Category of modules

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Short description: Category in mathematics

In algebra, given a ring R, the category of left modules over R is the category whose objects are all left modules over R and whose morphisms are all module homomorphisms between left R-modules. For example, when R is the ring of integers Z, it is the same thing as the category of abelian groups. The category of right modules is defined in a similar way.

Note: Some authors use the term module category for the category of modules. This term can be ambiguous since it could also refer to a category with a monoidal-category action.[1]


The categories of left and right modules are abelian categories. These categories have enough projectives[2] and enough injectives.[3] Mitchell's embedding theorem states every abelian category arises as a full subcategory of the category of modules.

Projective limits and inductive limits exist in the categories of left and right modules.[4]

Over a commutative ring, together with the tensor product of modules ⊗, the category of modules is a symmetric monoidal category.

Category of vector spaces

The category K-Vect (some authors use VectK) has all vector spaces over a field K as objects, and K-linear maps as morphisms. Since vector spaces over K (as a field) are the same thing as modules over the ring K, K-Vect is a special case of R-Mod (some authors use ModR), the category of left R-modules.

Much of linear algebra concerns the description of K-Vect. For example, the dimension theorem for vector spaces says that the isomorphism classes in K-Vect correspond exactly to the cardinal numbers, and that K-Vect is equivalent to the subcategory of K-Vect which has as its objects the vector spaces Kn, where n is any cardinal number.


The category of sheaves of modules over a ringed space also has enough injectives (though not always enough projectives).

See also


  1. "module category in nLab". 
  2. trivially since any module is a quotient of a free module.
  3. Dummit & Foote, Ch. 10, Theorem 38.
  4. Bourbaki, § 6.


  • Bourbaki. "Algèbre linéaire". Algèbre. 
  • Dummit, David; Foote, Richard. Abstract Algebra. 
  • Mac Lane, Saunders (September 1998). Categories for the Working Mathematician. Graduate Texts in Mathematics. 5 (second ed.). Springer. ISBN 0-387-98403-8. 

External links