Differential graded category

In mathematics, especially homological algebra, a differential graded category, often shortened to dg-category or DG category, is a category whose morphism sets are endowed with the additional structure of a differential graded formula_1-module.
In detail, this means that formula_2, the morphisms from any object "A" to another object "B" of the category is a direct sum 
and there is a differential "d" on this graded group, i.e., for each "n" there is a linear map 
which has to satisfy formula_5. This is equivalent to saying that formula_2 is a cochain complex. Furthermore, the composition of morphisms
formula_7 is required to be a map of complexes, and for all objects "A" of the category, one requires formula_8.
Further properties.
The category of small dg-categories can be endowed with a model category structure such that weak equivalences are those functors that induce an equivalence of derived categories.
Given a dg-category "C" over some ring "R", there is a notion of smoothness and properness of "C" that reduces to the usual notions of smooth and proper morphisms in case "C" is the category of quasi-coherent sheaves on some scheme "X" over "R".
Relation to triangulated categories.
A DG category "C" is called pre-triangulated if it has a suspension functor
formula_20 and a class of distinguished triangles compatible with the
suspension, such that its homotopy category Ho("C") is a triangulated category. 
A triangulated category "T" is said to have a "dg enhancement" "C" if "C"
is a pretriangulated dg category whose homotopy category is equivalent to "T". dg enhancements of an exact functor between triangulated categories are defined similarly. In general, there need not exist dg enhancements of triangulated categories or functors between them, for example stable homotopy category can be shown not to arise from a dg category in this way. However, various positive results do exist, for example the derived category "D"("A") of a Grothendieck abelian category "A" admits a unique dg enhancement.