Concept

Category of modules

Summary
In algebra, given a ring R, the category of left modules over R is the whose 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 . The category of right modules is defined in a similar way. One can also define the category of bimodules over a ring R but that category is equivalent to the category of left (or right) modules over the enveloping algebra of R (or over the opposite of that). Note: Some authors use the term for the category of modules. This term can be ambiguous since it could also refer to a category with a . The categories of left and right modules are . These categories have enough projectives and enough injectives. Mitchell's embedding theorem states every abelian category arises as a of the category of modules of some ring. Projective limits and inductive limits exist in the categories of left and right modules. Over a commutative ring, together with the tensor product of modules ⊗, the category of modules is a . A monoid object of the category of modules over a commutative ring R is exactly an associative algebra over R. See also: compact object (a compact object in the R-mod is exactly a finitely presented module). FinVect The 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 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).