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| 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.
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| ==Cohomology spectral sequence==
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| Let ''f'' : ''X'' → ''B'' be a Serre fibration of topological spaces, and let ''F'' be the [[Fiber (mathematics)|fiber]]. The Serre cohomology spectral sequence is the following:
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| :<math> E_2^{p,q} = H^p(B, H^q(F)) \Rightarrow H^{p+q}(X).</math>
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| Here, at least under standard simplifying conditions, the coefficient group in the ''E''<sub>2</sub>-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.
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| 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 equivalence|homotopy equivalent]]. In particular, their cohomology is isomorphic, so the choice of "the" fiber does not give any ambiguity.
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| The [[Spectral_sequence#Convergence.2C_degeneration.2C_and_abutment|abutment]] means integral cohomology of the total space ''X''.
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| This spectral sequence can be derived from an [[Spectral_sequence#Exact_couples|exact couple]] built out of the [[Exact_sequence|long exact sequences]] of the cohomology of the pair (''X<sub>p</sub>'', ''X''<sub>''p''−1</sub>), where ''X<sub>p</sub>'' is the restriction of the fibration over the ''p''-skeleton of ''B''. More precisely, using [[Spectral_sequence#Exact_couples|this notation]],
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| : <math>A = \bigoplus_{p,q} H^q(X_p)</math>, {{pad|2em}} <math>E_1^{p,q} = C = \bigoplus_{p,q} H^q(X_p, X_{p-1}),</math>
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| ''f'' is defined by restricting each piece on ''X<sub>p</sub>'' to ''X''<sub>''p''−1</sub>, ''g'' is defined using the coboundary map in the [[Eilenberg–Steenrod_axioms#Formal_definition|long exact sequence of the pair]], and ''h'' is defined by restricting (''X<sub>p</sub>'', ''X''<sub>''p''−1</sub>) to ''X<sub>p</sub>''.
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| There is a multiplicative structure
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| :<math>E_r^{p,q} \times E_r^{s,t} \to E_r^{p+s,q+t},</math>
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| coinciding on the ''E''<sub>2</sub>-term with (−1)<sup>''qs''</sup> times the cup product, and with respect to which the differentials ''d''<sub>r</sub> are [[Derivation (abstract algebra)#Graded derivations|(graded) derivation]]s inducing the product on the ''E''<sub>''r''+1</sub>-page from the one on the ''E<sub>r</sub>''-page.
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| == Homology spectral sequence ==
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| Similarly to the cohomology spectral sequence, there is one for homology:
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| :<math> E^2_{p,q} = H_p(B, H_q(F)) \Rightarrow H_{p+q}(X).</math>
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| where the notations are dual to the ones above.
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| It is actually a special case of a more general spectral sequence, namely the Serre spectral sequence for fibrations of [[simplicial set]]s. If ''f'' is a fibration of simplicial sets (a [[Kan fibration]]), such that π<sub>1</sub>(''B'') the first homotopy group of the simplicial set ''B'', vanishes, there is a spectral sequence exactly as above. (Applying the functor which associates to any [[topological space]] its simplices to a fibration of topological spaces, one recovers the above sequence).
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| == Example Computations ==
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| === A Basic Pathspace Fibration ===
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| We begin first with a basic example; consider the path space fibration
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| :<math> \Omega \mathbf{S}^{n+1}\to \mathbf{PS}^{n+1}\to \mathbf{S}^{n+1}. </math>
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| 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''<sup>∞</sup> page (the homology of the total space) to control what can happen on the ''E''<sup>2</sup> page. So recall that
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| :<math> E^2_{p,q} = H_p(\mathbf{S}^{n+1}; H_q(\Omega \mathbf{S}^{n+1})). </math>
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| Thus we know when ''q'' = 0, we are just looking at the regular integer valued homology groups ''H<sub>p</sub>''('''S'''<sup>''n''+1</sup>) which has value '''Z''' 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''<sup>∞</sup>, everything becomes 0 except for the group at ''p'' = ''q'' = 0. The only way this can happen is if there is an isomorphism from ''H''<sub>''n''+1</sub>('''S'''<sup>''n''+1</sup>; ''H''<sub>0</sub>(''F'')) = '''Z''' 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''<sup>''n''+1</sup> with codomain ''H''<sub>0</sub>('''S'''<sup>''n''+1</sup>; ''H<sub>n</sub>''(''F'')) = '''Z''' However, putting a '''Z''' in this group means there must be a '''Z''' at ''H''<sub>''n''+1</sub>('''S'''<sup>''n''+1</sup>; ''H<sub>n</sub>''(''F'')). Inductively repeating this process shows that ''H<sub>i</sub>''(Ω'''S'''<sup>''n''+1</sup>) has value '''Z''' at integer multiples of ''n'' and 0 everywhere else.
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| === The Cohomology Ring of Complex Projective Space ===
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| We compute the cohomology of '''CP'''<sup>''n''</sup> using the fibration:
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| :<math> \mathbf{S}^1\hookrightarrow \mathbf{S}^{2n+1}\to \mathbf{CP}^n </math>
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| Now, on the ''E''<sub>2</sub> 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 '''Z'''. 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''<sup>2</sup> by the Leibniz rule telling us that the 4,0 coordinate must be ''x''<sup>2</sup> since there can be no nontrivial homology until degree 2''n''+1. Repeating this argument inductively until 2''n''+1 gives ''ix<sup>n</sup>'' in coordinate 2''n'',1 which must then be the only generator of '''Z''' 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 '''CP'''<sup>''n''</sup> and it tells us that the answer is '''Z'''[''x'']/''x''<sup>''n''+1</sup>.
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| In the case of infinite complex projective space, taking limits gives the answer '''Z'''[''x''].
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| === The Fourth Homotopy Group of the Three Sphere ===
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| A more sophisticated application of the Serre spectral sequence is the computation π<sub>4</sub>('''S'''<sup>3</sup>) = '''Z'''/2'''Z'''. This particular example illustrates a systematic technique which one can use in order to deduce information about the higher homotopy groups of spheres. We consider the following fibration which is an isomorphism on π<sub>3</sub>
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| :<math>X\rightarrow \mathbf{S}^3\rightarrow K(\mathbf{Z},3).\ </math>
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| where ''K''(π, ''n'') is an Eilenberg-Maclane space. We then further convert the map ''X'' → '''S'''<sup>3</sup> 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 Ω''K''('''Z''', 3) = ''K''('''Z''', 2). But we know that ''K''('''Z''', 2) = '''CP'''<sup>∞</sup>. Now we look at the cohomological Serre spectral sequence: we suppose we have a generator for the degree 3 cohomology of '''S'''<sup>3</sup> called ''i''. Since there is nothing in degree 3 in the total homology, we know this must be killed by an isomorphism. But the only thing that can map to it is the generator ''a'' of the homology of '''CP'''<sup>∞</sup> so we have ''d''(''a'') = ''i''. Therefore by the cup product structure, the generator in degree 4, ''a<sup>2</sup>'' maps to the generator ''ia'' by multiplication by 2 and that the generator of cohomology in degree 6 maps to ''ia<sup>2</sup>'' by multiplication by 3 etc. In particular we find that ''H''<sub>4</sub>(''X'') = '''Z'''/2'''Z'''. 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 ''H<sub>4</sub>(''X'') = π<sub>4</sub>(''X'') by the Hurewicz theorem telling us that ''π<sub>4</sub>('''S'''<sup>3</sup>) = '''Z'''/2'''Z'''.
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| == See also ==
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| * [[Gysin sequence]]
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| == References ==
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| The Serre spectral sequence is covered in most textbooks on algebraic topology, e.g.
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| * [[Allen Hatcher]], ''[http://www.math.cornell.edu/~hatcher/SSAT/SSATpage.html The Serre spectral sequence]''
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| * [[Edwin Spanier]], ''Algebraic topology'', Springer
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| An elegant construction is due to
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| * A. Dress, ''Zur Spektralsequenz einer Faserung'', Inventiones Mathematicae 3, p. 172-178 (1967)
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| The case of simplicial sets is treated in
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| * P. Goerss, R. Jardine, ''Simplicial homotopy theory'', Birkhäuser
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| [[Category:Algebraic topology]]
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| [[Category:Spectral sequences]]
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