Iskovskikh Seminar November 13, 2025 18:00, Moscow, Steklov Mathematical Institute, room 530
On the construction of “new” semiorthogonal decompositions and Yoneda
embeddings for some subcategories of the derived category of quasicoherent
sheaves
Abstract:
The talk is devoted to the construction of new admissible subcategories and
semiorthogonal decompositions from original ones.
If $(\mathcal{LA},\mathcal{RA})$ is a semi-orthogonal decomposition of the category $\underline{C}_0$ of
compact objects in a compactly generated triangulated category $\underline{C}$
(respectively, $(\mathcal{LA},\mathcal{RA})$ is a $t$-structure and a weight structure invariant
under $[1]$, the subcategory $\mathcal{LA}$ is left admissible in $\underline{C}_0$, and $\mathcal{RA}$ is
right admissible), then there exists an “orthogonal” semi-orthogonal
decomposition $(\mathcal{LA}{{}^{\perp}},\mathcal{RA}{{}^{\perp}}$) of $\underline{C}$ itself; the classes $\mathcal{RA}{{}^{\perp}}$
and $\mathcal{RA}{{}^{\perp}}$ are characterized by the absence of nonzero morphisms into
them from elements of $\mathcal{RA}$ and $\mathcal{RA}$, respectively.
Further, if $\underline{C}'$ is a subcategory of $\underline{C}$ that can be characterized in
terms of morphisms from $\underline{C}_0$, then this semi-orthogonal decomposition is
restricted to $\underline{C}'$. If $\underline{C}'$ is in some sense dual to $\underline{C}_0$, then we
obtain a bijection between the semiorthogonal decompositions of $\underline{C}_0$ and $
\underline{C}'$.
These statements easily generalize to semiorthogonal decompositions of
arbitrary length. Recent results of A. Ne'eman (and the speaker) allow us to
apply them to various derived categories of quasicoherent sheaves on a scheme
$X$ that is proper over the spectrum of a Noetherian ring $R$. This gives a
one-to-one correspondence between the semiorthogonal decompositions of the
categories $D_{perf}(X)$ and $D^b(\operatorname{coh}(X))$; The latter extend
to $D^-(\operatorname{coh}(X))$, $D^+_{coh}(X)$, $D_{coh}(X)$, and $D_{qcoh}
(X)$$D^+_{coh}(\operatorname{Qcoh}(X))$, $D_{coh}(\operatorname{Qcoh}(X))$, and
$D(\operatorname{Qcoh}(X))$ (if very weak additional assumptions are
satisfied).
If time allows, I will explain that the transition from $(\mathcal{LA},\mathcal{RA})$ to
($\mathcal{LA}{{}^{\perp}},\mathcal{RA}{{}^{\perp}}$) is a special case of the orthogonality of weight and $t$-
structures, which is precisely what inspired these arguments in the talk.