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arXiv 1208.2806math.AGmath.CVmath.KT
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椭球面上代数联络的显式公式

Explicit formulas for algebraic connections on ellipsoid surfaces

Helge Øystein Maakestad

AI总结:

本文提出一种基于幂等矩阵和投射基构造任意阶代数微分算子及代数联络显式公式的新方法,并应用于椭球面模与实二维球面余切丛,证明所构造联络非平坦且曲率迹为零,进而证明任意复仿射正则超曲面的底层复流形为 Calabi-Yau 流形。

摘要:

本文旨在给出一种新方法,用于在交换含幺环 $A$ 上的有限生成投射模 $E$ 上构造任意阶代数微分算子的显式公式。此外,我们给出了一类椭球面上的有限生成投射模上代数联络的显式公式。我们构造的联络是非平坦的,且曲率的迹等于零。我们利用定义模 $E$ 的幂等矩阵 $M$ 来构造这些公式。这样的幂等矩阵 $M$ 由定义模 $E$ 的“投射基” $B$ 构造而来。对于 $E$ 的一个投射基 $B$,我们构造一个联络 $\nabla_B$。联络 $\nabla_B$ 的曲率 $R_{\nabla_B}$ 由一个李积给出:$R_{\nabla_B}(x,y):=[\nabla_B(x)(M), \nabla_B(y)(M)]$,其中涉及矩阵 $M$,并且这个李积一般不为零。因此曲率公式表明,大多数有限秩投射模不具有平坦的代数联络。我们还给出了实二维球面的余切丛 $Ω$ 上非平坦代数联络的显式公式。余切丛 $Ω$ 在拓扑上是非平凡的,并且它是否具有平坦的代数联络尚不清楚。在 deRham 上同调中所有高阶陈类均为零:对所有的 $i \geq 1$,$c_i(Ω)=0$。我们将该构造与非阿贝尔扩张以及另一篇相关论文中引入的精化示性类 $c(Ω)$ 联系起来。类 $c(Ω)$ 是利用联络 $\nabla_B$ 定义的,但它不依赖于联络的选取。类 $c(-)$ 取值于一个 torsor 中。本文引入的方法证明了任意复仿射正则超曲面的底层复流形是 Calabi-Yau 流形。这是因为其典范丛是平凡的。

英文摘要:

The aim of this paper is to give a new method to construct explicit formulas for algebraic differential operators of any order on a finitely generated projective module $E$ on a commutative unital ring $A$. We moreover give explicit formulas for algebraic connections on a class of finitely generated projective modules on ellipsoid surfaces. The connections we construct are non-flat with trace of curvature equal to zero. We construct these formulas using an idempotent matrix $M$ defining the module $E$. Such an idempotent matrix $M$ is constructed from a "projective basis" $B$ defining the module $E$. Associated to a projective basis $B$ for $E$ we construct a connection $\nabla_B$. The curvature $R_{\nabla_B}$ of the connection $\nabla_B$ is given by a Lie product: $R_{\nabla_B}(x,y):=[\nabla_B(x)(M), \nabla_B(y)(M)]$ involving the matrix $M$, and this Lie product is non-zero in general. Hence the curvature formula indicates that most projective finite rank modules do not have a flat algebraic connection. We also give an explicit formula for a non-flat algebraic connection on the cotangent bundle $Ω$ of the real 2-sphere. The cotangent bundle $Ω$ is topologically non-trivial and it is not clear if it has a flat algebraic connection. All higher Chern classes in deRham cohomology are zero: $c_i(Ω)=0$ for all $i \geq 1$. We relate the construction to non-abelian extensions and a refined characteristic class $c(Ω)$ introduced in another paper on the subject. The class $c(Ω)$ is defined using the connection $\nabla_B$ but it is independent of choice of connection. The class $c(-)$ lives in a torsor. The methods introduced in the paper prove that the underlying complex manifold of any complex affine regular hypersurface is a Calabi-Yau manifold. This is because its canonical bundle is trivial.

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