摘要翻译:
设X是P^R中维数为n的光滑射影簇。本文研究了一般线性投影pi:x-->p^{n+c}的纤维,其中c>0。当n较小时,任何纤维的度数都以n/c+1为界,这是经典的,但当n>>0时,这就失效了。我们描述了一个新的纤维不变量,它在许多情况下与度一致,并且总是以n/c+1为界。这意味着,例如,如果我们把一个光纤写成格式Y'和Y''的不相交并,使得Y'是Y的局部完全交分量的并,那么degy'+degy''_red<=n/c+1,这个公式可以进一步加强。对于任意正整数l,我们的方法也给出了由X的l-割线扫出的P^R子簇的一个锐利界,并讨论了高维高割线线性空间的一个相应界。这些结果推广了Ziv Ran的“维数+2割线引理”。
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英文标题:
《Fibers of Generic Projections》
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作者:
Roya Beheshti, David Eisenbud
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最新提交年份:
2009
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分类信息:
一级分类:Mathematics 数学
二级分类:Algebraic Geometry 代数几何
分类描述:Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology
代数簇,叠,束,格式,模空间,复几何,量子上同调
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一级分类:Mathematics 数学
二级分类:Commutative Algebra 交换代数
分类描述:Commutative rings, modules, ideals, homological algebra, computational aspects, invariant theory, connections to algebraic geometry and combinatorics
交换环,模,理想,同调代数,计算方面,不变理论,与代数几何和组合学的联系
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英文摘要:
Let X be a smooth projective variety of dimension n in P^r. We study the fibers of a general linear projection pi: X --> P^{n+c}, with c > 0. When n is small it is classical that the degree of any fiber is bounded by n/c+1, but this fails for n >> 0. We describe a new invariant of the fiber that agrees with the degree in many cases and is always bounded by n/c+1. This implies, for example, that if we write a fiber as the disjoint union of schemes Y' and Y'' such that Y' is the union of the locally complete intersection components of Y, then deg Y'+deg Y''_red <= n/c+1 and this formula can be strengthened a little further. Our method also gives a sharp bound on the subvariety of P^r swept out by the l-secant lines of X for any positive integer l, and we discuss a corresponding bound for highly secant linear spaces of higher dimension. These results extend Ziv Ran's "Dimension+2 Secant Lemma".
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PDF链接:
https://arxiv.org/pdf/0806.1928