Singularity categories via the derived quotient

Abstract: Given a noncommutative partial resolution $A=\mathrm{End}R(R\oplus M)$ of a Gorenstein singularity $R$, we show that the relative singularity category $\Delta_R(A)$ of Kalck-Yang is controlled by a certain connective dga $A/^{\mathbb{L}}\kern -2pt AeA$, the derived quotient of Braun-Chuang-Lazarev. We think of $A/^{\mathbb{L}}\kern -2pt AeA$ as a kind of `derived exceptional locus' of the partial resolution $A$, as we show that it can be thought of as the universal dga fitting into a suitable recollement. This theoretical result has geometric consequences. When $R$ is an isolated hypersurface singularity, it follows that the singularity category $D\mathrm{sg}(R)$ is determined completely by $A/^{\mathbb{L}}\kern -2pt AeA$, even when $A$ has infinite global dimension. Thus our derived contraction algebra classifies threefold flops, even those $X \to \mathrm{Spec} (R)$ where $X$ has only terminal singularities. This gives a solution to the strongest form of the derived Donovan-Wemyss conjecture, which we further show is the best possible classification result in this singular setting.