Quotient of an abelian category explained

l{A}

by a Serre subcategory

l{B}

is the abelian category

l{A}/l{B}

which, intuitively, is obtained from

l{A}

by ignoring (i.e. treating as zero) all objects from

l{B}

. There is a canonical exact functor

Q\colonl{A}\tol{A}/l{B}

whose kernel is

lB

, and

l{A}/l{B}

is in a certain sense the most general abelian category with this property.

Forming Serre quotients of abelian categories is thus formally akin to forming quotients of groups. Serre quotients are somewhat similar to quotient categories, the difference being that with Serre quotients all involved categories are abelian and all functors are exact. Serre quotients also often have the character of localizations of categories, especially if the Serre subcategory is localizing.

Definition

Formally,

lA/lB

is the category whose objects are those of

lA

and whose morphisms from X to Y are given by the direct limit (of abelian groups)

\mathrm_(X,Y):=\varinjlim \mathrm_\mathcal A(X', Y/Y')

where the limit is taken over subobjects

X'\subseteqX

and

Y'\subseteqY

such that

X/X'\in\cal{B}

and

Y'\in\cal{B}

. (Here,

X/X'

and

Y/Y'

denote quotient objects computed in

lA

.) These pairs of subobjects are ordered by

(X',Y')\preccurlyeq(X'',Y'')\LongleftrightarrowX''\subseteqX'andY'\subseteqY''

.

Composition of morphisms in

l{A}/l{B}

is induced by the universal property of the direct limit.

The canonical functor

Q\colonlA\tolA/lB

sends an object X to itself and a morphism

f\colonX\toY

to the corresponding element of the direct limit with X′ = X and Y′ = 0.

An alternative, equivalent construction of the quotient category uses what is called a "calculus of fractions" to define the morphisms of

lA/lB

. Here, one starts with the class

S

of those morphisms in

lA

whose kernel and cokernel both belong to

l{B}

. This is a multiplicative system in the sense of Gabriel-Zisman, and one can localize the category

lA

at the system

S

to obtain

lA/lB:=lA[S-1]

.[1]

Examples

Let

k

be a field and consider the abelian category

{\rmMod}(k)

of all vector spaces over

k

. Then the full subcategory

{\rmmod}(k)

of finite-dimensional vector spaces is a Serre-subcategory of

{\rmMod}(k)

. The Serre quotient

\cal{C}={\rmMod}(k)/{\rmmod}(k)

has as objects the

k

-vector spaces, and the set of morphisms from

X

to

Y

in

\cal{C}

is \/\ (which is a quotient of vector spaces). This has the effect of identifying all finite-dimensional vector spaces with 0, and of identifying two linear maps whenever their difference has finite-dimensional image. This example shows that the Serre quotient can behave like a quotient category.

For another example, take the abelian category Ab of all abelian groups and the Serre subcategory of all torsion abelian groups. The Serre quotient here is equivalent to the category

\operatorname{Mod}(\Bbb{Q})

of all vector spaces over the rationals, with the canonical functor

Ab\to\operatorname{Mod}(\Bbb{Q})

given by tensoring with

\Bbb{Q}

. Similarly, the Serre quotient of the category of finitely generated abelian groups by the subcategory of finitely generated torsion groups is equivalent to the category of finite-dimensional vectorspaces over

\Bbb{Q}

.[2] Here, the Serre quotient behaves like a localization.

Properties

The Serre quotient

lA/lB

is an abelian category, and the canonical functor

Q\colonl{A}\tol{A}/l{B}

is exact and surjective on objects. The kernel of

Q

is

lB

, i.e.,

Q(X)

is zero in

l{A}/l{B}

if and only if

X

belongs to

l{B}

.

The Serre quotient and canonical functor are characterized by the following universal property: if

lC

is any abelian category and

F\colonlA\tolC

is an exact functor such that

F(X)

is a zero in

lC

for each object

X\inlB

, then there is a unique exact functor

\overline{F}\colonlA/lB\tolC

such that

F=\overline{F}\circQ

.[3]

Given three abelian categories

l{A}

,

l{B}

,

l{C}

, we have

lA/lB\conglC

if and only if

there exists an exact and essentially surjective functor

F\colonlA\tolC

whose kernel is

l{B}

and such that for every morphism

f:FX\toFY

in

lC

there exist morphisms

\phi:W\toX

and

\psi:W\toY

in

lA

so that

F\phi

is an isomorphism and

f=(F\psi)\circ(F\phi)-1

.

Theorems involving Serre quotients

Serre's description of coherent sheaves on a projective scheme

According to a theorem by Jean-Pierre Serre, the category

\operatorname{coh}(X)

of coherent sheaves on a projective scheme

X=\operatorname{Proj}(R)

(where

R

is a commutative noetherian graded ring, graded by the non-negative integers and generated by degree-0 and finitely many degree-1 elements, and

\operatorname{Proj}(R)

refers to the Proj construction) can be described as the Serre quotient

\operatorname(X) \cong \operatorname^\Bbb(R)\ / \ \operatorname_^\Bbb(R)

where

\operatorname{mod}\Bbb{Z}(R)

denotes the category of finitely-generated graded modules over

R

and
\Bbb{Z}(R)
\operatorname{mod}
tor
is the Serre subcategory consisting of all those graded modules

M

which are 0 in all degrees that are high enough, i.e. for which there exists

n0\in\Bbb{N}

such that

Mn=0

for all

n\geqn0

.[4] [5]

A similar description exists for the category of quasi-coherent sheaves on

X=\operatorname{Proj}(R)

, even if

R

is not noetherian.

Gabriel–Popescu theorem

l{A}

is equivalent to a Serre quotient of the form

\operatorname{Mod}(R)/\cal{B}

, where

\operatorname{Mod}(R)

denotes the abelian category of right modules over some unital ring

R

, and

\cal{B}

is some localizing subcategory of

\operatorname{Mod}(R)

.[6]

Quillen's localization theorem

l{C}

a sequence of abelian groups

Kn(lC),n\geq0

, and this assignment is functorial in

l{C}

. Quillen proved that, if

l{B}

is a Serre subcategory of the abelian category

l{A}

, there is a long exact sequence of the form[7]

\cdots \to K_n(\mathcal B) \to K_n(\mathcal A) \to K_n(\mathcal)\to K_(\mathcal B)\to\cdots\to K_0(\mathcal)\to 0

Notes and References

  1. https://stacks.math.columbia.edu/tag/02MN Section 12.10
  2. Web site: 109.76 The category of modules modulo torsion modules . The Stacks Project.
  3. Gabriel, Pierre, Des categories abeliennes, Bull. Soc. Math. France 90 (1962), 323-448.
  4. Book: Görtz . Ulrich . Algebraic Geometry I: Schemes: With Examples and Exercises . Wedhorn . Torsten . Springer Nature . 2020 . 9783658307332. 2nd . 381 . en . Remark 13.21.
  5. Web site: Proposition 30.14.4 . The Stacks Project.
  6. N. Popesco . P. Gabriel . 1964. Caractérisation des catégories abéliennes avec générateurs et limites inductives exactes . Comptes Rendus de l'Académie des Sciences . 258 . 4188–4190.
  7. Quillen . Daniel . 1973 . Higher algebraic K-theory: I . Higher K-Theories . Lecture Notes in Mathematics . 341 . en . Springer . 85–147 . 10.1007/BFb0067053 . 978-3-540-06434-3.