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For the sorting of stars in astronomy, see Stellar classification

In homological algebra, especially in algebraic topology or group cohomology, a spectral sequence is a means of computing homology groups by taking successive approximations. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics Homological algebra is the branch of Mathematics which studies homology in a general algebraic setting Algebraic topology is a branch of Mathematics which uses tools from Abstract algebra to study Topological spaces The basic goal is to find algebraic In Abstract algebra, Homological algebra, Algebraic topology and Algebraic number theory, as well as in applications to Group theory proper Spectral sequences are a generalization of exact sequences, and since their introduction by Jean Leray in 1946, they have become an important research tool, particularly in homotopy theory. Jean Leray ( 7 November 1906 &ndash 10 November 1998) was a French Mathematician, who worked on both Partial differential Year 1946 ( MCMXLVI) was a Common year starting on Tuesday (link will display full 1946 calendar of the Gregorian calendar. In Topology, two continuous functions from one Topological space to another are called homotopic ( Greek homos = identical However, they have a reputation for being abstruse and difficult to comprehend.

Contents

Discovery and motivation

Motivated by problems in algebraic topology, Jean Leray introduced the notion of a sheaf and found himself faced with the problem of computing sheaf cohomology. Algebraic topology is a branch of Mathematics which uses tools from Abstract algebra to study Topological spaces The basic goal is to find algebraic In Mathematics, a sheaf is a tool for systematically tracking locally defined data attached to the Open sets of a Topological space. In Mathematics, sheaf cohomology is the aspect of Sheaf theory, concerned with sheaves of Abelian groups that applies Homological algebra to To compute sheaf cohomology, Leray introduced a computational technique now known as the Leray spectral sequence. In Mathematics, the Leray spectral sequence was a pioneering example in Homological algebra, introduced in 1946 by Jean Leray. This gave a relationship between cohomology groups of a sheaf and cohomology groups of the pushforward of the sheaf. In Mathematics, in the field of Sheaf theory and especially in Algebraic geometry, the direct image functor generalizes the notion of a Section of It was not a direct relation. Instead, Leray found that the cohomology groups of the pushforward formed a natural chain complex, so that he could take the cohomology of the cohomology. In Mathematics, a chain complex is a construct originally used in the field of Algebraic topology. The result was not the cohomology of the original sheaf, but it was closer, and it too formed a natural chain complex. Leray found that he could repeat this process, and that each step got him closer to the cohomology groups of the original sheaf. Taking the limit of the iterated cohomologies gave him exactly the cohomology groups of the original sheaf. This allowed Leray to compute sheaf cohomology.

It was soon realized that Leray's computational technique was an example of a more general phenomenon. Spectral sequences were found in diverse situations, and they gave intricate relationships among homology and cohomology groups coming from geometric situations such as fibrations and from algebraic situations involving derived functors. In Mathematics, especially Algebraic topology, a fibration is a continuous mapping pE\to B\ satisfying the In Mathematics, certain Functors may be derived to obtain other functors closely related to the original ones While their theoretical importance has decreased since the introduction of derived categories, they are still the most effective computational tool available. In Mathematics, the derived category D ( C) of an Abelian category C is a construction of Homological algebra introduced This is true even when many of the terms of the spectral sequence are incalculable.

Unfortunately, because of the large amount of information carried in spectral sequences, they are difficult to grasp. This information is usually contained in a rank three lattice of abelian groups or modules. An abelian group, also called a commutative group, is a group satisfying the additional requirement that the product of elements does not depend on their order (the In Abstract algebra, the concept of a module over a ring is a generalization of the notion of Vector space, where instead of requiring the scalars In most cases that can be computed, the spectral sequence eventually collapses, meaning that going out further in the sequence produces no new information. This does not always happen, however, and then it becomes necessary to use tricks to extract useful information. Even in these cases, however, it is still possible to get useful information from a spectral sequence.

Formal definition

Fix an abelian category, such as a category of modules over a ring. In Mathematics, an abelian category is a category in which Morphisms and objects can be added and in which kernels and Cokernels exist A spectral sequence is a choice of a nonnegative integer r0 and a collection of three sequences:

  1. For all integers r ≥ r0, an object Er, called a sheet (as in a sheet of paper),
  2. Endomorphisms dr : Er → Er satisfying dr o dr = 0, called boundary maps or differentials,
  3. Isomorphisms of Er+1 with H(Er), the homology of Er with respect to dr. Paper is thin material mainly used for writing upon printing upon or packaging

Usually the isomorphisms between Er+1 and H(Er) are suppressed, and we write equalities instead. Er+1 is sometimes called the derived object of Er.

The most elementary example is a chain complex C•. In Mathematics, a chain complex is a construct originally used in the field of Algebraic topology. C• is an object in an abelian category of chain complexes, and it comes with a differential d. Let r0 = 0, and let E0 be C•. This forces E1 to be the complex H(C•): At the i'th location this is the i'th homology group of C•. The only natural differential on this new complex is the zero map, so we let d1 = 0. This forces E2 to equal E1, and again our only natural differential is the zero map. Putting the zero differential on all the rest of our sheets gives a spectral sequence whose terms are:

The terms of this spectral sequence stabilize at the first sheet because its only nontrivial differential was on the zeroth sheet. Consequently we can get no more information at later steps. Usually, to get useful information from later sheets, we need extra structure on the Er.

In the ungraded situation described above, r0 is irrelevant, but in practice most spectral sequences occur in the category of doubly graded modules over a ring R (or doubly graded sheaves of modules over a sheaf of rings). In Abstract algebra, the concept of a module over a ring is a generalization of the notion of Vector space, where instead of requiring the scalars In Mathematics, a ring is an Algebraic structure which generalizes the algebraic properties of the Integers though the rational, real In Mathematics, a sheaf is a tool for systematically tracking locally defined data attached to the Open sets of a Topological space. The degree of the boundary maps depends on r and is fixed by convention. For a homological spectral sequence, the terms are written E^r_{p,q} and the differentials have bidegree (-r,r-1). For a cohomological spectral sequence, the terms are written E^{p,q}_r and the differentials have bidegree (r,1-r). (These choices of bidegree occur naturally in practice; see the example of a double complex below. ) Depending upon the spectral sequence, the boundary map on the first sheet can have a degree which corresponds to r = 0, r = 1, or r = 2. For example, for the spectral sequence of a filtered complex, described below, r0 = 0, but for the Grothendieck spectral sequence, r0 = 2. In Mathematics, in the field of Homological algebra, the Grothendieck spectral sequence is a technique that allows one to compute the Derived functors of Usually r0 is zero, one, or two.

A morphism of spectral sequences E → E' is by definition a collection of maps fr : Er → E'r which commute with the given isomorphisms between cohomology of the r-th step and the (r+1)-st step of E and E' , respectively. The category of spectral sequences is an abelian category.

Exact couples

The most powerful technique for the construction of spectral sequences is William Massey's method of exact couples. William Schumacher Massey (born 1920 is an American mathematician known for his work in Algebraic topology. Exact couples are particularly common in algebraic topology, where there are many spectral sequences for which no other construction is known. In fact, all known spectral sequences can be constructed using exact couples. Despite this they are unpopular in abstract algebra, where most spectral sequences come from filtered complexes. To define exact couples, we begin again with an abelian category. As before, in practice this is usually the category of doubly graded modules over a ring. An exact couple is a pair of objects A and C, together with three homomorphisms between these objects: f : A → A, g : A → C and h : C → A subject to certain exactness conditions:

We will abbreviate this data by (A, C, f, g, h). In Mathematics, the image of a preimage under a given function is the set of all possible function outputs when taking each element of the preimage In the various branches of Mathematics that fall under the heading of Abstract algebra, the kernel of a Homomorphism measures the degree to which the homomorphism Exact couples are usually depicted as triangles. We will see that C corresponds to the E0 term of the spectral sequence and that A is some auxiliary data.

To pass to the next sheet of the spectral sequence, we will form the derived couple. We set:

From here it is straightforward to check that (A', C', f', g', h') is an exact couple. C' corresponds to the E1 term of the spectral sequence. We can iterate this procedure to get exact couples (A(n), C(n), f(n), g(n), h(n)). We let En be C(n) and dn be g(n) o h(n). This gives a spectral sequence.

Visualization

The E2 sheet of a spectral sequence
The E2 sheet of a spectral sequence

A doubly graded spectral sequence has a tremendous amount of data to keep track of, but there is a common visualization technique which makes the structure of the spectral sequence clearer. We have three indices, r, p, and q. For each r, imagine that we have a sheet of graph paper. On this sheet, we will take p to be the horizontal direction and q to be the vertical direction. At each lattice point we have the object E_r^{p,q}.

It is very common for n = p + q to be another natural index in the spectral sequence. n runs diagonally, northwest to southeast, across each sheet. In the homological case, the differentials have bidegree (-r,r-1), so they decrease n by one. In the cohomological case, n is increased by one. When r is zero, the differential moves objects one space to the right or left. This is similar to the differential on a chain complex. When r is one, the differential moves objects one space down or up. When r is two, the differential moves objects just like a knight's move in chess. The knight (♘ ♞ sometimes referred to by players as a 'horse' is a piece in the Game of Chess, representing a Knight (armoured cavalry Chess is a recreational and competitive Game played between two players. For higher r, the differential acts like a generalized knight's move.

Examples of spectral sequences

The spectral sequence of a filtered complex

A very common type of spectral sequence comes from a filtered cochain complex. In Mathematics, a filtration is an Indexed set Si of Subobjects of a given Algebraic structure S, with the index This is a cochain complex C• together with a set of subcomplexes FpC•, where p ranges across all integers. (In practice, p is usually bounded on one side. ) We require that the boundary map is compatible with the filtration; this means that d(FpCn) ⊆ FpCn+1. We assume that the filtration is descending, i. e. , FpC• ⊇ Fp+1C•. We will number the terms of the cochain complex by n. Later, we will also assume that the filtration is Hausdorff or separated, that is, the intersection of the set of all FpC• is zero, and that the filtration is exhaustive, that is, the union of the set of all FpC• is the entire chain complex C•.

The filtration is useful because it gives a measure of nearness to zero: As p increases, FpC• gets closer and closer to zero. We will construct a spectral sequence from this filtration where boundaries and cycles in later sheets get closer and closer to boundaries and cycles in the original complex. This spectral sequence will be doubly graded. One of the grades will be the filtration degree p. The other is called the complementary degree and is denoted q. The complementary degree satisfies the relation p + q = n. (We use the complementary degree instead of the location in the chain complex because this is more natural in the common case of the spectral sequence of a double complex, explained below. )

We will construct this spectral sequence by hand. C• has only a single grading and a filtration, so we first construct a doubly graded object from C•. To get the second grading, we will take the associated graded object with respect to the filtration. We will write it in an unusual way which will be justified at the E1 step:

Z_{-1}^{p,q} = Z_0^{p,q} = F^p C^{p+q}
B_0^{p,q} = 0
E_0^{p,q} = \frac{Z_0^{p,q}}{B_0^{p,q} + Z_{-1}^{p+1,q-1}} = \frac{F^p C^{p+q}}{F^{p+1} C^{p+q}}
E_0 = \bigoplus_{p,q\in\bold{Z}} E_0^{p,q}

Since we assumed that the boundary map was compatible with the filtration, E0 is a doubly graded object and there is a natural doubly graded boundary map d0 on E0. To get E1, we take the homology of E0.

\bar{Z}_1^{p,q} = \ker d_0^{p,q} : E_0^{p,q} \rightarrow E_0^{p,q+1} = \ker d_0^{p,q} : F^p C^{p+q}/F^{p+1} C^{p+q} \rightarrow F^p C^{p+q+1}/F^{p+1} C^{p+q+1}
\bar{B}_1^{p,q} = \mbox{im } d_0^{p,q-1} : E_0^{p,q-1} \rightarrow E_0^{p,q} = \mbox{im } d_0^{p,q-1} : F^p C^{p+q-1}/F^{p+1} C^{p+q-1} \rightarrow F^p C^{p+q}/F^{p+1} C^{p+q}
E_1^{p,q} = \frac{\bar{Z}_1^{p,q}}{\bar{B}_1^{p,q}} = \frac{\ker d_0^{p,q} : E_0^{p,q} \rightarrow E_0^{p,q+1}}{\mbox{im } d_0^{p,q-1} : E_0^{p,q-1} \rightarrow E_0^{p,q}}
E_1 = \bigoplus_{p,q\in\bold{Z}} E_1^{p,q} = \bigoplus_{p,q\in\bold{Z}} \frac{\bar{Z}_1^{p,q}}{\bar{B}_1^{p,q}}

Notice that \bar{Z}_1^{p,q} and \bar{B}_1^{p,q} can be written as the images in E_0^{p,q} of

Z_1^{p,q} = \ker d_0^{p,q} : F^p C^{p+q} \rightarrow C^{p+q+1}/F^{p+1} C^{p+q+1}
B_1^{p,q} = (\mbox{im } d_0^{p,q-1} : F^{p-1} C^{p+q-1} \rightarrow C^{p+q}) \cap F^p C^{p+q}

and that we then have

E_1^{p,q} = \frac{Z_1^{p,q}}{B_1^{p,q} + Z_0^{p+1,q-1}}.

Z_1^{p,q} is exactly the stuff which the differential pushes up one level in the filtration, and B_1^{p,q} is exactly the image of the stuff which the differential pushes up one level in the filtration. This suggests that we should choose Z_r^{p,q} to be the stuff which the differential pushes up r levels in the filtration and B_r^{p,q} to be image of the stuff which the differential pushes up r levels in the filtration. In other words, the spectral sequence should satisfy

Z_r^{p,q} = \ker d_0^{p,q} : F^p C^{p+q} \rightarrow C^{p+q+1}/F^{p+r} C^{p+q+1}
B_r^{p,q} = (\mbox{im } d_0^{p,q-r} : F^{p-r} C^{p+q-1} \rightarrow C^{p+q}) \cap F^p C^{p+q}
E_r^{p,q} = \frac{Z_r^{p,q}}{B_r^{p,q} + Z_r^{p+1,q-1}}

and we should have the relationship

B_r^{p,q} = d_0^{p,q}(Z_r^{p-r,q+r-1}).

For this to make sense, we must find a differential on each Er which gives the Er+1 we stated above. Since we have written Z_r^{p,q} as a subobject of Cp + q, we get a differential on E_r^{p,q} by restricting the differential on Cp + q:

d_r^{p,q} : E_r^{p,q} \rightarrow E_r^{p+r,q-r+1}

Now it is straightforward to check that the homology of Er with respect to this differential is Er+1, so this gives a spectral sequence. Unfortunately, the differential is not very explicit. Determining differentials or finding ways to work around them is one of the main challenges to successfully applying a spectral sequence.

The spectral sequence of a double complex

Another common spectral sequence is the spectral sequence of a double complex. A double complex is a collection of objects Ci,j for all integers i and j together with two differentials, d I and d II. d I is assumed to decrease i, and d II is assumed to decrease j. Furthermore, we assume that the differentials anticommute, so that d I d II + d II d I = 0. Our goal is to compare the iterated homologies H^I_i(H^{II}_j(C_{\bull,\bull})) and H^{II}_j(H^I_i(C_{\bull,\bull})). We will do this by filtering our double complex in two different ways. Here are our filtrations:

(C_{i,j}^I)_p = \left\{\begin{matrix} 0 & \mbox{if } i < p \\ C_{i,j} & \mbox{if } i \ge p \end{matrix}\right.
(C_{i,j}^{II})_p = \left\{\begin{matrix} 0 & \mbox{if } j < p \\ C_{i,j} & \mbox{if } j \ge p \end{matrix}\right.

To get a spectral sequence, we will reduce to the previous example. We define the total complex T(C•,•) to be the complex whose n'th term is \bigoplus_{i+j=n} C_{i,j} and whose differential is d I + d II. This is a complex because d I and d II are anticommuting differentials. The two filtrations on Ci,j give two filtrations on the total complex:

T_n(C_{\bull,\bull})^I_p = \bigoplus_{i+j=n \atop i < p} C_{i,j}
T_n(C_{\bull,\bull})^{II}_p = \bigoplus_{i+j=n \atop j < p} C_{i,j}

To show that these spectral sequences give information about the iterated homologies, we will work out the E0, E1, and E2 terms of the I filtration on T(C•,•). The E0 term is clear:

{}^IE^0_{p,q} = T_n(C_{\bull,\bull})^I_p / T_n(C_{\bull,\bull})^I_{p+1} = \bigoplus_{i+j=n \atop i < p} C_{i,j} \Big/ \bigoplus_{i+j=n \atop i < p+1} C_{i,j} = C_{p,q}

To find the E1 term, we need to determine d I + d II on E0. Notice that the differential must have degree 1 with respect to n, so we get a map

d^I_{p,q} + d^{II}_{p,q} : T_n(C_{\bull,\bull})^I_p / T_n(C_{\bull,\bull})^I_{p+1} = C_{p,q} \rightarrow T_{n-1}(C_{\bull,\bull})^I_p / T_{n-1}(C_{\bull,\bull})^I_{p+1} = C_{p,q-1}

Consequently, the differential on E0 is the map Cp,q → Cp,q-1 induced by d I + d II. But d I has the wrong degree to induce such a map, so d I must be zero on E0. That means the differential is exactly d II, so we get

{}^IE^1_{p,q} = H^{II}_q(C_{p,\bull}).

To find E2, we need to determine

d^I_{p,q} + d^{II}_{p,q} : H^{II}_q(C_{p,\bull}) \rightarrow H^{II}_q(C_{p+1,\bull})

Because E1 was exactly the homology with respect to d II, d II is zero on E1. Consequently, we get

{}^IE^2_{p,q} = H^I_p(H^{II}_q(C_{\bull,\bull})).

Using the other filtration gives us a different spectral sequence with a similar E2 term:

{}^{II}E^2_{p,q} = H^{II}_q(H^{I}_p(C_{\bull,\bull})).

What remains is to find a relationship between these two spectral sequences. It will turn out that as r increases, the two sequences will become similar enough to allow useful comparisons.

Convergence, degeneration, and abutment

In the elementary example that we began with, the sheets of the spectral sequence were constant once r was at least 1. In that setup it makes sense to take the limit of the sequence of sheets: Since nothing happens after the zeroth sheet, the limiting sheet E∞ is the same as E1.

In more general situations, limiting sheets often exist and are always interesting. They are one of the most powerful aspects of spectral sequences. We say that a spectral sequence E_r^{p,q} converges to or abuts to E_\infty^{p,q} if there is an r(p, q) such that for all r ≥ r(p, q), the differentials d_r^{p-r,q+r-1} and d_r^{p,q} are zero. This forces E_r^{p,q} to be isomorphic to E_\infty^{p,q} for large r. In symbols, we write:

E_r^{p,q} \Rightarrow_p E_\infty^{p,q}

The p indicates the filtration index. It is very common to write the E_2^{p,q} term on the left-hand side of the abutment, because this is the most useful term of most spectral sequences.

In most spectral sequences, the E_\infty term is not naturally a doubly graded object. Instead, there are usually E_\infty^n terms which come with a natural filtration F^\bullet E_\infty^n. In these cases, we set E_\infty^{p,q} = \mbox{gr}_p E_\infty^{p+q} = F^pE_\infty^{p+q}/F^{p+1}E_\infty^{p+q}. We define convergence in the same way as before, but we write

E_r^{p,q} \Rightarrow_p E_\infty^n

to mean that whenever p + q = n, E_r^{p,q} converges to E_\infty^{p,q}.

The simplest situation in which we can determine convergence is when the spectral sequences degenerates. We say that the spectral sequences degenerates at sheet r if, for any s ≥ r, the differential ds is zero. This implies that Er ≅ Er+1 ≅ Er+2 ≅ . . . In particular, it implies that Er is isomorphic to E∞. This is what happened in our first, trivial example of an unfiltered chain complex: The spectral sequence degenerated at the first sheet. In general, if a doubly-graded spectral sequence is zero outside of a horizontal or vertical strip, the spectral sequence will degenerate, because later differentials will always go to or from an object not in the strip.

The spectral sequence also converges if E_r^{p,q} vanishes for all p less than some p0 and for all q less than some q0. If p0 and q0 can be chosen to be zero, this is called a first-quadrant spectral sequence. This sequence converges because each object is a fixed distance away from the edge of the non-zero region. Consequently, for a fixed p and q, the differential on later sheets always maps E_r^{p,q} from or to the zero object; more visually, the differential leaves the quadrant where the terms are nonzero. The spectral sequence need not degenerate, however, because the differential maps might not all be zero at once. Similarly, the spectral sequence also converges if E_r^{p,q} vanishes for all p greater than some p0 and for all q greater than some q0.

The five-term exact sequence of a spectral sequence relates certain low-degree terms and E∞ terms. The five-term exact sequence or exact sequence of low-degree terms is a sequence of terms related to the first step of a Spectral sequence.

Examples of degeneration

The spectral sequence of a filtered complex, continued

Notice that we have a chain of inclusions:

Z_0^{p,q} \supe Z_1^{p,q} \supe Z_2^{p,q}\supe\cdots\supe B_2^{p,q} \supe B_1^{p,q} \supe B_0^{p,q}

We can ask what happens if we define

Z_\infty^{p,q} = \bigcap_{r=0}^\infty Z_r^{p,q},
B_\infty^{p,q} = \bigcup_{r=0}^\infty B_r^{p,q},
E_\infty^{p,q} = Z_\infty^{p,q}/B_\infty^{p,q}.

E_\infty^{p,q} is a natural candidate for the abutment of this spectral sequence. Convergence is not automatic, but happens in many cases. In particular, if the filtration is finite and consists of exactly r nontrivial steps, then the spectral sequence degenerates after the r'th sheet. Convergence also occurs if the complex and the filtration are both bounded below or both bounded above.

To describe the abutment of our spectral sequence in more detail, notice that we have the formulas:

Z_\infty^{p,q} = \bigcap_{r=0}^\infty Z_r^{p,q} = \bigcap_{r=0}^\infty \ker(F^p C^{p+q} \rightarrow C^{p+q+1}/F^{p+r} C^{p+q+1})
B_\infty^{p,q} = \bigcup_{r=0}^\infty B_r^{p,q} = \bigcap_{r=0}^\infty (\mbox{im } d^{p,q-r} : F^{p-r} C^{p+q-1} \rightarrow C^{p+q}) \cap F^p C^{p+q}

To see what this implies for Z_\infty^{p,q} recall that we assumed that the filtration was separated. This implies that as r increases, the kernels shrink, until we are left with Z_\infty^{p,q} = \ker(F^p C^{p+q} \rightarrow C^{p+q+1}). For B_\infty^{p,q}, recall that we assumed that the filtration was exhaustive. This implies that as r increases, the images grow until we reach B_\infty^{p,q} = \mbox{im }(C^{p+q-1} \rightarrow C^{p+q}) \cap F^p C^{p+q}. We conclude

E_\infty^{p,q} = \mbox{gr}_p H^{p+q}(C^\bull),

that is, the abutment of the spectral sequence is the p'th graded part of the p+q'th homology of C. If our spectral sequence converges, then we conclude that:

E_r^{p,q} \Rightarrow_p H^{p+q}(C^\bull)

Long exact sequences

Using the spectral sequence of a filtered complex, we can derive the existence of long exact sequences. In Mathematics, especially in Homological algebra and other applications of Abelian category theory as well as in Differential geometry and Group Choose a short exact sequence of cochain complexes 0 → A• → B• → C• → 0, and call the first map f• : A• → B•. We get natural maps of homology objects Hn(A•) → Hn(B•) → Hn(C•), and we know that this is exact in the middle. We will use the spectral sequence of a filtered complex to find the connecting homomorphism and to prove that the resulting sequence is exact. To start, we filter B•:

F0Bn = Bn
F1Bn = An
F2Bn = 0

This gives:

E^{p,q}_0 = \frac{F^p B^{p+q}}{F^{p+1} B^{p+q}} = \left\{\begin{matrix} 0 & \mbox{if } p < 0 \mbox{ or } p > 1 \\ C^q & \mbox{if } p = 0 \\ A^{q+1} & \mbox{if } p = 1 \end{matrix}\right.
E^{p,q}_1 = \left\{\begin{matrix} 0 & \mbox{if } p < 0 \mbox{ or } p > 1 \\ H^q(C^\bull) & \mbox{if } p = 0 \\ H^{q+1}(A^\bull) & \mbox{if } p = 1 \end{matrix}\right.

The differential has bidegree (1, 0), so d0,q : Hq(C•) → Hq+1(A•). These are the connecting homomorphisms from the snake lemma, and together with the maps A• → B• → C•, they give a sequence:

\cdots\rightarrow H^q(B^\bull) \rightarrow H^q(C^\bull) \rightarrow H^{q+1}(A^\bull) \rightarrow H^{q+1}(B^\bull) \rightarrow\cdots

It remains to show that this sequence is exact at the A and C spots. In Mathematics, particularly Homological algebra, the snake lemma, a statement valid in every Abelian category, is the crucial tool used to construct the Notice that this spectral sequence degenerates at the E2 term because the differentials have bidegree (2, −1). Consequently, the E2 term is the same as the E∞ term:

E^{p,q}_2 \cong \mbox{gr}_p H^{p+q}(B^\bull) = \left\{\begin{matrix} 0 & \mbox{if } p < 0 \mbox{ or } p > 1 \\ H^q(B^\bull)/H^q(A^\bull) & \mbox{if } p = 0 \\ \mbox{im } H^{q+1}f^\bull : H^{q+1}(A^\bull) \rightarrow H^{q+1}(B^\bull) &\mbox{if } p = 1 \end{matrix}\right.

But we also have a direct description of the E2 term as the homology of the E1 term. These two descriptions must be isomorphic:

H^q(B^\bull)/H^q(A^\bull) \cong \ker d^1_{0,q} : H^q(C^\bull) \rightarrow H^{q+1}(A^\bull)
\mbox{im } H^{q+1}f^\bull : H^{q+1}(A^\bull) \rightarrow H^{q+1}(B^\bull) \cong H^{q+1}(A^\bull) / (\mbox{im } d^1_{0,q} : H^q(C^\bull) \rightarrow H^{q+1}(A^\bull))

The former gives exactness at the C spot, and the latter gives exactness at the A spot.

The spectral sequence of a double complex, continued

Using the abutment for a filtered complex, we find that:

H^I_p(H^{II}_q(C_{\bull,\bull})) \Rightarrow_p H^{p+q}(T(C_{\bull,\bull}))
H^{II}_q(H^I_p(C_{\bull,\bull})) \Rightarrow_q H^{p+q}(T(C_{\bull,\bull}))

In general, the two gradings on Hp+q(T(C•,•)) are distinct. Despite this, it is still possible to gain useful information from these two spectral sequences.

Commutativity of Tor

Let M and N be R-modules. Recall that the derived functors of the tensor product are denoted Tor. In higher Mathematics, the Tor functors of Homological algebra are the Derived functors of the Tensor product functor Tor is defined using a projective resolution of its first argument. However, it turns out that Tori(M, N) = Tori(N, M). While this can be verified without a spectral sequence, it is very easy with spectral sequences.

Choose projective resolutions P• and Q• of M and N, respectively. Consider these as complexes which vanish in negative degree having differentials d and e, respectively. We can construct a double complex whose terms are Ci,j = Pi ⊗ Qj and whose differentials are d ⊗ 1 and (−1)j(1 ⊗ e). (The factor of −1 is so that the differentials anticommute. ) Since projective modules are flat, taking the tensor product with a projective module commutes with taking homology, so we get:

H^I_p(H^{II}_q(P_\bull \otimes Q_\bull)) = H^I_p(P_\bull \otimes H^{II}_q(Q_\bull))
H^{II}_q(H^I_p(P_\bull \otimes Q_\bull)) = H^{II}_q(Q_\bull \otimes H^I_p(P_\bull))

Since the two complexes are resolutions, their homology vanishes outside of degree zero. In degree zero, we are left with

H^I_p(P_\bull \otimes N) = \mbox{Tor}_p(M,N)
H^{II}_q(Q_\bull \otimes M) = \mbox{Tor}_q(N,M)

In particular, the E^2_{p,q} terms vanish except along the lines p = 0 (for one spectral sequence) and q = 0 (for the other). This implies that the spectral sequence degenerates at the second sheet. We get isomorphisms:

\mbox{Tor}_p(M,N) \cong E^\infty_{p,q} = \mbox{gr}_p H^{p+q}(T(C_{\bull,\bull}))
\mbox{Tor}_q(N,M) \cong E^\infty_{p,q} = \mbox{gr}_q H^{p+q}(T(C_{\bull,\bull}))

Finally, when p and q are equal, we get isomorphisms of the two right-hand sides, even after accounting for their different gradings, and the commutativity of Tor follows.

Further examples

Some notable spectral sequences are:

References

Historical references

Modern references

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