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Questions about the branch of algebra that deals with groups.

47 votes
1 answer
2k views

Transitivity on $\mathbb{N}_0$ -- a 42 problem

Let $r(m)$ denote the residue class $r+m\mathbb{Z}$, where $0 \leq r < m$. Given disjoint residue classes $r_1(m_1)$ and $r_2(m_2)$, let the class transposition $\tau_{r_1(m_1),r_2(m_2)}$ be the permu …
Stefan Kohl's user avatar
  • 19.6k
33 votes
2 answers
1k views

Richness of the subgroup structure of p-groups

Given a prime $p$ and $n \in \mathbb{N}$, let $f_p(n)$ be the smallest number such that there is a group of order $p^{f_p(n)}$ which all groups of order $p^n$ embed into. What is the asymptotic growth …
Stefan Kohl's user avatar
  • 19.6k
32 votes
3 answers
3k views

Order of products of elements in symmetric groups

Let $n \in \mathbb{N}$. Is it true that for any $a, b, c \in \mathbb{N}$ satisfying $1 < a, b, c \leq n-2$ the symmetric group ${\rm S}_n$ has elements of order $a$ and $b$ whose product has order $c$ …
Stefan Kohl's user avatar
  • 19.6k
30 votes
2 answers
2k views

Fractal-like structures arising from the action of a group on $\mathbb{Z}^2$

Let $G := \langle a, b, c \rangle < {\rm Sym}(\mathbb{Z}^2)$ be the group generated by the permutation $$ a: \ (m,n) \ \mapsto \ (m-n,m) $$ of order $6$ and the involutions $$ b: \ (m,n) \ \mapsto …
Stefan Kohl's user avatar
  • 19.6k
27 votes

Are there $n$ groups of order $n$ for some $n>1$?

A "near-miss" is $N(19328) = 19324$, while the only $n \leq 2000$ such that $|N(n)-n| \leq 25$ are $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$, $10$, $11$, $12$, $13$, $14$, $15$, $16$, $17$, $18$, $1 …
Stefan Kohl's user avatar
  • 19.6k
22 votes
1 answer
1k views

Numbers of distinct products obtained by permuting the factors

Let $n \in \mathbb{N}$. Is it true that for every $k \in \{1, \dots, n!\}$ there are some group $G$ and pairwise distinct elements $g_1, \dots, g_n \in G$ such that the set $\{g_{\sigma(1)} \cdot \ \d …
Stefan Kohl's user avatar
  • 19.6k
22 votes

Small-index subgroups of SL(3,Z)

In order to answer the question we need a finite presentation of ${\rm SL}(3,\mathbb{Z})$ and a general method to find all subgroups of index $\leq n$ of a finitely presented group: A finite present …
Stefan Kohl's user avatar
  • 19.6k
21 votes
3 answers
930 views

Primes occurring as orders of elements of a finitely presented group

Is it true that given a finitely presented group $G$, either all primes or only finitely many of them occur as orders of elements of $G$?
Stefan Kohl's user avatar
  • 19.6k
21 votes
1 answer
562 views

Partitions of ${\rm Sym}(\mathbb{N})$ induced by convergent, but not absolutely convergent s...

Let $(a_n) \subset \mathbb{R}$ be a sequence such that the series $\sum_{n=1}^\infty a_n$ converges, but does not converge absolutely. Then there is a partition of the symmetric group ${\rm Sym}(\math …
Stefan Kohl's user avatar
  • 19.6k
18 votes

Why do sporadic simple groups have so few conjugacy classes?

This is also rather an expanded comment. -- Since for purely arithmetical reasons, $\ln(\ln(|G|))$ is a lower bound for the number $k(G)$ of conjugacy classes of a finite group $G$, maybe $$ f(G) := …
Stefan Kohl's user avatar
  • 19.6k
17 votes

Is there an infinite group with exactly two conjugacy classes?

The answer to the question is yes, even if one additionally requires the group to be finitely generated. In this case, the question is Problem 9.10 in the Kourovka Notebook, and has been answered in: …
Stefan Kohl's user avatar
  • 19.6k
17 votes
0 answers
965 views

Groups generated by 3 involutions

Let $r(m)$ denote the residue class $r+m\mathbb{Z}$, where $0 \leq r < m$. Given disjoint residue classes $r_1(m_1)$ and $r_2(m_2)$, let the class transposition $\tau_{r_1(m_1),r_2(m_2)}$ be the permu …
Stefan Kohl's user avatar
  • 19.6k
16 votes
1 answer
913 views

Is it true that every f.g. infinite simple group has exponential growth?

Is it true that every finitely generated infinite simple group has exponential (word-)growth? Remark: As Mark Sapir has pointed out, the question whether every finitely generated group of subexponent …
Stefan Kohl's user avatar
  • 19.6k
16 votes

Conjugacy classes of $\mathrm{SL}_2(\mathbb{Z})$

The conjugacy classes of elements of ${\rm SL}(2,\mathbb{Z})$ with given trace are counted in: S. Chowla, J. Cowles and M. Cowles: On the number of conjugacy classes in SL(2,Z). Journal of Number …
Stefan Kohl's user avatar
  • 19.6k
15 votes
1 answer
1k views

Free subgroups of $\mathrm{GL}(2,\mathbb{Z})$

Is there a bound $B$ such that every 2-generator subgroup $G = \langle a, b \rangle \le {\rm GL}(2,\mathbb{Z})$ whose generators do not satisfy a relation of length $\leq B$ is free? If it exists, su …
Stefan Kohl's user avatar
  • 19.6k

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