Consider an algebraic variety over a base scheme and a faithful base change . Given an admissible subcategory in the bounded derived category of coherent sheaves on , we construct an admissible subcategory in the bounded derived category of coherent sheaves on the fiber product , called the base change of , in such a way that the following base change theorem holds: if a semiorthogonal decomposition of the bounded derived category of is given then the base changes of its components form a semiorthogonal decomposition of the bounded derived category of the fiber product. As an intermediate step we construct a compatible system of semiorthogonal decompositions of the unbounded derived category of quasicoherent sheaves on and of the category of perfect complexes on . As an application we prove that the projection functors of a semiorthogonal decomposition are kernel functors.

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