nLab spectral sequence

Context

Homological algebra

homological algebra

and

nonabelian homological algebra

diagram chasing

Theorems

Stable Homotopy theory

stable homotopy theory

Contents

Idea

The notion of spectral sequence is an algorithm or computational tool in homological algebra and more generally in homotopy theory which allows to compute chain homology groups/homotopy groups of bi-graded objects from the homology/homotopy of the two graded components.

Notably there is a spectral sequence for computing the homology of the total complex of a double complex from the homology of its row and column complexes separately. This in turn allows to compute derived functors of composite functors $G\circ F$ from the double complex $\mathbb{R}^\bullet G (\mathbb{R}^\bullet F(-))$ obtained by non-totally deriving the two functors separately (called the Grothendieck spectral sequence). By choosing various functors $F$ and $G$ here this gives rise to various important classes of examples of spectral sequences, see below.

More concretely, a homology spectral sequence is a sequence of graded chain complexes that provides the higher order corrections to the naïve idea of computing the homology of the total complex $Tot(V)_\bullet$ of a double complex $V_{\bullet, \bullet}$: by first computing those of the vertical differential, then those of the horizontal differential induced on these vertical homology groups (or the other way around). This simple idea in general does not produce the correct homology groups of $Tot(V)_\bullet$, but it does produce a “first-order approximation” to them, in a useful sense. The spectral sequence is the sequence of higher-order corrections that make this naive idea actually work.

Being, therefore, an iterative perturbative approximation scheme of bigraded differential objects, fully-fledged spectral sequences can look a bit intricate. However, a standard experience in mathematical practice is that for most problems of practical interest the relevant spectral sequence “perturbation series” yields the exact result already at the second stage. This reduces the computational complexity immensely and makes spectral sequences a wide-spread useful computational tool.

Despite their name, there seemed to be nothing specifically “spectral” about spectral sequences, for any of the technical meanings of the word spectrum. Together with the concept, this term was introduced by Jean Leray and has long become standard, but was never really motivated (see p. 5 of Chow). But then, by lucky coincidence it turns out in the refined context of stable (∞,1)-category theory/stable homotopy theory that spectral sequences frequently arise by considering the homotopy groups of sequences of spectra. This is discussed at spectral sequence of a filtered stable homotopy type.

While therefore spectral sequences are a notion considered in the context of homological algebra and more generally in stable homotopy theory, there is also an “unstable” or nonabelian variant of the notion in plain homotopy theory, called homotopy spectral sequence.

Definition

We give the general definition of a (co)homology spectral sequence. For motivation see the example Spectral sequence of a filtered complex below.

Throughout, let $\mathcal{A}$ be an abelian category.

Spectral sequence

Definition

A cohomology spectral sequence in $\mathcal{A}$ is

• a family $(E^{p,q}_r)$ of objects in $\mathcal{A}$, for all integers $p,q,r$ with $r\geq 1$

(for a fixed $r$ these are said to form the $r$-th page of the spectral sequence)

• for each $p,q,r$ as above a morphism (called the differential)

$d^{p,q}_r:E^{p,q}_r\to E^{p+r,q-r+1}_r$

satisfying $d_r^2 = 0$ (more precisely, $d_r^{p+r,q-r+1}\circ d_r^{p,q} = 0$)

• isomorphisms $\alpha_r^{p,q}: H^{p,q}(E_r)\to E^{p,q}_{r+1}$ where the chain cohomology is given by

$H^{p,q}(E_r) = \mathrm{ker} d^{p,q}_r/ \mathrm{im} d^{p-r,q+r-1}_r \,.$

Analogously a homology spectral sequence is collection of objects $(E_{p,q}^r)$ with the differential $d_r$ of degree $(-r,r-1)$.

Convergence

Definition

Let $\{E^r_{p,q}\}_{r,p,q}$ be a spectral sequence such that for each $p,q$ there is $r(p,q)$ such that for all $r \geq r(p,q)$ we have

$E^{r \geq r(p,q)}_{p,q} \simeq E^{r(p,q)}_{p,q} \,.$

Then one says equivalently that

1. $E^\infty \coloneqq \{E^\infty_{p,q}\}_{p,q} \coloneqq \{ E^{r(p,q)}_{p,q} \}_{p,q}$

is the limit term of the spectral sequence;

2. the spectral sequence abuts to $E^\infty$.

Example

If for a spectral sequence there is $r_s$ such that all differentials on pages after $r_s$ vanish, $\partial^{r \geq r_s} = 0$, then $\{E^{r_s}\}_{p,q}$ is limit term for the spectral sequence. One says in this cases that the spectral sequence degenerates at $r_s$.

Proof

By the defining relation

$E^{r+1}_{p,q} \simeq ker(\partial^r_{p-r,q+r-1})/im(\partial^r_{p,q}) = E^r_{pq}$

the spectral sequence becomes constant in $r$ from $r_s$ on if all the differentials vanish, so that $ker(\partial^r_{p,q}) = E^r_{p,q}$ for all $p,q$.

Example

If for a spectral sequence $\{E^r_{p,q}\}_{r,p,q}$ there is $r_s \geq 2$ such that the $r_s$th page is concentrated in a single row or a single column, then the the spectral sequence degenerates on this pages, example 1, hence this page is a limit term, def. 2. One says in this case that the spectral sequence collapses on this page.

Proof

For $r \geq 2$ the differentials of the spectral sequence

$\partial^r \colon E^r_{p,q} \to E^r_{p-r, q+r-1}$

have domain and codomain necessarily in different rows an columns (while for $r = 1$ both are in the same row and for $r = 0$ both coincide). Therefore if all but one row or column vanish, then all these differentials vanish.

Definition

A spectral sequence $\{E^r_{p,q}\}_{r,p,q}$ is said to converge to a graded object $H_\bullet$ with filtering $F_\bullet H_\bullet$, traditionally denoted

$E^r_{p,q} \Rightarrow H_\bullet \,,$

if the associated graded complex $\{G_p H_{p+q}\}_{p,q} \coloneqq \{F_p H_{p+q} / F_{p-1} H_{p+q}\}$ of $H$ is the limit term of $E$, def. 2:

$E^\infty_{p,q} \simeq G_p H_{p+q} \;\;\;\;\;\;\; \forall_{p,q} \,.$
Remark

In practice spectral sequences are often referred to via their first non-trivial page, often also the page at which it collapses, def. 2, often the second page. Then one often uses notation such as

$E^2_{p,q} \Rightarrow H_\bullet$

to be read as “There is a spectral sequence whose second page is as shown on the left and which converges to a filtered object as shown on the right.”

Remark

In applications one is interested in computing the $H_n$ and uses spectral sequences converging to this as tools for approximating $H_n$ in terms of the given filtration.

Therefore usually spectral sequences are required to converge in each degree, or even that for each pair $(p,q)$ there exists an $r_0$ such that for all $r\geq r_0$, $d_r^{p-r,q+r-1} = 0$.

Remark

If $(E^r)$ collapses at $r$, then it converges to $H_\bullet$ with $H_n$ being the unique entry $E_{p,q}^r$ on the non-vanishing row/column with $p+q = n$.

Boundedness

Definition

A spectral sequence $\{E^r_{p,q}\}$ is called a bounded spectral sequence if for all $n,r \in \mathbb{Z}$ the number of non-vanishing terms of the form $E^r_{k,n-k}$ is finite.

Example

A spectral sequence $\{E^r_{p,q}\}$ is called

• a first quadrant spectral sequence if all terms except possibly for $p,q \geq 0$ vanish;

• a third quadrant spectral sequence if all terms except possibly for $p,q \leq 0$ vanish.

Such spectral sequences are bounded, def. 4.

Proposition

A bounded spectral sequence, def. 4, has a limit term, def. 2.

Proof

First notice that if a spectral sequence has at most $N$ non-vanishing terms of total degree $n$ on page $r$, then all the following pages have at most at these positions non-vanishing terms, too, since these are the homologies of the previous terms.

Therefore for a bounded spectral sequence for each $n$ there is $L(n) \in \mathbb{Z}$ such that $E^r_{p,n-p} = 0$ for all $p \leq L(n)$ and all $r$. Similarly there is $T(n) \in \mathbb{Z}$ such $E^r_{n-q,q} = 0$ for all $q \leq T(n)$ and all $r$.

We claim then that the limit term of the bounded spectral sequence is in position $(p,q)$ given by the value $E^r_{p,q}$ for

$r \gt max( p-L(p+q-1), q + 1 - L(p+q+1) ) \,.$

This is because for such $r$ we have

1. $E^r_{p-r, q+r-1} = 0$ because $p-r \lt L(p+q-1)$, and hence the kernel $ker(\partial^r_{p-r,q+r-1}) = 0$ vanishes;

2. $E^r_{p+r, q-r+1} = 0$ because $q-r + 1 \lt T(p+q+1)$, and hence the image $im(\partial^r_{p,q}) = 0$ vanishes.

Therefore

\begin{aligned} E^{r+1}_{p,q} &= ker(\partial^r_{p-r,q+r-1})/im(\partial^r_{p,q}) \\ & \simeq E^r_{p,q}/0 \\ & \simeq E^r_{p,q} \end{aligned} \,.

Examples

The basic class of examples are

which compute the cohomology of a filtered complex from the cohomologies of its associated graded objects.

From this one obtains as a special case the class of

which compute the cohomology of the total complex of a double complex using the two canonical filtrations of this by row- and by column-degree.

From this in turn one obtains as a special case the class of

which compute the derived functor $\mathbb{R}^\bullet(G \circ F (-))$ of the composite of two functors from the spectral sequence of the double complex $\mathbb{R}^\bullet (F (\mathbb{R}^\bullet G (-)))$.

Many special cases of this for various choices of $F$ and $G$ go by special names, this we tabulate at

Spectral sequence of a filtered complex

The fundamental example of a spectral sequence, from which essentially all the other examples arise as special cases, is the spectral sequence of a filtered complex. (See there for details). Or more generally in stable homotopy theory: the spectral sequence of a filtered stable homotopy type.

If a cochain complex $C^\bullet$ is equipped with a filtration $F^\bullet C^\bullet$, there is an induced filtration $F^\bullet H(C)$ of its cohomology groups, according to which levels of the filtration contain representatives for the various cohomology classes.

A filtration $F$ also gives rise to an associated graded object $Gr(F)$, whose grades are the successive level inclusion cokernels. Generically, the operations of grading and cohomology do not commute:

$Gr(F^\bullet H^\bullet(C)) \neq H^\bullet (Gr(F^\bullet) C) \,.$

But the spectral sequence associated to a filtered complex $F^\bullet C^\bullet$, passes through $H^\bullet (Gr(F^\star) C)$ in the page $E_{(1)}$ and in good cases converges to $Gr(F^* H^\bullet(C))$.

Spectral sequence of a double complex

The total complex of a double complex is naturally filtered in two ways: by columns and by rows. By the above spectral sequence of a filtered complex this gives two different spectral sequences associated computing the cohomology of a double complex from the cohomologies of its rows and columns. Many other classes of spectral sequences are special cases of this cases, notably the Grothendieck spectral sequence and its special cases.

This is discussed at spectral sequence of a double complex.

Spectral sequences for hyper-derived functors

From the spectral sequence for a double complex? one obtains as a special case a spectral sequence that computes hyper-derived functors.

(…)

Grothendieck spectral sequence

The Grothendieck spectral sequence computes the composite of two derived functors from the two derived functors separately.

Let $\mathcal{A} \stackrel{F}{\to} \mathcal{B} \stackrel{G}{\to} \mathcal{C}$ be two left exact functors between abelian categories.

Write $R^p F : \mathcal{D} \to Ab$ for the cochain cohomology of the derived functor of $F$ in degree $p$ etc. .

Theorem

If $F$ sends injective objects of $\mathcal{A}$ to $G$-acyclic objects in $\mathcal{B}$ then for each $A \in \mathcal{A}$ there is a first quadrant cohomology spectral sequence

$E_r^{p,q} := (R^p G \circ R^q F)(A)$

that converges to the right derived functor of the composite functor

$E_r^{p,q} \Rightarrow R^{p+q} (G \circ F)(A).$

Moreover

1. the edge maps in this spectral sequence are the canonical morphisms

$R^p G (F A) \to R^p (G \circ F)(A)$

induced from applying $F$ to an injective resolution $A \to \hat A$ and the isomorphism

$R^q (G \circ F)(A) \to G(R^q F (A)) \,.$
2. the exact sequence of low degree terms is

$0 \to (R^1 G)(F(A)) \to R^1(G \circ F)(A) \to F(R^1(G(A))) \to (R^2 F)(G(A)) \to R^2(G \circ F)(A)$

This is called the Grothendieck spectral sequence.

Proof

Since for $A \to \hat A$ an injective resolution of $A$ the complex $F(\hat A)$ is a chain complex not concentrated in a single degree, we have that $R^p (G \circ F)(A)$ is equivalently the hyper-derived functor evaluation $\mathbb{R}^p(G) (F(A))$.

Therefore the second spectral sequence discussed at hyper-derived functor spectral sequences converges as

$(R^p G)H^q(F(\hat A)) \Rightarrow R^p (G \circ F)(A) \,.$

Now since by construction $H^q(F(\hat A)) = R^q F(A)$ this is a spectral sequence

$(R^p G)(R^q F) A) \Rightarrow R^p (G \circ F)(A) \,.$

This is the Grothendieck spectral sequence.

Special Grothendieck spectral sequences

Leray spectral sequence

The Leray spectral sequence is the special case of the Grothendieck spectral sequence for the case where the two functors being composed are a push-forward of sheaves of abelian groups along a continuous map $f : X \to Y$ followed by the push-forward $X \to *$ to the point. This yields a spectral sequence that computes the abelian sheaf cohomology on $X$ in terms of the abelian sheaf cohomology on $Y$.

Theorem

Let $X, Y$ be suitable sites and $f : X \to Y$ be a morphism of sites. () Let $\mathcal{C} = Ch_\bullet(Sh(X,Ab))$ and $\mathcal{D} = Ch_\bullet(Sh(Y,Ab))$ be the model categories of complexes of sheaves of abelian groups. The direct image $f_*$ and global section functor $\Gamma_Y$ compose to $\Gamma_X$:

$\Gamma_X : \mathcal{C} \stackrel{f_*}{\to} \mathcal{D} \stackrel{\Gamma_Y}{\to} Ch_\bullet(Ab) \,.$

Then for $A \in Sh(X,Ab)$ a sheaf of abelian groups on $X$ there is a cohomology spectral sequence

$E_r^{p,q} := H^p(Y, R^q f_* A)$

that converges as

$E_r^{p,q} \Rightarrow H^{p+q}(X, A)$

and hence computes the cohomology of $X$ with coefficients in $A$ in terms of the cohomology of $Y$ with coefficients in the push-forward of $A$.

Base change spectral sequence for $Tor$ and $Ext$

For $R$ a ring write $R$Mod for its category of modules. Given a homomorphism of ring $f : R_1 \to R_2$ and an $R_2$-module $N$ there are composites of base change along $f$ with the hom-functor and the tensor product functor

$R_1 Mod \stackrel{\otimes_{R_1} R_2}{\to} R_2 Mod \stackrel{\otimes_{R_2} N}{\to} Ab$
$R_1 Mod \stackrel{Hom_{R_1 Mod}(-,R_2)}{\to} R_2 Mod \stackrel{Hom_{R_2}(-,N)}{\to} Ab \,.$

The derived functors of $Hom_{R_2}(-,N)$ and $\otimes_{R_2} N$ are the Ext- and the Tor-functors, respectively, so the Grothendieck spectral sequence applied to these composites yields base change spectral sequence for these.

Exact couples

The above examples are all built on the spectral sequence of a filtered complex. An alternatively universal construction builds spectral sequences from exact couples.

An exact couple is an exact sequence of three arrows among two objects

$E \overset{j}{\to} D \overset{\varphi}{\to} D \overset{k}{\to} E \overset{j}{\to}.$

These creatures construct spectral sequences by a two-step process:

• first, the composite $d \coloneqq k j \colon E\to E$ is nilpotent, in that $d^2=0$
• second, the homology $E'$ of $(E,d)$ supports a map $j':E'\to \varphi D$, and receives a map $k':\varphi D\to E'$. Setting $D'=\varphi D$, by general reasoning
$E' \overset{j'}{\to} D' \overset{\varphi}{\to} D' \overset{k'}{\to} E' \overset{j'}{\to} \,.$

is again an exact couple.

The sequence of complexes $(E,d),(E',d'),\dots$ is a spectral sequence, by construction.

Examples of exact couples can be constructed in a number of ways. Importantly, any short exact sequence involving two distinct chain complexes provides an exact couple among their total homology complexes, via the Mayer-Vietoris long exact sequence; in particular, applying this procedure to the relative homology of a filtered complex gives precisely the spectral sequence of the filtered complex described (???) somewhere else on this page. For another example, choosing a chain complex of flat modules $(C^\dot,d)$, tensoring with the short exact sequence

$\mathbb{Z}/p\mathbb{Z} \to \mathbb{Z}/p^2\mathbb{Z} \to \mathbb{Z}/p\mathbb{Z}$

gives the exact couple

$H^\bullet(d,\mathbb{Z}/p^2\mathbb{Z}) \overset{[\cdot]}{\to} H^\bullet(d,\mathbb{Z}/p\mathbb{Z}) \overset{\beta}{\to} H^\bullet(d,\mathbb{Z}/p\mathbb{Z}) \overset{p}{\to}H^\bullet(d,\mathbb{Z}/p^2\mathbb{Z})\cdots$

in which $\beta$ is the mod-$p$ Bockstein homomorphism.

The exact couple recipe for spectral sequences is notable in that it doesn’t mention any grading on the objects $D,E$; trivially, an exact couple can be specified by a short exact sequence $\coker \varphi\to E\to \ker\varphi$, although this obscures the focus usually given to $E$. In applications, a bi-grading is usually induced by the context, which also specifies bidegrees for the initial maps $j,k,\varphi$, leading to the conventions mentioned earlier.

List of examples

More

The following list of examples orders the various classes of spectral sequences by special cases: items further to the right are special cases of items further to the left.

Here is a more random list (using material from Wikipedia). Eventually to be merged with the above.

Properties

Basic lemmas

Lemma

(mapping lemma)

If $f : (E_r^{p,q} \to (F_r^{p,q}))$ is a morphism of spectral sequences such that for some $r$ we have that $f_r : E_r^{p,q} \toF_r^{p,q}$ is an isomorphism, then also $f_s$ is an isomorphism for all $s \geq r$.

Lemma

(classical convergence theorem)

(…)

This is recalled in (Weibel, theorem 5.51).

Definition

A first quadrant spectral sequence is one for wich all pages are concentrated in the first quadrant of the $(p,q)$-plane, in that

$((p \lt 0) or (q \lt 0)) \;\; \Rightarrow E_r^{p,q} = 0 \,.$
Observation

If the $r$th page is concentrated in the first quadrant, then so the $(r+1)st$ page. So if the first one is, then all are.

Observation

Every first quadrant spectral sequence converges at $(p,q)$ from $r \gt max(p,q+1)$ on

$E_{max(p,q+1)+1}^{p,q} = E_\infty^{p,q} \,.$
Observation

If a first quadrant spectral sequence converges

$E_r^{p,q} \Rightarrow H^{p+q}$

then each $H^n$ has a filtration of length $n+1$

$0 = F^{n+1}H^n \subset F^n H^n \subset \cdots \subset F^1 H^n \subset F^0 H^n = H^n$

and we have

• $F^n H^n \simeq E_\infty^{n,0}$

• $H^n/F^1 H^n \simeq E_\infty^{0,n}$.

References

Abelian/stable theory

An elementary pedagogical introduction is in

• Timothy Chow, You could have invented spectral sequences, Notices of the AMS (2006) (pdf)

Standard textbook references are

• John McCleary, A User’s Guide to Spectral Sequences, Cambridge University Press

chapter 5 of

• Charles Weibel, An introduction to homological algebra Cambridge studies in advanced mathematics 38 (1994)

and section 14 of

• Raoul Bott, Loring Tu, Differential forms in algebraic topology, Graduate Texts in Mathematics 82, Springer 1982. xiv+331 pp.

and

A textbook with a focus on applications in algebraic topology is

The general discussion in the context of stable (∞,1)-category theory (the spectral sequence of a filtered stable homotopy type) is in section 1.2.2 of

A review Master thesis is

• Jennifer Orlich, Spectral sequences and an application (pdf)

Reviews of and lecture notes on standard definitions and facts about spectral sequences include

• Matthew Greenberg, Spectral sequences (pdf)

• Michael Hutchings, Introduction to spectral sequences (pdf)

• Daniel Murfet, Spectral sequences (pdf)

• Neil Strickland, Spectral sequences (pdf)

• Ravi Vakil, Spectral Sequences: Friend or Foe? (pdf)

• Brandon Williams, Spectral sequences (pdf)

Original articles incluce

• A. Romero, J. Rubio, F. Sergeraert, Computing spectral sequences (pdf)

Nonabelian / unstable theory

Homotopy spectral sequences in model categories are discussed in

Spectral sequences in general categories with zero morphisms are discussed in

History

• John McCleary, A history of spectral sequences: Origins to 1953, in History of Topology, edited by Ioan M. James, North Holland (1999) 631–663

Revised on June 16, 2014 03:26:08 by Urs Schreiber (88.128.80.80)