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Questions on group theory which concern finite groups.

15 votes

The mysterious significance of local subgroups in finite group theory

There is indeed a strong analogy between the study of $p$-local subgroups and the theory of buildings, at least for groups of Lie type. More precisely, if $G$ is a finite group of Lie type over a fiel …
Tom De Medts's user avatar
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3 votes
Accepted

How to classify homomorphisms from $\operatorname{PSL}(2,p)$ to $\operatorname{PGL}(n,2)$ wh...

The map $T \colon \mathrm{PSL}_2(p) \to \operatorname{Sym}(\mathbb{F}_{2^n}) \colon f \mapsto T_f$ does not have its image in $\mathrm{GL}_n(2)$ for other Mersenne primes $p = 2^n - 1$, unlike the cas …
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1 vote

Twisted root subgroups in twisted Chevalley groups (reference request)

As indicated in Martin Seysen's comment, this construction can be found in Carter's book "Simple Groups of Lie type". More precisely, this is Proposition 13.6.3, and your "naive approach" is indeed ex …
Tom De Medts's user avatar
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24 votes

Order of product of group elements

The following theorem (which does not take the order $N$ of the group $G$ into account) shows that all possible combinations of $a$, $b$ and the order of $xy$ are possible. See Theorem 1.64 from Milne …
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9 votes
2 answers
441 views

Embedding $\mathrm{PGL}(n,q^h)$ in $\mathrm{PGL}(nh,q)$

It is not very hard to see that for each prime power $q$ and natural numbers $n,h$, we have an embedding $$\iota \colon \mathrm{GL}(n,q^h) \hookrightarrow \mathrm{GL}(nh, q),$$ obtained by choosing a …
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4 votes

Orthogonal Groups over finite fields

I think it's worth adding that there is a very detailed analysis of the orthogonal groups over arbitrary fields (not just finite fields, and including characteristic 2) in Dieudonné's "La Géométrie de …
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3 votes
Accepted

Example of a finite group

The group $G = C_2 \times C_2 \times C_3 \times C_3$ has this property (for $p=3$).
Tom De Medts's user avatar
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37 votes
2 answers
2k views

Order-increasing bijection from arbitrary groups to cyclic groups

In his answer to this previous MO question, Gjergji Zaimi referred to the statement that for every finite group $G$ of order $n$, there is a bijection $\sigma \colon G \to \mathbb{Z}/n\mathbb{Z}$ sati …
Tom De Medts's user avatar
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7 votes
Accepted

Cyclic subgroups of finite abelian groups

I think that you can find the formulas that you are looking for in the paper "An arithmetic method of counting the subgroups of a finite abelian group" by Marius Tarnauceanu, Bull. Math. Soc. Sci. Mat …
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9 votes

Spherical building of an exceptional group of Lie type

In the case of groups of rank 2, such as your examples $\mathrm{SL}_3(\mathbb{F}_2)$ or $\mathsf{G}_2(3)$, the building is rather easy to describe (either as an incidence geometry or as a bipartite gr …
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5 votes

Classifications of finite simple objects

Somewhat related to Igor Pak's comment is the classification of the finite irreducible Coxeter groups. Of course they are not "simple" as groups, but the irreducibility seems the natural replacement f …
4 votes
1 answer
442 views

Finding groups of odd order without non-cyclic nilpotent quotients

I hope that my question is appropriate for MO, since it might turn out te be mainly a question about GAP or other group theory software. Is there an algorithm to produce all non-nilpotent groups o …
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12 votes
Accepted

Subgroups of groups of Square-free order

Yes, $G$ always contains a cyclic subgroup of composite order. Note that all Sylow subgroups of $G$ are cyclic, i.e. $G$ is a Zassenhaus metacyclic group. Such groups have a very precise structure: th …
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11 votes

Automorphisms of non-abelian groups of order $ p^3$

The former group can be seen as the group of unitriangular $3 \times 3$-matrices over the field with $p$ elements: $$G = \left\{ \begin{pmatrix} 1 & * & * \\ 0 & 1 & * \\ 0 & 0 & 1 \end{pmatrix} \righ …
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1 vote
Accepted

A problem in Finite Group Theory

You were almost there: Since $N$ and $F(G)$ are two normal subgroups intersecting trivially, they commute. But now take a non-trivial element $a \in A \cap F(G)$; then by the previous observation, $a$ …
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