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arXiv 1603.09705math.AGmath.COmath.RT
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基于表示论的完备齐次簇

Complete Homogeneous Varieties via Representation Theory

Francesco Cavazzani

摘要:

给定一个具有稳定子$H$的代数簇$X\subset\mathbb{P}^N$,商$PGL_{N+1}/H$可以解释为$X$的所有$PGL_{N+1}$ -平移的参数空间。我们定义$X$是一个$\textit{homogeneous variety}$,如果$H$作用于它传递,并满足一些其他性质,如$H$是半单的。齐次簇的一些例子是二次超曲面,有理正规曲线,Veronese和Segre嵌入。在这种情况下,我们构造了参数空间$PGL_{N+1}/H$的新的紧化,得到了将$PGL_{N+1}$紧化为\textit{complete collineations}的经典已知空间,并取G.I.T.商由$H$,我们将调用\textit{complete homogeneous varieties}的结果空间;这扩展了[36]中二次超曲面的相同构造。我们建立了一些属性,这些空间:特别是,我们发现一个公式的体积因子,只依赖于尺寸的$H$ -不变量的不可约表示的$SL_{N+1}$。然后,我们开发了一些工具,在不变量理论,组合数学和样条近似计算这样的不变量,并进行了整个计算的情况下,$SL_2$ -不变量的不可约表示的$SL_4$,这给我们明确的价值观的体积函数的情况下,$X$是扭曲的立方。之后,我们将注意力集中在扭曲的立方的情况下,给出了更明确的描述,这些紧化,包括与以前已知的模空间的关系。最后,对体积函数在解决计数问题中的应用作了一些探讨。

英文摘要:

Given an algebraic variety $X\subset\mathbb{P}^N$ with stabilizer $H$, the quotient $PGL_{N+1}/H$ can be interpreted a parameter space for all $PGL_{N+1}$-translates of $X$. We define $X$ to be a $\textit{homogeneous variety}$ if $H$ acts on it transitively, and satisfies a few other properties, such as $H$ being semisimple. Some examples of homogeneous varieties are quadric hypersurfaces, rational normal curves, and Veronese and Segre embeddings. In this case, we construct new compactifications of the parameter spaces $PGL_{N+1}/H$, obtained compactifying $PGL_{N+1}$ to the classically known space of \textit{complete collineations}, and taking the G.I.T. quotient by $H$, and we will call the result space of \textit{complete homogeneous varieties}; this extends the same construction for quadric hypersurfaces in [36]. We establish a few properties of these spaces: in particular, we find a formula for the volume of divisors that depends only on the dimension of $H$-invariants in irreducible representations of $SL_{N+1}$. We then develop some tools in invariant theory, combinatorics and spline approximation to calculate such invariants, and carry out the entire calculations for the case of $SL_2$-invariants in irreducible representations of $SL_4$, that gives us explicit values for the volume function in the case of $X$ being a twisted cubic. Afterwards, we focus our attention on the case of twisted cubics, giving a more explicit description of these compactifications, including the relation with the previously known moduli spaces. In the end, we make some conjectures about how the volume function might be used in solving some enumerative problems.

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