Clifford代数是根系和Coxeter群的自然框架。群论：Coxeter，保形群和模群
Clifford algebra is the natural framework for root systems and Coxeter groups. Group theory: Coxeter, conformal and modular groups
摘要：
在本文中，我们证明了Clifford代数是根系和反射群以及相关群如共形群和模群的自然框架：存在于这些空间上的度量总是可以用来构造相应的Clifford代数。通过Cartan-Dieudonné定理，所有感兴趣的变换都可以写成反射的乘积，从而通过与Clifford代数多向量的“重叠”。这些多向量群可以用于在不同的群中执行具体的计算，例如各种类型的多面体群，并且我们详细地处理四面体群$A_3$的例子。顺便说一句，这给出了一个建设性的结果，从每个3D根系中导出一个四维根系，这取决于以下事实：旋量组提供了旋转的双重覆盖，3D旋量空间具有4D欧几里得内积，并且相对于这个内积，旋量组可以被示出为在反射下闭合。特别是4D根系/Coxeter群诱导以这种方式正是例外的，与三维旋的观点也解释了他们不寻常的自同构群。这种结构简化了阿诺德的三位一体，并把麦凯对应到一个更广泛的框架。我们最后讨论了将共形几何代数方法推广到二维共形群和模群，这在共形场论、弦论和模形式理论中可能有有趣的新应用。
英文摘要：
In this paper, we make the case that Clifford algebra is the natural framework for root systems and reflection groups, as well as related groups such as the conformal and modular groups: The metric that exists on these spaces can always be used to construct the corresponding Clifford algebra. Via the Cartan-Dieudonné theorem all the transformations of interest can be written as products of reflections and thus via `sandwiching' with Clifford algebra multivectors. These multivector groups can be used to perform concrete calculations in different groups, e.g. the various types of polyhedral groups, and we treat the example of the tetrahedral group $A_3$ in detail. As an aside, this gives a constructive result that induces from every 3D root system a root system in dimension four, which hinges on the facts that the group of spinors provides a double cover of the rotations, the space of 3D spinors has a 4D euclidean inner product, and with respect to this inner product the group of spinors can be shown to be closed under reflections. In particular the 4D root systems/Coxeter groups induced in this way are precisely the exceptional ones, with the 3D spinorial point of view also explaining their unusual automorphism groups. This construction simplifies Arnold's trinities and puts the McKay correspondence into a wider framework. We finally discuss extending the conformal geometric algebra approach to the 2D conformal and modular groups, which could have interesting novel applications in conformal field theory, string theory and modular form theory.
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