Group Actions on Trees and Boundaries of Trees

几何群论连接了数学的两个基本领域，即群论和几何。 一个群可以被认为是一个物体的对称性集合，比如一个水分子或一个魔方。 一个群可以表示许多几何或拓扑空间的对称性。 如果选择的空间足够好，可以充分理解，空间的特征可以揭示群体固有的属性。 也可以采取相反的方法。 人们通常可以通过研究描述其对称性的群来发现拓扑或几何空间的性质。 该项目旨在探索群体之间的这些联系以及他们行动的空间。 该项目的更广泛的影响包括通过各种方案和网络继续进行指导，面向初中和高中学生的公共宣传，以及通过组织会议传播知识。该项目的重点主要是在无限树和无限树的边界上行动的团体。 这包括大类的群体;例如，它包含所有剩余有限群，但也有许多已知的例子无限简单的群体。 该项目的第一个目标是扩展PI过去的工作，以更好地理解无限简单群的宇宙。 在过去的10年里已经看到了涌入的新的和令人惊讶的定理照亮了理解的各种群体在这一类。 PI将研究扩展的Thompson家族中的群，使用它们在根树上的部分作用，它们在Stein-Farley复形上的完全作用，以及它们嵌入到康托空间的同胚群中。 该项目的第二个目标是更好地理解分支和自动机组。 在过去的40年里，这类群体已经成为一个丰富的外来但易处理的群体来源。 PI将使用自动机理论来研究增长和扭转问题，并将长期发展的分支群理论应用于分支群的最大子群的一般理论。该项目的最后一个重点是来自拓扑结构的组属性。 PI将开发分支覆盖拓扑理论的几何群论模拟，并探索同调稳定性和拓扑有限性的一般理论之间的联系，首先通过一些大的映射类组的某些自然子群，通过利用与曲线复合体相关的高度连接的复合体上的动作。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估。

Geometric group theory connects two foundational fields of mathematics, namely group theory and geometry. A group can be thought of as the set of symmetries of an object such as a water molecule or a Rubik's Cube. A single group can represent the symmetries of many geometric or topological spaces. If the chosen space is nice enough and can be sufficiently well understood, the characteristics of the spaces can reveal properties inherent to the group. One can take the opposite approach as well. Often one can detect properties of a topological or geometric space by studying groups which describe their symmetries. This project aims to explore these connections between groups and the spaces on which they act. Broader impacts of the project include continued mentoring through various programs and networks, public outreach for middle and high school students, and dissemination of knowledge through conference organization.The focus of this project is primarily on groups acting on infinite trees and on boundaries of infinite trees. This includes large classes of groups; for instance it contains all residually finite groups but also many of the known examples of infinite simple groups. The first goal of the project is to extend the PI's past work to better understand the universe of infinite simple groups. The last 10 years have seen an influx of new and surprising theorems illuminating the understanding of the variety of groups in this class. The PI will study groups in the extended Thompson family using their partial actions on a rooted tree, their full action on the Stein-Farley complex, and their embeddings into the homeomorphism group of the Cantor space. The second goal of the project is to better understand branch and automata groups. Over the last forty years, this class of groups has served as a rich source of exotic yet tractable groups. The PI will use automata theory to investigate questions of growth and torsion and apply the long developed theory of branch groups to work towards a general theory of maximal subgroups of branch groups. The final focus of the project is on group properties coming from topological constructions. The PI will develop a geometric group theory analog of the topological theory of branch coverings as well as explore connections between the general theory of homological stability and topological finiteness properties, first through certain natural subgroups of some big mapping class groups by exploiting actions on highly connected complexes related to the curve complex.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.