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For the purposes of this question I will work over $\mathbb{C}$. Consider on $T=\mathbb{P}^1$ the bundle $E=O_{T}^{\oplus 3}\oplus O_T(-1)^{\oplus 2}$ and $\mathbb{P}E_T$ the associated projective bundle on $T$. Let $[s,t]$ be coordinates on $T$. Let $w_0,w_1,w_2$ be a frame for $O_T^{\oplus 3}$ and $u_4,u_5$ sections of $O_T(-1)^2$ with poles at $s=0$. Set $w_4=su_4$, $w_5=su_5$, and let $X\subset \mathbb{P}E$ be the scroll given by the vanishing of the rank 2 minors of $$ \left( \begin{array}{} w_1 & w_2 & w_4\\ w_0 & w_1 & w_3 \end{array} \right) $$ Given $t\in T$, does there exist $m$ such that the $m$-Hilbert point of $X_t$ is not stable for the action of $SL$ on $H^0(X_t,O_{\mathbb{P}E}(1)|_{X_t})$?

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  • $\begingroup$ Hi Carlo, This question should still be improved. You are answering Tim's comment at mathoverflow.net/questions/28037 but it would be much better to post a complete, self-contained question. You should look over the FAQ again: try to make the title into a question, etc. Also, I'll try to clean up the TeX, but I don't know what \frd is supposed to be (googling didn't turn anything up). $\endgroup$ Commented Jun 13, 2010 at 18:49
  • $\begingroup$ While cleaning up the TeX, I also made some grammar changes to try to put everything into sentences. This, of course, required guessing some of the meaning --- syntax is part of semantics. If I made an error, please roll back, and I apologize. I just guessed that \frd in \mathbb{P}E\frd T was supposed to make the T a subscript, and so I replaced it with an underscore _ protected by back-ticks. It takes some getting used to writing TeX here on MO, because you really should protect every TeX command with back-ticks, so that MarkDown doesn't process it before jsMath gets a chance. $\endgroup$ Commented Jun 13, 2010 at 18:58

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