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Timeline for Cone over the Veronese surface

Current License: CC BY-SA 3.0

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Jan 8, 2015 at 10:00 history edited Francesco Polizzi CC BY-SA 3.0
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Jan 8, 2015 at 6:59 comment added Francesco Polizzi Ok, now I completely agree with you. I (re)corrected the answer. Thanks.
Jan 8, 2015 at 6:51 history edited Francesco Polizzi CC BY-SA 3.0
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Jan 7, 2015 at 23:59 comment added Sándor Kovács I am sticking with $-5/2$, but not saying you're wrong. It's a question of what we're calling $\mathscr O_X(1)$. See my answer to your comment after my answer for more details. I think we agree in the essence of the result.
Jan 7, 2015 at 20:50 history edited Francesco Polizzi CC BY-SA 3.0
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Jan 7, 2015 at 20:39 history edited Francesco Polizzi CC BY-SA 3.0
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Jan 7, 2015 at 20:30 comment added Francesco Polizzi By the way, I think that the final answer is $b/a = -5/1$, see my comment above. Cheers!
Jan 7, 2015 at 20:27 history edited Francesco Polizzi CC BY-SA 3.0
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Jan 7, 2015 at 20:07 comment added Francesco Polizzi Sandor, of course you are right. For some reason, I did not notice that my result was in contradiction with the rationality of $X$ :-) I've edited the answer. Thank you for correcting me (and HNY!)
Jan 4, 2015 at 21:42 vote accept CommunityBot
Jan 4, 2015 at 21:42
Jan 4, 2015 at 21:35 vote accept CommunityBot
Jan 4, 2015 at 21:42
Jan 2, 2015 at 21:16 comment added Sándor Kovács There is one more place to nitpick actually: In your final conclusion you are assuming that the Picard group of $X$ is torsion-free. This is of course true (see an easy argument in my answer) but perhaps deserves mentioning. Otherwise you only get that their ratio is $2:3$. Cheers! (and HNY!)
Jan 2, 2015 at 21:15 comment added Sándor Kovács Francesco: This could not possibly be true. Your computation of $d$ shows that $X$ has terminal singularities, while $a,b>0$ implies that $X$ is canonically polarized, so that would imply that $Y$ is of general type. At the same time it is clearly rational. The error you made is very simple actually: I think that $K_Y\cdot \ell=-2$ and not $2$. Once you make that correction everything falls into place and you get the correct value $b/a=-5/2$.
Jan 1, 2015 at 15:42 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 20:08 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 17:22 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 16:53 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 16:46 history undeleted Francesco Polizzi
Dec 30, 2014 at 16:46 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 16:04 history deleted Francesco Polizzi via Vote
Dec 30, 2014 at 16:04 history edited Francesco Polizzi CC BY-SA 3.0
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Dec 30, 2014 at 15:38 history answered Francesco Polizzi CC BY-SA 3.0