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Let $X$ be a degree $10$, genus $6$ index $1$ and Picard number $1$ smooth Fano threefold. There are two versions of Semi-orthogonal decompositions. The First version is $$D^b(X)=\langle\mathrm{Ku}(X),\mathcal{E}_X,\mathcal{O}_X\rangle$$ where $\mathcal{E}_X$ is the stable bundle of rank 2 with character $2-H+L+\frac{1}{3}P$, it is a pull back of tautological rank 2 sub-bundle of $\mathrm{Gr}(2,5)$. The second version of semi-orthogonal decomposition is $$D^b(X)=\langle\mathcal{A}_X,\mathcal{O}_X,\mathcal{E}_X^{\vee}\rangle$$ where $\mathcal{E}_X^{\vee}$ is the dual vector bundle of $\mathcal{E}_X$. I understand that both of them are coming from full exceptional collections of vector bundles on $Y\rightarrow\mathrm{Gr}(2,5)$ where $Y$ is a codimension 3 linear section. $$D^b(Y)=\langle\mathcal{U}_Y,\mathcal{O}_Y,\mathcal{U}_Y(1),\mathcal{O}_Y(1)\rangle.$$ This is a rectangular Lefschetz decomposition with $\mathcal{B}=\langle\mathcal{U}_Y,\mathcal{O}_Y\rangle$. Now since $X\rightarrow Y$ is either an embedding or a double cover, Then $D^b(X)$ TEMPhas the first type semi-orthogonal decomposition. If one consider the "rotate" of full exceptional (or by consecutively right mutations) $$D^b(Y)=\langle\mathcal{O}_Y,\mathcal{U}_Y(1),\mathcal{O}_Y(1),\mathcal{U}_Y(2)\rangle,$$ this is exactly $$\langle\mathcal{O}_Y,\mathcal{U}_Y^{\vee},\mathcal{O}_Y(1),\mathcal{U}_Y^{\vee}(1)\rangle$$ This is another rectangular Lefschetz decomposition with $\mathcal{B}'=\langle\mathcal{O}_Y,\mathcal{U}_Y^{\vee}\rangle$. Then from this, we TEMPhas $D^b(X)$ with the second type semi-orthogonal decomposition. By a result of A.Kuznetsov, either $\mathrm{Ku}(X)$ or $\mathcal{A}_X$ TEMPhas the same shape Serre functor, which is given by $\tau\circ[2]$, where $\tau$ is an involution functor, if $X$ is special, $\tau$ is induced by double cover.

I was wondering whether $\mathcal{A}_X$ is equivalent to $\mathrm{Ku}(X)$? I think they should be equivalent by several mutation functors (as in $Y_5$), but when I do right mutation $\mathrm{R}_{\mathcal{O}_X}\mathcal{E}_X$, I get a rank 3 vector bundle, I do not know how to proceed from here.

But it looks like the subcategory $\langle\mathcal{E}_X,\mathcal{O}_X\rangle$ is anti-equivalent to $\langle\mathcal{O}_X,\mathcal{E}^{\vee}\rangle$ by derived dual functor. So $\mathrm{Ku}(X)$ is anti-equivalent to $\mathcal{A}_X$?

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    $\begingroup$ The two categories are equivalent, see section 2.8 in A. Kuznetsov, A. Perry, "Derived categories of Gushel-Mukai varieties", Compositio Mathematica, V. 154 (2018), N. 7, pp. 1362--1406, available at mi-ras.ru/~akuznet/publications/… $\endgroup$
    – Sasha
    Commented Sep 17, 2020 at 12:57
  • $\begingroup$ @Sasha, thanks so much! That would be very nice! In fact, for the computation, I found that Ku(X) is better for special Gushel-Mukai while A_X is better for ordinary Gushel-Mukai in some sense. But they are equivalent, this is so nice. $\endgroup$
    – user41650
    Commented Sep 17, 2020 at 13:36

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