TheoremComplete

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 D(A)=K(A)/Kac(A)D(\mathcal{A}) = K(\mathcal{A})/K_{\mathrm{ac}}(\mathcal{A}) is the primary example.


Statement

Definition5.9Thick Subcategory

A full triangulated subcategory ST\mathcal{S} \subseteq \mathcal{T} is thick (or epaisse) if it is closed under direct summands: if XYSX \oplus Y \in \mathcal{S}, then X,YSX, Y \in \mathcal{S}.

Theorem5.5Verdier Localization

Let T\mathcal{T} be a triangulated category and ST\mathcal{S} \subseteq \mathcal{T} a thick subcategory. Then the Verdier quotient T/S\mathcal{T}/\mathcal{S}, defined as the localization of T\mathcal{T} at the class of morphisms ff whose cone lies in S\mathcal{S}, is a triangulated category. The localization functor Q:TT/SQ : \mathcal{T} \to \mathcal{T}/\mathcal{S} is exact (preserves distinguished triangles and commutes with the shift).

Moreover, kerQ=S\ker Q = \mathcal{S} (the objects sent to zero are exactly those in S\mathcal{S}).


Examples

ExampleDerived category

D(A)=K(A)/Kac(A)D(\mathcal{A}) = K(\mathcal{A}) / K_{\mathrm{ac}}(\mathcal{A}), where Kac(A)K_{\mathrm{ac}}(\mathcal{A}) is the thick subcategory of acyclic complexes. The quasi-isomorphisms are exactly the morphisms whose cones are acyclic.

ExampleBounded derived category

Db(A)=Kb(A)/Kacb(A)D^b(\mathcal{A}) = K^b(\mathcal{A}) / K^b_{\mathrm{ac}}(\mathcal{A}). Similarly for D+D^+ and DD^-.

ExampleSingularity category

Dsg(R)=Db(mod-R)/Perf(R)D_{\mathrm{sg}}(R) = D^b(\mathrm{mod}\text{-}R) / \mathrm{Perf}(R): the singularity category is the Verdier quotient by perfect complexes. It is trivial iff RR has finite global dimension.

ExampleLocalization sequence

For a closed subvariety ZXZ \subseteq X with open complement UU, there is a localization sequence DZb(X)Db(X)Db(U)D^b_Z(X) \to D^b(X) \to D^b(U) where DZb(X)D^b_Z(X) consists of complexes supported on ZZ. The Verdier quotient Db(X)/DZb(X)Db(U)D^b(X)/D^b_Z(X) \simeq D^b(U).

ExampleMorphisms in the quotient

Morphisms in T/S\mathcal{T}/\mathcal{S} are represented by "roofs": XsXfYX \xleftarrow{s} X' \xrightarrow{f} Y where the cone of ss lies in S\mathcal{S}. Two roofs are equivalent if they can be connected by a common refinement.

ExampleDerived category of sheaves with support

Dcb(X)/DZb(X)Dcb(U)D^b_c(X) / D^b_Z(X) \simeq D^b_c(U) for a closed subset ZZ with complement UU. This is the sheaf-theoretic localization sequence, crucial for excision arguments.

ExampleQuotient by compact objects

In a compactly generated triangulated category T\mathcal{T}, the quotient T/Tc\mathcal{T}/\mathcal{T}^c (modding out compact objects) captures the "large-scale" behavior. This perspective is used in the theory of Brown representability.

ExampleTorsion quotient

For R=ZR = \mathbb{Z}, the quotient Db(Z-mod)/Db(torsion)D^b(\mathbb{Z}\text{-}\mathrm{mod}) / D^b(\mathrm{torsion}) is equivalent to Db(Q-Vect)D^b(\mathbb{Q}\text{-}\mathrm{Vect}): modding out torsion modules gives rational vector spaces.

ExampleQuotient and Serre subcategories

For an abelian category A\mathcal{A} and a Serre subcategory S\mathcal{S}, Db(A)/Db(S)Db(A/S)D^b(\mathcal{A})/D^b(\mathcal{S}) \simeq D^b(\mathcal{A}/\mathcal{S}) under suitable hypotheses. This relates Verdier localization to Gabriel localization.

ExampleStable module category as Verdier quotient

For a self-injective algebra AA, the stable module category mod-A=mod-A/proj-A\underline{\mathrm{mod}}\text{-}A = \mathrm{mod}\text{-}A / \mathrm{proj}\text{-}A can be realized as a Verdier quotient of the bounded derived category.

ExampleLocalization and K-theory

The localization sequence STT/S\mathcal{S} \to \mathcal{T} \to \mathcal{T}/\mathcal{S} induces a long exact sequence in algebraic K-theory: Kn(S)Kn(T)Kn(T/S)Kn1(S)\cdots \to K_n(\mathcal{S}) \to K_n(\mathcal{T}) \to K_n(\mathcal{T}/\mathcal{S}) \to K_{n-1}(\mathcal{S}) \to \cdots.

ExampleUniversal property

The Verdier quotient T/S\mathcal{T}/\mathcal{S} is universal among exact functors TT\mathcal{T} \to \mathcal{T}' that send S\mathcal{S} to zero: any such functor factors uniquely through Q:TT/SQ : \mathcal{T} \to \mathcal{T}/\mathcal{S}.


RemarkVerdier vs Bousfield

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 T/S\mathcal{T}/\mathcal{S} is equivalent to the right orthogonal S\mathcal{S}^\perp when a suitable adjunction exists.