Decomposition theorem

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In mathematics, especially algebraic geometry the decomposition theorem is a set of results concerning the cohomology of algebraic varieties.

Statement

Decomposition for smooth proper maps

The first case of the decomposition theorem arises via the hard Lefschetz theorem which gives isomorphisms, for a smooth proper map of relative dimension d between two projective varieties[1]

Here is the fundamental class of a hyperplane section, is the direct image (pushforward) and is the n-th derived functor of the direct image. This derived functor measures the n-th cohomologies of , for . In fact, the particular case when Y is a point, amounts to the isomorphism

This hard Lefschetz isomorphism induces canonical isomorphisms

Moreover, the sheaves appearing in this decomposition are local systems, i.e., locally free sheaves of Q-vector spaces, which are moreover semisimple, i.e., a direct sum of local systems without nontrivial local subsystems.

Decomposition for proper maps

The decomposition theorem generalizes this fact to the case of a proper, but not necessarily smooth map between varieties. In a nutshell, the results above remain true when the notion of local systems is replaced by perverse sheaves.

The hard Lefschetz theorem above takes the following form: there is an isomorphism in the derived category of sheaves on Y:

where is the total derived functor of and is the i-th truncation with respect to the perverse t-structure.

Moreover, there is an isomorphism

where the summands are semi-simple perverse-sheaves, meaning they are direct sums of push-forwards of intersection cohomology sheaves.

If X is not smooth, then the above results remain true when is replaced by the intersection cohomology complex .

Proofs

The decomposition theorem was first proved by Beilinson, Bernstein, and Deligne.[2] Their proof is based on the usage of weights on l-adic sheaves in positive characteristic. A different proof using mixed Hodge modules was given by Saito. A more geometric proof, based on the notion of semismall maps was given by de Cataldo and Migliorini.[3]

For semismall maps, the decomposition theorem also applies to Chow motives.[4]

Applications of the Decomposition Theorem

Cohomology of a Rational Lefschetz Pencil

Consider a rational morphism from a smooth quasi-projective variety given by . If we set the vanishing locus of as then there is an induced morphism . We can compute the cohomology of from the intersection cohomology of and subtracting off the cohomology from the blowup along . This can be done using the perverse spectral sequence

References

  1. ^ Deligne, Pierre (1968), "Théoreme de Lefschetz et critères de dégénérescence de suites spectrales", Publ. Math., Inst. Hautes Étud. Sci., 35: 107–126, Zbl 0159.22501, doi:10.1007/BF02698925 
  2. ^ Beilinson, Alexander A.; Bernstein, Joseph; Deligne, Pierre (1982). "Faisceaux pervers". Astérisque (in French). Société Mathématique de France, Paris. 100. 
  3. ^ de Cataldo, Mark Andrea; Migliorini, Luca (2005). "The Hodge theory of algebraic maps". Annales Scientifiques de l'École Normale Supérieure. 38 (5): 693–750. doi:10.1016/j.ansens.2005.07.001. 
  4. ^ de Cataldo, Mark Andrea; Migliorini, Luca (2004), "The Chow motive of semismall resolutions", Math. Res. Lett., 11 (2-3): 151–170, MR 2067464, doi:10.4310/MRL.2004.v11.n2.a2 

Survey Articles

  • de Cataldo, Mark, Perverse sheaves and the topology of algebraic varieties Five lectures at the 2015 PCMI (PDF) 
  • de Cataldo, Mark; Milgiorini, Luca, The Decomposition Theorem, Perverse Sheaves, and the Topology of Algebraic Maps (PDF) 

Pedagogical References

  • Hotta, Ryoshi; Takeuchi, Kiyoshi; Tanisaki, Toshiyuki, D-Modules, Perverse Sheaves, and Representation Theory 
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