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arXiv 1705.04598math.GRcs.LO
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自动机半群中的可判定性问题

Decidability problems in automaton semigroups

Laurent Bartholdi

摘要:

我们考虑自相似半群的可判定性问题,特别是在$X^*$的自动变换半群。我们描述的算法回答的字问题,并限制其复杂性下的一些额外的假设。我们给出了一个部分算法,决定在一个自动机,给定$x,y$,是否恩格尔身份($[\cdots[[x,y],y],\dots,y]=1$足够长的换向器序列)所产生的一组是满意的。这个算法成功地证明了Grigorchuk的$2$ -群不是Engel群。接下来,我们考虑识别恩格尔元素,即元素$y$,使得映射$x\mapsto[x,y]$吸引到$\{1\}$的问题。虽然这个问题似乎棘手的一般情况下,我们证明,它是可判定的Grigorchuk的组:恩格尔元素正是那些为了最多$2$。我们在正文中包括了大量的未决问题。我们的计算是使用计算机代数系统“Gap”中的软件包“Fr”实现的。

英文摘要:

We consider decidability problems in self-similar semigroups, and in particular in semigroups of automatic transformations of $X^*$. We describe algorithms answering the word problem, and bound its complexity under some additional assumptions. We give a partial algorithm that decides in a group generated by an automaton, given $x,y$, whether an Engel identity ($[\cdots[[x,y],y],\dots,y]=1$ for a long enough commutator sequence) is satisfied. This algorithm succeeds, importantly, in proving that Grigorchuk's $2$-group is not Engel. We consider next the problem of recognizing Engel elements, namely elements $y$ such that the map $x\mapsto[x,y]$ attracts to $\{1\}$. Although this problem seems intractable in general, we prove that it is decidable for Grigorchuk's group: Engel elements are precisely those of order at most $2$. We include, in the text, a large number of open problems. Our computations were implemented using the package "Fr" within the computer algebra system "Gap".

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。

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