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S Dec 22, 2015 at 10:04 history bounty ended CommunityBot
S Dec 22, 2015 at 10:04 history notice removed CommunityBot
Dec 14, 2015 at 12:19 comment added Bernie Have you had a look at "Geometry of the moduli spaces of sheaves" by Huybrechts and Lehn? They construct these coarse moduli spaces for polarized projective schemes. They also construct a relative version: a coarse moduli scheme for sheaves on the fibers of a projective morphisms with a relative ample line bundle. Maybe this is a good starting point, see ncatlab.org/nlab/files/HuybrechtsLehn.pdf
S Dec 14, 2015 at 8:40 history bounty started gradstudent
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Dec 14, 2015 at 8:40 history edited gradstudent CC BY-SA 3.0
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Dec 14, 2015 at 6:19 history edited gradstudent CC BY-SA 3.0
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Dec 10, 2015 at 14:29 comment added gradstudent Prof. @JasonStarr, and when you said there is a projective coarse moduli space of semistable objects, is that for a single $(A,\theta)$ or is it over $\mathcal{A}_g$?
Dec 10, 2015 at 13:44 history edited gradstudent CC BY-SA 3.0
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Dec 10, 2015 at 13:44 comment added gradstudent I am sorry, I meant the universal sheaf only!
Dec 10, 2015 at 13:18 comment added Jason Starr Even for a single $(A,\theta)$, there is not a moduli scheme $\mathcal{M}(r,c_i)$ for that $A$. There is an Artin stack. If you specify a stability condition, there is frequently a projective coarse moduli space of semistable objects. Also, if memory serves, the Brauer group of $\mathcal{A}_g$ equals $\mathbb{Q}/\mathbb{Z}$ (I think I proved that here some years ago). Since this is nontrivial, likely there do not exist universal sheaves over those coarse moduli spaces (I am not sure why you refer to such a universal sheaf as a Poincare "line bundle).
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