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I have been studying exceptional sheaves and their mutations on Del Pezzo surfaces (specifically, on $\mathbb{P}^1 \times \mathbb{P}^1$). Given an exceptional pair $(E,F)$ of sheaves there are a priori three different kinds of mutations that can occur:

  1. "regular/division": when $E\otimes \text{Hom}(E,F) \twoheadrightarrow F$

  2. "rebound/recoil": when $E\otimes \text{Hom}(E,F) \hookrightarrow F$

  3. "extension": when $\text{Ext}^1(E,F) \neq 0$

However, I have never seen an example of the second "rebound/recoil" type. Drezet proves (see note after Theorem 6) that these do not exist on $\mathbb{P}^2$. Is there an example of these on another Del Pezzo surface that I have missed, or is there a conjecture stating that none exist?

Also, if an example does exist, does there exist an example where $(E,F)$ is in a thread of a helix?

Notes:

-A sheaf on a surface is exceptional if $\text{Ext}^\star(E,E) = \text{End}(E,E) = \mathbb{C}$.

-$(E,F)$ is an exceptional pair if $\text{Ext}^\star(F,E)=0$

-the left mutation of $F$ though $E$, denoted $L_E F$, is defined by the triangle $L \longrightarrow E\otimes \text{Hom}^\bullet(E,F) \stackrel{\text{eval}}{\longrightarrow} F \longrightarrow L[1]$, and either $L$ or $L[1]$ is chosen as the mutation depending on if the mutation is of type 1,2, or 3. Note: it is also possible to define $L_F E$ always as $L$ or always as $L[1]$

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  • $\begingroup$ Your definition of an exceptional pair is very special. A usual one asks only for $Ext^\bullet(F, E)=0$. A pair is called strong if $Ext^i(E, F)=0$ for all $i>0$. Also when one defines mutations, $Hom$ is taken in the derived sense. $\endgroup$ Commented Jun 22, 2012 at 23:19
  • $\begingroup$ Thank you for your comments! You are correct that a priori my definition is special, however on a Del Pezzo surface it turns out to be equivalent (see [Helix Theory](cdsweb.cern.ch/record/737489/files/sis-2004-233.ps) - Section 4.4, page 30). I will update the definition so that I do not assume this. You are also correct about $Hom$ - I have made the adjustment. $\endgroup$
    – EricM
    Commented Jun 25, 2012 at 16:35
  • $\begingroup$ The link to springerlink.com is broken. I'm also unable to find any snapshot saved on the Wayback Machine. $\endgroup$ Commented Apr 17, 2023 at 10:16

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