On an algorithm to decide whether a free group is a free factor of another

A classification of metacyclic groups by group invariants

We obtain a new classification of the finite metacyclic group in terms of group invariants. We present an algorithm to compute these invariants, and hence to decide if two given finite metacyclic groups are isomorphic, and another algorithm which computes all the metacyclic groups of a given order. A GAP implementation of these algorithms is given.

$BS(n, 1)$ 中的扭共轭

Twisted conjugacy in $BS(n, 1)$

本文解决可解 Baumslag-Solitar 群 $BS(n,1)$ 的扭共轭问题，提出判定算法，并证明其自同构群轨道可判定。

代数长度为3的拟正4-辫子的算法识别

Algorithmic recognition of quasipositive 4-braids of algebraic length three

本文基于Garside理论提出多项式时间算法，识别代数长度为3的拟正4-辫子，并将两因子共轭幂判定推广到任意股数。

多循环群与投射有限同构

Polycyclic groups and profinite isomorphism

针对几乎多循环群类，给出了判断两个群是否具有同构的投射有限完备化的算法。

On the automorphism group of generalized Baumslag-Solitar groups

A generalized Baumslag-Solitar group (GBS group) is a finitely generated group $G$ which acts on a tree with all edge and vertex stabilizers infinite cyclic. We show that Out(G) either contains non-abelian free groups or is virtually nilpotent of class at most 2. It has torsion only at finitely many primes. One may decide algorithmically whether Out(G) is virtually nilpotent or not. If it is, one may decide whether it is virtually abelian, or finitely generated. The isomorphism problem is solvable among GBS groups with Out(G) virtually nilpotent. If $G$ is unimodular (virtually $F_n \times Z$)…

Computing fixed closures in free groups

Let $F$ be a finitely generated free group. We present an algorithm such that, given a subgroup $H\leqslant F$, decides whether $H$ is the fixed subgroup of some family of automorphisms, or family of endomorphisms of $F$ and, in the affirmative case, finds such a family. The algorithm combines both combinatorial and geometric methods.

The isomorphism problem for profinite completions of residually finite groups

We consider pairs of finitely presented, residually finite groups $u:P\hookrightarrow Γ$. We prove that there is no algorithm that, given an arbitrary such pair, can determine whether or not the associated map of profinite completions $\hat{u}: \widehat{P} \to \widehatΓ$ is an isomorphism. Nor do there exist algorithms that can decide whether $\hat{u}$ is surjective, or whether $\widehat{P}$ is isomorphic to $\widehatΓ$.

On several problems about automorphisms of the free group of rank two

Let $F_n$ be a free group of rank $n$. In this paper we discuss three algorithmic problems related to automorphisms of $F_2$. A word $u$ of $F_n$ is called positive if $u$ does not have negative exponents. A word $u$ in $F_n$ is called potentially positive if $ϕ(u)$ is positive for some automorphism $ϕ$ of $F_n$. We prove that there is an algorithm to decide whether or not a given word in $F_2$ is potentially positive, which gives an affirmative solution to problem F34a in [1] for the case of $F_2$. Two elements $u$ and $v$ in $F_n$ are said to be boundedly translation equivalent if the ratio …

Group isomorphism is nearly-linear time for most orders

We show that there is a dense set $\ourset\subseteq \mathbb{N}$ of group orders and a constant $c$ such that for every $n\in \ourset$ we can decide in time $O(n^2(\log n)^c)$ whether two $n\times n$ multiplication tables describe isomorphic groups of order $n$. This improves significantly over the general $n^{O(\log n)}$-time complexity and shows that group isomorphism can be tested efficiently for almost all group orders $n$. We also show that in time $O(n^2 (\log n)^c)$ it can be decided whether an $n\times n$ multiplication table describes a group; this improves over the known $O(n^3)$ comp…

Efficient computations with counting functions on free groups and free monoids

We present efficient algorithms to decide whether two given counting functions on non-abelian free groups or monoids are at bounded distance from each other and to decide whether two given counting quasimorphisms on non-abelian free groups are cohomologous. We work in the multi-tape Turing machine model with non-constant time arithmetic operations. In the case of integer coefficients we construct an algorithm of linear space and time complexity (assuming that the rank is at least $3$ in the monoid case). In the case of rational coefficients we prove that the time complexity is $O(N\log N)$, wh…

Embedded spheres in S^2\times S^1#...#S^2\times S^1

We give an algorithm to decide which elements of pi_2(S^2\times S^1#...#S^2\times S^1) can be represented by embedded spheres. Such spheres correspond to splittings of the free group on k generators. Equivalently our algorithm decides whether, for a handlebody N, an element in pi_2(N,\partial N) can be represented by an embedded disc. We also give an algorithm to decide when classes in $π_2(S^2\times S^1#...#S^2\times S^1)$ can be represented by disjoint embedded spheres. We introduce the splitting complex of a free group which is analogous to the complex of curves of a surface. We show that t…

Iteration of functions and contractibility of acyclic 2-complexes

We show that there can be no algorithm to decide whether infinite recursively described acyclic aspherical 2-complexes are contractible. We construct such a complex that is contractible if and only if the Collatz conjecture holds.

Testing spherical transitivity in iterated wreath products of cyclic groups

We give a partial solution a question of Grigorchuk, Nekrashevych, Sushchanskii and Šunik by giving an algorithm to test whether a finite state element of an infinite iterated (permutational) wreath product $\hat G = \mathbb Z/k\mathbb Z\wr \mathbb Z/k\mathbb Z\wr \mathbb Z/k\mathbb Z\wr >...$ of cyclic groups of order $n$ acts spherically transitively. We can also decide whether two finite state spherically transitive elements of $\hat G$ are conjugate. For general infinite iterated wreath products, an algorithm is presented to determine whether two finite state automorphisms have the same im…

另一个可遍历群

Another ambitable group

研究拓扑群是否必为预紧或可遍历的问题，构造了一个可遍历但不满足已知充分条件的拓扑群。

On Tarski's problem for virtually free groups

We give a complete classification of finitely generated virtually free groups up to $\forall\exists$-elementary equivalence. As a corollary, we give an algorithm that takes as input two finite presentations of virtually free groups, and decides whether these groups have the same $\forall\exists$-theory or not.