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Questions about the branch of algebra that deals with groups.

19 votes
Accepted

Number of finite index subgroups in a free abelian group

Yes. This is given by OEIS sequence A160870. The number of subgroups of index $n$ in $\mathbf{Z}^m$ is there denoted $T(n,m)$. There is a recursive formula in terms of the divisors of $n$ given at thi …
Carl-Fredrik Nyberg Brodda's user avatar
6 votes
Accepted

Is there a residually nilpotent one-relator group that is not residually a finite p-group fo...

The answer is yes, such a group exists: this is the main result of [1]. In fact, one can take the Baumslag-Solitar group $\operatorname{BS}(p^r, -p^r) = \langle a, b \mid ba^{p^r}b^{-1} = a^{-p^r} \ra …
Carl-Fredrik Nyberg Brodda's user avatar
7 votes
0 answers
177 views

Completeness of automorphism groups of free metabelian groups

I am not very familiar with free metabelian groups, so I apologise in advance if this is trivial. A group $G$ is said to be complete if every automorphism of $G$ is inner. In this case, $\operatorname …
Carl-Fredrik Nyberg Brodda's user avatar
4 votes
Accepted

Examples of "natural" finitely generated groups with an undecidable conjugacy problem

Chuck Miller in [Miller, Charles F., III On group-theoretic decision problems and their classification. Annals of Mathematics Studies, No. 68. Princeton University Press, Princeton, N.J.; University o …
Carl-Fredrik Nyberg Brodda's user avatar
4 votes

Compute generators of $\Gamma_0(N)$

sage should have what you need. Check under the documentation for the modular group. Specifically under generators you can find a couple of working examples for finding the generating set for $\Gamm …
Carl-Fredrik Nyberg Brodda's user avatar
3 votes

Subgroups of one-relator groups

It is known that a right-angled Artin group $A(\Gamma)$ embeds in some one-relator group if and only if $\Gamma$ is a forest. In fact, any such $A(\Gamma)$ embeds in the one-relator group $\operatorna …
Carl-Fredrik Nyberg Brodda's user avatar
3 votes

Embedding of Baumslag-Solitar group into a certain group

A few minutes of running GAP shows that the group has order $6751269$. Hence, as the final edit of the accepted answer points out, the group is finite, and no Baumslag-Solitar group embeds in it.
Carl-Fredrik Nyberg Brodda's user avatar
4 votes
Accepted

Computable change in minimum word length of subgroup elements

I'll put my comment here, so that the question has an answer. The function $\operatorname{len}_H$ is what is called an actual distortion function (for $H$ in $G$) by Margolis, Meakin & Šuniḱ (see [1]) …
Carl-Fredrik Nyberg Brodda's user avatar
7 votes

Centre of orbifold fundamental group of torus (Klein bottle) with one cone point

The groups have trivial center, as pointed out by Sam Nead. Another, more combinatorial, way to show this is to apply the algorithm from [1], which decides whether any given one-relator group has a no …
Carl-Fredrik Nyberg Brodda's user avatar
9 votes
Accepted

Is the isomorphism problem solvable for torsion-free groups?

Novikov's centrally-symmetric group $\mathfrak{A}_P$ is a torsion-free group with undecidable word problem, constructed in [1]. Novikov did not prove it is torsion-free but, as Adian points out in [Ad …
Carl-Fredrik Nyberg Brodda's user avatar
9 votes
3 answers
490 views

Residually solvable Bianchi groups

Let $d$ be a square-free positive integer, and let $\mathcal{O}_d$ be the ring of integers of the quadratic imaginary number field $\mathbb{Q}(\sqrt{-d})$. Consider the Bianchi group $\Gamma_d = \oper …
Carl-Fredrik Nyberg Brodda's user avatar
5 votes

Prove these are not surface groups

Here's an argument that uses only basic combinatorial group theory (Reidemeister-Schreier). Let $n \geq 2$, and let $G = \langle a_1, b_1, \dots, a_g, b_g | [a_1, b_1]^n [a_2, b_2] \cdots [a_g, b_g] \ …
Carl-Fredrik Nyberg Brodda's user avatar
6 votes
Accepted

Is there a finitely generated residually finite group with solvable word problem that does n...

A preprint by E. Rauzy appeared today on the arXiv, and gives a negative answer to this question. In other words (if the proof is correct), there exists a f.g. residually finite group with decidable w …
Carl-Fredrik Nyberg Brodda's user avatar
5 votes
2 answers
425 views

Centre of group with deficiency at least two (Progress on Murasugi's conjecture)

In 1965, Murasugi [1] conjectured that any finitely presented group with deficiency at least two has trivial centre. The year before, he had proved it true for one-relator groups, and in [1] he proved …
Carl-Fredrik Nyberg Brodda's user avatar
6 votes
0 answers
623 views

Minimum Simple Burger-Mozes Type Group

Burger and Mozes constructed (Burger and Mozes - Lattices in products of trees) infinite, finitely presented, torsion-free simple groups which split as amalgams of two finitely generated free groups o …
Carl-Fredrik Nyberg Brodda's user avatar

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