柏拉图与交错2-群
Platonic and alternatinc 2-groups
摘要：
我们回顾舒尔的工作，普遍的中央扩展和发展类似的理论范畴扩展的群体。我们证明了弦2-群在这个意义上是普遍的，并详细研究了它们对自旋群的有限子群的限制。特别令人感兴趣的是3-sphere Spin（3）的子群，以及交替群的自旋双覆盖，它们的范畴扩张被证明是由稳定的3-stem控制的。
英文摘要：
We recall Schur's work on universal central extensions and develop the analogous theory for categorical extensions of groups. We prove that the String 2-groups are universal in this sense and study in detail their restrictions to the finite subgroups of the Spin groups. Of particular interest are subgroups of the 3-sphere Spin(3), as well as the spin double covers of the alternating groups, whose categorical extensions turn out to be governed by the stable 3-stem.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
