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In
mathematics, specifically in
category theory, an '''additive category''' is a
preadditive category C such that any finitely many objects
A1,...,
An of
C have a
biproduct A1 ⊕ ··· ⊕
An in
C.
(Recall that a category
C is preadditive if all its morphism sets are
Abelian groups and morphism composition is
bilinear, i.e. if
C is
enriched over the
monoidal category of Abelian groups; and recall that a biproduct in a preadditive category is both a
finite product and a finite
coproduct.)
Warning:
The term "additive category" is sometimes applied to ''any'' preadditive category; but Wikipedia does not follow this older practice.
== Examples ==
The original example of an additive category is the category
Ab of
Abelian groups with group homomorphisms.
Ab is preadditive because it is a
closed monoidal category, and the biproduct in
Ab is the finite
direct sum.
Other common examples:
* The category of (left)
modules over a
ring R, in particular:
** the category of
vector spaces over a
field K.
* The algebra of
matrices over a ring, thought of as a category as described below.
These will give you an idea of what to think of; for more examples, follow the links to '''Special cases''' below.
== Elementary properties ==
Every additive category is of course a
preadditive category, and many basic properties of these categories are described under that subject.
This article concerns itself with the properties that exist specifically because of the existence of biproducts.
First note that because nullary biproducts exist, every additive category has a
zero object, commonly denoted simply "0".
Given objects
A and
B in an additive category, we can use
matrices to study the biproducts of
A and
B with themselves.
Specifically, if we define the ''biproduct power''
An to be the
n-fold biproduct
A ⊕ ··· ⊕
A and
Bm similarly, then the morphisms from
An to
Bm can be described as
m-by-
n matrices whose entries are morphisms from
A to
B.
For a concrete example, consider the category of
real vector spaces, so that
A and
B are individual vector spaces.
(There is no need for
A and
B to have
finite dimensions, although of course the numbers
m and
n must be finite.)
Then an element of
An may be represented as an
n-by-
1 column vector whose entries are elements of
A:
<math>
\begin{pmatrix} a_{1} \\ a_{2} \\ \vdots \\ a_{n} \end{pmatrix}
</math>
and a morphism from
An to
Bm is an
m-by-
n matrix whose entries are morphisms from
A to
B:
<math>
\begin{pmatrix} f_{1,1} & f_{1,2} & \cdots & f_{1,n} \\
f_{2,1} & f_{2,2} & \cdots & f_{2,n} \\
\vdots & \vdots & \cdots & \vdots \\
f_{m,1} & f_{m,2} & \cdots & f_{m,n} \end{pmatrix}
</math>
Then this morphism matrix acts on the column vector by the usual rules of matrix multiplication to give an element of
Bm, represented by an
m-by-1 column vector with entries from
B:
<math>
\begin{pmatrix} f_{1,1} & f_{1,2} & \cdots & f_{1,n} \\
f_{2,1} & f_{2,2} & \cdots & f_{2,n} \\
\vdots & \vdots & \cdots & \vdots \\
f_{m,1} & f_{m,2} & \cdots & f_{m,n} \end{pmatrix}
\begin{pmatrix} a_{1} \\ a_{2} \\ \vdots \\ a_{n} \end{pmatrix}
= \begin{pmatrix} f_{1,1}(a_{1}) + f_{1,2}(a_{2}) + \cdots + f_{1,n}(a_{n}) \\
f_{2,1}(a_{1}) + f_{2,2}(a_{2}) + \cdots + f_{2,n}(a_{n}) \\
\cdots \\
f_{m,1}(a_{1}) + f_{m,2}(a_{2}) + \cdots + f_{m,n}(a_{n}) \end{pmatrix}
</math>
Even in the setting of an abstract additive category, where it makes no sense to speak of elements of the objects
An and
Bm, the matrix representation of the morphism is still useful, because
matrix multiplication correctly reproduces composition of morphisms.
Thus additive categories can be seen as the most general context in which the algebra of matrices makes sense.
Recall that the morphisms from a single object
A to itself form the
endomorphism ring End(
A).
Then morphisms from
An to
Am are
m-by-
n matrices with entries from the ring End(
A).
Conversely, given any
ring R, we can form a category
Mat(
R) by taking objects
An indexed by the set of
natural numbers (including
zero) and letting the
hom-set of morphisms from
An to
Am be the
set of
m-by-
n matrices over
R.
If we define morphism composition to be multiplication of matrices, then
Mat(
R) becomes an additive category, and
An will be the biproduct power (
A1)
n.
In this way, matrices over a ring are seen to form an additive category, just as an individual ring formed a preadditive category (which in this case is End(
A1)).
If we interpret the object
An as the left
module Rn, then this ''matrix category'' becomes a
subcategory of the category of left modules over
R.
This may be confusing in the special case where
m or
n is
zero, because we usually don't think of matrices with 0 rows or 0 columns.
However, this concept makes sense — such matrices have 0 entries are determined uniquely by their size alone — and while they are rather degenerate, they do need to be included to get an additive category, since an additive category must have a zero object 0.
Thinking about such matrices can be useful in one way, however — they highlight the fact that given any objects
A and
B in an additive category, there is exactly one morphism from 0 to
B (just as there is exactly one 1-by-0 matrix with entries in End(
B)) and exactly one morphism from
A to 0 (just as there is exactly one 0-by-1 matrix with entries in End(
A)) -- this is just what it means to say that 0 is a zero object.
Furthermore, the
zero morphism from
A to
B is the composition of these morphisms, as can be calculated by multiplying the degenerate matrices.
== Additive functors ==
Recall that a functor
F:
C →
D between preadditive categories is ''additive'' if it is an Abelian
group homomorphism on each
hom-set in
C.
But if the categories are additive, then an additive functor can also be characterised as any functor that preserves
biproduct diagrams.
That is, if
B is a biproduct of
A1,...,
An in
C with projection morphisms
pj and injection morphisms
ij, then
F(
B) should be a biproduct of
F(
A1),...,
F(
An) in
D with projection morphisms
F(
pj) and injection morphisms
F(
ij).
Almost all functors studied between additive categories are additive.
In fact, it is a theorem that all
adjoint functors between additive categories must be additive functors, and most interesting functors studied in all of category theory are adjoints.
== Special cases ==
* A ''
pre-Abelian category'' is an additive category in which every morphism has a
kernel and a
cokernel.
* An ''
Abelian category'' is a pre-Abelian category such that every
monomorphism and
epimorphism is
normal.
The additive categories most commonly studied are in fact Abelian categories; for example,
Ab is an Abelian category.
== Sources ==
*
Nicolae Popescu;
1973;
Abelian Categories with Applications to Rings and Modules; Academic Press, Inc.; out of print
** goes over all of this very slowly
