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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 …
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 …
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 …
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 …
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 …
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 …
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.
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]) …
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 …
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 …
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 …
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] \ …
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 …
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 …
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 …