coboundary map in the long exact sequence of the pair, and *h* is defined by restricting to .

There is a multiplicative structure

coinciding on the *E*_{2}-term with (−1)^{qs} times the cup product, and with respect to which the differentials *d*_{r} are (graded) derivations inducing the product on the *E*_{r+1}-page from the one on the *E _{r}*-page.

## Homology spectral sequence

Similarly to the cohomology spectral sequence, there is one for homology:

where the notations are dual to the ones above.

## Example computations

### Hopf Fibration

Recall that the Hopf fibration is given by . The *E*_{2}-page of the Leray-Serre Spectral sequence reads

The differential *d*_{2+i} goes down and right. Thus the only differential which is not necessarily 0 is *d*^{0,1}_{2}, because the rest have domain or codomain *0* (since they are 0 on the *E*_{2}-page). In particular, this sequence degenerates at *E*_{2}=*E*_{∞}. The *E*_{3}-page reads

The spectral sequence abuts to i.e. Evaluating at the interesting parts, we have and Knowing the cohomology of both are zero, so the differential is an isomorphism.

### Sphere bundle on a complex projective variety

Given a complex *n*-dimensional projective variety X there is a canonical family of line bundles for coming from the embedding . This is given by the global sections which send

If we construct a rank r vector bundle which is a finite whitney sum of vector bundles we can construct a sphere bundle whose fibers are the spheres . Then, we can use the Serre spectral sequence along with the Euler class to compute the integral cohomology of S. The -page is given by . We see that the only non-trivial differentials are given on the -page and are defined by cupping with the Euler class . In this case it is given by the top chern class of . For example, consider the vector bundle for X a K3 surface. Then, the spectral sequence reads as

The differential for is the square of the Lefschetz class. In this case, the only non-trivial differential is then

We can finish this computation by noting the only nontrivial cohomology groups are

### Basic pathspace fibration

We begin first with a basic example; consider the path space fibration

We know the homology of the base and total space, so our intuition tells us that the Serre spectral sequence should be able to tell us the homology of the loop space. This is an example of a case where we can study the homology of a fibration by using the *E*^{∞} page (the homology of the total space) to control what can happen on the *E*^{2} page. So recall that

Thus we know when *q* = 0, we are just looking at the regular integer valued homology groups *H _{p}*(

*S*

^{n+1}) which has value in degrees 0 and

*n*+1 and value 0 everywhere else. However, since the path space is contractible, we know that by the time the sequence gets to

*E*

^{∞}, everything becomes 0 except for the group at

*p*=

*q*= 0. The only way this can happen is if there is an isomorphism from to another group. However, the only places a group can be nonzero are in the columns

*p*= 0 or

*p*=

*n*+1 so this isomorphism must occur on the page

*E*

^{n+1}with codomain However, putting a in this group means there must be a at

*H*

_{n+1}(

*S*

^{n+1};

*H*(

_{n}*F*)). Inductively repeating this process shows that

*H*(Ω

_{i}*S*

^{n+1}) has value at integer multiples of

*n*and 0 everywhere else.

### Cohomology ring of complex projective space

We compute the cohomology of using the fibration:

Now, on the *E*_{2} page, in the 0,0 coordinate we have the identity of the ring. In the 0,1 coordinate, we have an element *i* that generates However, we know that by the limit page, there can only be nontrivial generators in degree 2*n*+1 telling us that the generator *i* must transgress to some element *x* in the 2,0 coordinate. Now, this tells us that there must be an element *ix* in the 2,1 coordinate. We then see that *d*(*ix*) = *x*^{2} by the Leibniz rule telling us that the 4,0 coordinate must be *x*^{2} since there can be no nontrivial homology until degree 2*n*+1. Repeating this argument inductively until 2*n*+1 gives *ix ^{n}* in coordinate 2

*n*,1 which must then be the only generator of in that degree thus telling us that the 2

*n*+1,0 coordinate must be 0. Reading off the horizontal bottom row of the spectral sequence gives us the cohomology ring of and it tells us that the answer is

In the case of infinite complex projective space, taking limits gives the answer

### Fourth homotopy group of the three-sphere

A more sophisticated application of the Serre spectral sequence is the computation This particular example illustrates a systematic technique which one can use in order to deduce information about the higher homotopy groups of spheres. Consider the following fibration which is an isomorphism on

where is an Eilenberg–MacLane space. We then further convert the map to a fibration; it is general knowledge that the iterated fiber is the loop space of the base space so in our example we get that the fiber is But we know that Now we look at the cohomological Serre spectral sequence: we suppose we have a generator for the degree 3 cohomology of called *i*. Since there is nothing in degree 3 in the total cohomology, we know this must be killed by an isomorphism. But the only element that can map to it is the generator *a* of the cohomology ring of , so we have . Therefore by the cup product structure, the generator in degree 4, , maps to the generator by multiplication by 2 and that the generator of cohomology in degree 6 maps to by multiplication by 3, etc. In particular we find that But now since we killed off lower homotopy groups of *X* (i.e., groups in dimension less than 4) by using the iterated fibration, we know that by the Hurewicz theorem, telling us that

**Corollary**:

Proof: Take the long exact sequence of homotopy groups for the Hopf fibration .

## See also

## References

The Serre spectral sequence is covered in most textbooks on algebraic topology, e.g.

- Allen Hatcher,
*The Serre spectral sequence* - Edwin Spanier,
*Algebraic topology*, Springer

Also

- James Davis, Paul Kirk,
*Lecture notes in algebraic topology*gives many nice applications of the Serre spectral sequence.

An elegant construction is due to

- Andreas Dress,
*Zur Spektralsequenz einer Faserung*, Inventiones Mathematicae**3**(1967), 172–178.

The case of simplicial sets is treated in

- Paul Goerss, Rick Jardine,
*Simplicial homotopy theory*, Birkhäuser