We introduce the notion of a rank function on a triangulated category
𝒞
{\mathcal{C}}
which generalizes the Sylvester
rank function in the case when
𝒞
=
𝖯𝖾𝗋𝖿
(
A
)
{\mathcal{C}=\mathsf{Perf}(A)}
is the perfect derived category of a ring A. We show that rank functions are closely related
to functors into simple triangulated categories and classify
Verdier quotients into simple triangulated categories in terms of particular rank functions called localizing.
If
𝒞
=
𝖯𝖾𝗋𝖿
(
A
)
{\mathcal{C}=\mathsf{Perf}(A)}
as above, localizing rank functions also classify
finite homological epimorphisms from A into differential graded skew-fields or, more generally, differential graded Artinian rings. To establish these results, we develop the theory of derived localization of differential graded algebras at thick subcategories of their perfect derived categories. This is a far-reaching generalization of Cohn’s matrix localization of rings and has independent interest.