Semiorthogonal decomposition


In mathematics, a semiorthogonal decomposition is a way to divide a triangulated category into simpler pieces. One way to produce a semiorthogonal decomposition is from an exceptional collection, a special sequence of objects in a triangulated category. For an algebraic variety X, it has been fruitful to study semiorthogonal decompositions of the bounded derived category of coherent sheaves,.

Semiorthogonal decomposition

Alexei Bondal and Mikhail Kapranov defined a semiorthogonal decomposition of a triangulated category to be a sequence of strictly full triangulated subcategories such that:
The notation is used for a semiorthogonal decomposition.
Having a semiorthogonal decomposition implies that every object of has a canonical "filtration" whose graded pieces are in the subcategories. That is, for each object T of, there is a sequence
of morphisms in such that the cone of is in, for each i. Moreover, this sequence is unique up to a unique isomorphism.
One can also consider "orthogonal" decompositions of a triangulated category, by requiring that there are no morphisms from to for any. However, that property is too strong for most purposes. For example, for an smooth projective variety X over a field, the bounded derived category of coherent sheaves never has a nontrivial orthogonal decomposition, whereas it may have a semiorthogonal decomposition, by the examples below.
A semiorthogonal decomposition of a triangulated category may be considered as analogous to a finite filtration of an abelian group. Alternatively, one may consider a semiorthogonal decomposition as closer to a split exact sequence, because the exact sequence of triangulated categories is split by the subcategory, mapping isomorphically to.
Using that observation, a semiorthogonal decomposition implies a direct sum splitting of Grothendieck groups:
For example, when is the bounded derived category of coherent sheaves on a smooth projective variety X, can be identified with the Grothendieck group of algebraic vector bundles on X. In this geometric situation, using that comes from a dg-category, a semiorthogonal decomposition actually gives a splitting of all the algebraic K-groups of X:
for all i.

Admissible subcategory

One way to produce a semiorthogonal decomposition is from an admissible subcategory. By definition, a full triangulated subcategory is left admissible if the inclusion functor has a left adjoint functor, written. Likewise, is right admissible if the inclusion has a right adjoint, written, and it is admissible if it is both left and right admissible.
A right admissible subcategory determines a semiorthogonal decomposition
where
is the right orthogonal of in. Conversely, every semiorthogonal decomposition arises in this way, in the sense that is right admissible and. Likewise, for any semiorthogonal decomposition, the subcategory is left admissible, and, where
is the left orthogonal of.
If is the bounded derived category of a smooth projective variety over a field k, then every left or right admissible subcategory of is in fact admissible. By results of Bondal and Michel Van den Bergh, this holds more generally for any regular proper triangulated category that is idempotent-complete.
Moreover, for a regular proper idempotent-complete triangulated category, a full triangulated subcategory is admissible if and only if it is regular and idempotent-complete. These properties are intrinsic to the subcategory. For example, for X a smooth projective variety and Y a subvariety not equal to X, the subcategory of of objects supported on Y is not admissible.

Exceptional collection

Let k be a field, and let be a k-linear triangulated category. An object E of is called exceptional if Hom = k and Hom = 0 for all nonzero integers t, where is the shift functor in.
The triangulated subcategory generated by an exceptional object E is equivalent to the derived category of finite-dimensional k-vector spaces, the simplest triangulated category in this context.
Alexei Gorodentsev and Alexei Rudakov defined an exceptional collection to be a sequence of exceptional objects such that for all i < j and all integers t. In a proper triangulated category over k, such as the bounded derived category of coherent sheaves on a smooth projective variety, every exceptional collection generates an admissible subcategory, and so it determines a semiorthogonal decomposition:
where, and denotes the full triangulated subcategory generated by the object. An exceptional collection is called full if the subcategory is zero.
In particular, if X is a smooth projective variety such that has a full exceptional collection, then the Grothendieck group of algebraic vector bundles on X is the free abelian group on the classes of these objects:
A smooth complex projective variety X with a full exceptional collection must have trivial Hodge theory, in the sense that for all ; moreover, the cycle class map must be an isomorphism.

Examples

The original example of a full exceptional collection was discovered by Alexander Beilinson : the derived category of projective space over a field has the full exceptional collection
where O for integers j are the line bundles on projective space. Full exceptional collections have also been constructed on all smooth projective toric varieties, del Pezzo surfaces, many projective homogeneous varieties, and some other Fano varieties.
More generally, if X is a smooth projective variety of positive dimension such that the coherent sheaf cohomology groups are zero for i > 0, then the object in is exceptional, and so it induces a nontrivial semiorthogonal decomposition. This applies to every Fano variety over a field of characteristic zero, for example. It also applies to some other varieties, such as Enriques surfaces and some surfaces of general type.
On the other hand, many naturally occurring triangulated categories are "indecomposable". In particular, for a smooth projective variety X whose canonical bundle is basepoint-free, every semiorthogonal decomposition is trivial in the sense that or must be zero. For example, this applies to every variety which is Calabi–Yau in the sense that its canonical bundle is trivial.