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arXiv 1301.4669math.GR
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有限生成群空间的排序

Ordering the space of finitely generated groups

Laurent Bartholdi, Anna Erschler

摘要:

考虑顶点为有限生成群的同构类的定向图,若对H中的某个生成集T和G中的某个生成集序列Si,(G,Si)和(H,T)中半径为i的标记球重合,则有向图的边从G到H.给定一个幂零群群G,我们刻画了它在这个图中的连通分支:如果该连通分支至少包含一个无挠群,那么它由生成与G相同的群簇的那些群组成。图中的箭头定义了有限生成群的同构类集合上的一个前序。我们表明,偏序可以嵌入在这个预序当且仅当它是可实现的可数集下包含的子集。我们证明了每个可数群嵌入一个非均匀指数增长性群中。特别地,存在非均匀指数增长性的群,它们不是次指数增长性的剩余,并且不允许均匀嵌入到希尔伯特空间中。

英文摘要:

We consider the oriented graph whose vertices are isomorphism classes of finitely generated groups, with an edge from G to H if, for some generating set T in H and some sequence of generating sets S_i in G, the marked balls of radius i in (G,S_i) and in (H,T) coincide. Given a nilpotent group G, we characterize its connected component in this graph: if that connected component contains at least one torsion-free group, then it consists of those groups which generate the same variety of groups as G. The arrows in the graph define a preorder on the set of isomorphism classes of finitely generated groups. We show that a partial order can be imbedded in this preorder if and only if it is realizable by subsets of a countable set under inclusion. We show that every countable group imbeds in a group of non-uniform exponential growth. In particular, there exist groups of non-uniform exponential growth that are not residually of subexponential growth and do not admit a uniform imbedding into Hilbert space.

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