Verdier Localization
Verdier localization constructs the quotient of a triangulated category by a thick subcategory. The resulting quotient inherits a canonical triangulated structure. The derived category is the primary example.
Statement
A full triangulated subcategory is thick (or epaisse) if it is closed under direct summands: if , then .
Let be a triangulated category and a thick subcategory. Then the Verdier quotient , defined as the localization of at the class of morphisms whose cone lies in , is a triangulated category. The localization functor is exact (preserves distinguished triangles and commutes with the shift).
Moreover, (the objects sent to zero are exactly those in ).
Examples
, where is the thick subcategory of acyclic complexes. The quasi-isomorphisms are exactly the morphisms whose cones are acyclic.
. Similarly for and .
: the singularity category is the Verdier quotient by perfect complexes. It is trivial iff has finite global dimension.
For a closed subvariety with open complement , there is a localization sequence where consists of complexes supported on . The Verdier quotient .
Morphisms in are represented by "roofs": where the cone of lies in . Two roofs are equivalent if they can be connected by a common refinement.
for a closed subset with complement . This is the sheaf-theoretic localization sequence, crucial for excision arguments.
In a compactly generated triangulated category , the quotient (modding out compact objects) captures the "large-scale" behavior. This perspective is used in the theory of Brown representability.
For , the quotient is equivalent to : modding out torsion modules gives rational vector spaces.
For an abelian category and a Serre subcategory , under suitable hypotheses. This relates Verdier localization to Gabriel localization.
For a self-injective algebra , the stable module category can be realized as a Verdier quotient of the bounded derived category.
The localization sequence induces a long exact sequence in algebraic K-theory: .
The Verdier quotient is universal among exact functors that send to zero: any such functor factors uniquely through .
Verdier localization is the "quotient" approach (mod out a subcategory). Bousfield localization is the "reflection" approach (project onto a subcategory). They are related: the Verdier quotient is equivalent to the right orthogonal when a suitable adjunction exists.