Closed Subcategories of Quotient Categories

We study the spectrum of closed subcategories in a quasi-scheme, i.e. a Grothendieck category X . The closed subcategories are the direct analogs of closed subschemes in the commutative case, in the sense that when X is the category of quasi-coherent sheaves on a quasi-projective scheme S , then the closed subschemes of S correspond bijectively to the closed subcategories of X . Many interesting quasi-schemes, such as the noncommutative projective scheme Qgr--2.0ptB = Gr--2.0ptB/Tors--2.0ptB Qgr- B = Gr- B / Tors- B associated to a graded algebra B , arise as quotient categories of simpler abelian categories. In this paper, we will show how to describe the closed subcategories of any quotient category X / Y in terms of closed subcategories of X with special properties, when X is a category with a set of compact projective generators.