In this paper, we first prove for two differential graded algebras (DGAs) A, B which are derived equivalent to k-algebras Λ, Γ, respectively, that D(A⊗ k B) ≃ D(Λ⊗ k Γ).Secondly, for two quasi-compact and separated schemes X, Y and two algebras A, B over k which satisfy D(Qcoh(X)) ≃ D(A) and). Finally, we prove that if X is a quasi-compact and separated scheme over k, then D(Qcoh(X × P 1 )) admits a recollement relative to D(Qcoh(X)), and we describe the functors in the recollement explicitly. This recollement induces a recollement of bounded derived categories of coherent sheaves and a recollement of singularity categories. When the scheme X is derived equivalent to a DGA or algebra, then the recollement which we get corresponds to the recollement of DGAs in [14] or the recollement of upper triangular algebras in [7].for tensor products of algebras. Precisely, given a commutative ring R, let Λ be an R-algebra with a tilting complex P * whose endomorphism algebra is isomorphic to Γ and let Λ 0 be an R-algebra with a tilting complex Q * whose endomorphism algebra is isomorphic to Γ 0 . IfBeilinson [1] and Bernstein and Gelfand's [3] used derived categories to establish a beautiful relation between coherent sheaves on projective spaces and representations of certain finite-dimensional algebras. Their constructions had numerous generalizations (see [8,15]). It need very strong conditions for a variety or scheme to be derived equivalent to an algebra. But in [5], Bondal and Van den Bergh showed that a quasi-compact and separated scheme can be derived equivalent to a differential graded algebra (abbreviated below to DGA). Furthermore, the derived categories of DGAs, or more generally, DG categories, provide a conceptual framework for tilting theory (see [16,17]). So we first consider how derived equivalences occur for tensor products of DGAs which generalizes Rickard's result. We get Q)). Subsequently, we prove the following theorem.
Proposition 1.1. Let A and B be two DG k-algebras. If P and Q are compact generators of D(A) and D(B), respectively, then P ⊗ k Q is a compact generator of D(A ⊗ k B). In particular, D(A ⊗ k B) ≃ D(HomTheorem 1.2. Let A and B be two DG k-algebras, and Λ and Γ be two k-algebras.
If D(A) ≃ D(Λ) and D(B) ≃ D(Γ), then D(A ⊗Corresponding to every scheme X, there are two very important abelian categories, Coh(X) of coherent sheaves and Qcoh(X) of quasi-coherent sheaves, and then their derived categories. Parallel to the tensor products of algebras or DGAs, we consider the products of schemes and get: Theorem 1.3. Let X, Y be two quasi-compact and separated schemes over a field k and A, B two algebras also over k.Besides derived equivalences, recollements are also important in the study of algebraic geometry and representation theory. A recollement of triangulated categories is a diagram