Decomposition theorem

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 ${\displaystyle f:X\to Y}$ of relative dimension d between two projective varieties[1]

${\displaystyle -\cup \eta ^{i}:R^{d-i}f_{*}(\mathbb {Q} ){\stackrel {\cong }{\to }}R^{d+i}f_{*}(\mathbb {Q} ).}$

Here ${\displaystyle \eta }$ is the fundamental class of a hyperplane section, ${\displaystyle f_{*}}$ is the direct image (pushforward) and ${\displaystyle R^{n}f_{*}}$ is the n-th derived functor of the direct image. This derived functor measures the n-th cohomologies of ${\displaystyle f^{-1}(U)}$, for ${\displaystyle U\subset Y}$. In fact, the particular case when Y is a point, amounts to the isomorphism

${\displaystyle -\cup \eta ^{i}:H^{d-i}(X,\mathbb {Q} ){\stackrel {\cong }{\to }}H^{d+i}(X,\mathbb {Q} ).}$

This hard Lefschetz isomorphism induces canonical isomorphisms

${\displaystyle Rf_{*}(\mathbb {Q} ){\stackrel {\cong }{\to }}\bigoplus _{i=-d}^{d}R^{d+i}f_{*}(\mathbb {Q} )[-d-i].}$

Moreover, the sheaves ${\displaystyle R^{d+i}f_{*}\mathbb {Q} }$ 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 ${\displaystyle f:X\to Y}$ 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:

${\displaystyle {}^{p}H^{-i}(Rf_{*}\mathbb {Q} )\cong {}^{p}H^{+i}(Rf_{*}\mathbb {Q} ),}$

where ${\displaystyle Rf_{*}}$ is the total derived functor of ${\displaystyle f_{*}}$ and ${\displaystyle {}^{p}H^{i}}$ is the i-th truncation with respect to the perverse t-structure.

Moreover, there is an isomorphism

${\displaystyle Rf_{*}IC_{X}^{\bullet }\cong \bigoplus _{i}{}^{p}H^{i}(Rf_{*}IC_{X}^{\bullet })[-i].}$

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 ${\displaystyle \mathbb {Q} [\dim X]}$ is replaced by the intersection cohomology complex ${\displaystyle IC}$.

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 ${\displaystyle f:X\rightarrow \mathbb {P} ^{1}}$ from a smooth quasi-projective variety given by ${\displaystyle [f_{1}(x):f_{2}(x)]}$. If we set the vanishing locus of ${\displaystyle f_{1},f_{2}}$ as ${\displaystyle Y}$ then there is an induced morphism ${\displaystyle {\tilde {X}}=Bl_{Y}(X)\to \mathbb {P} ^{1}}$. We can compute the cohomology of ${\displaystyle X}$ from the intersection cohomology of ${\displaystyle Bl_{Y}(X)}$ and subtracting off the cohomology from the blowup along ${\displaystyle Y}$. This can be done using the perverse spectral sequence

${\displaystyle E_{2}^{l,m}=H^{l}(\mathbb {P} ^{1};{}^{\mathfrak {p}}{\mathcal {H}}^{m}(IC_{\tilde {X}}^{\bullet }(\mathbb {Q} ))\Rightarrow IH^{l+m}({\tilde {X}};\mathbb {Q} )\cong H^{l+m}(X;\mathbb {Q} )}$

References

1. Deligne, Pierre (1968), "Théoreme de Lefschetz et critères de dégénérescence de suites spectrales", Publ. Math. Inst. Hautes Études Sci., 35: 107–126, doi:10.1007/BF02698925, Zbl 0159.22501
2. Beilinson, Alexander A.; Bernstein, Joseph; Deligne, Pierre (1982). "Faisceaux pervers". Astérisque (in French). Société Mathématique de France, Paris. 100.CS1 maint: ref=harv (link)
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. arXiv:math/0306030. Bibcode:2003math......6030D. 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, arXiv:math/0204067, doi:10.4310/MRL.2004.v11.n2.a2, MR 2067464

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