Questions tagged [finitely-generated]
For questions regarding finitely generated groups, modules, and other algebraic structures. A structure is called finitely generated if there exists a finite subset that generates it.
843 questions
2 votes
0 answers
34 views
An inclusion-exclusion-like inequality for the Hirsch length
Let $G$ be a virtually polycyclic group with subgroups $H$ and $K$. Let $h(\,.)$ denote the Hirsch length. Does the inequality $$h(G) \geq h(H) + h(K) - h(H \cap K)$$ always hold? (I've found this ...
0 votes
0 answers
61 views
Invariance of the length and the factors (up to isomorphism) in the composition series of a module
I aim to break down the proof of the Jordan–Hölder theorem in the context of finitely generated $R$-modules. Let $R$ be a commutative ring with $1$, and let $M$ be an $R$-module. In the lecture, we ...
1 vote
1 answer
74 views
How can we show that $- \otimes M$ preserves all limits if and only if $M$ is finitely generated projective?
Let $R$ be a commutative ring with unity and let $R \operatorname{-Mod}$ be the category of $R$-modules. Question: How can we show that for an $R$-module $M$, the following are equivalent? The ...
0 votes
1 answer
68 views
question about generated modules, is my solution wrong? [closed]
Let $f, g : X\to Y$ be two homomorphisms of a module $X$ over $R$ into a module $Y$ over $R$ such that $f(s) = g(s)$ for every element $s$ of a subset $S$ of $X.$ Prove that $f(x) = g(x)$ for every ...
4 votes
1 answer
70 views
Solving systems of linear equations over finitely generated modules of non-PID rings
Consider solving a linear system of the form $M\mathbf{x} = \mathbf{b}$ where the coefficient matrix has entries in $\mathbb{Z}[\sqrt{-6}]$, which is a Dedekind domain but not a principal ideal domain....
2 votes
1 answer
129 views
Information required to determine a group given a minimal generating set
Say we have a finite group $G$ generated by $\langle g_1,g_2,\ldots ,g_n\rangle$ which is a minimal generating set. What information is required from this set in order to determine the whole group? ...
6 votes
1 answer
160 views
Seifert-Van Kampen and nontrivial elements in amalgamated products [duplicate]
As a comment to this question, I was encouraged to ask the following group-theoretic question separately. Consider the three groups $G$, $G'$ and $G''$, and the homomorphisms $$\phi: G \rightarrow G' \...
1 vote
1 answer
91 views
For a group $G$ generated by $S$, what is the normal closure of the group generated by $S^{-1}S$?
This is a follow-up to my recent question about $G'=\langle S^{-1} S\rangle$, where $S^{-1}S=\{s^{-1}t \vert s,t\in S\}$. $G'$ is a subgroup of $N = \{x\in G \vert x = s_1^{k_1} s_2^{k_2}\cdots s_r^{...
1 vote
1 answer
145 views
$1+ t\mathbb{F}_q[[t]]$ is finitely generated as topological group
Let $G=1+t\mathbb{F}_q[[t]]$ be the unipotent unit group of the ring $R=\mathbb{F}_q[[t]]$ of formal power series. This question is a continuation of a past question, and for more context see another ...
4 votes
1 answer
103 views
Action of group on connected CW complex implies finitely generated.
I'm studying finiteness properties of groups and in my research I found the following theorem, which is quite useful: $G$ is of type $F_n$ (i.e. there is a CW complex X with contractible universal ...
1 vote
0 answers
122 views
Why is the structure theorem for finitely generated abelian groups unique up to isomorphism?
Let $G$ be a finitely generated abelian group. Then, by the structure theorem, there exists an isomorphism of the form: $$ G \cong \mathbb{Z}^r \oplus \mathbb{Z}_{n_1} \oplus \mathbb{Z}_{n_2} \oplus \...
7 votes
1 answer
141 views
For a group $G$ generated by $S$, what can be said about the group generated by $S^{-1}S$?
Suppose $G$ is a group such that $G = \langle S \rangle$ for some finite subset $S\subset G$. Then let $G'=\langle S^{-1}S\rangle$, where $S^{-1}S = \{s^{-1} t \vert s,t \in S\}$. Of course, in ...
2 votes
1 answer
63 views
Is the intersection of co-finitely-generated subalgebras again co-finitely-generated?
Fix a commutative ring with unity $R$. All $R$-algebras are also assumed to be commutative and unital. Given an $R$-algebra $B$, a sub-$R$-algebra $A \subseteq B$ and finitely many elements $b_1, \...
6 votes
1 answer
137 views
Finding a presentation of the group generated by a translation and a rotation
In the group of isometries of the plane, let $r$ be a rotation by $\frac{2\pi}5$ radians and let $s$ be a translation. I'd like to find a finite presentation of the subgroup $G=\langle r,s\rangle$. ...
4 votes
2 answers
643 views
The term "finitely generated algebra"
Let $k$ be a field. I have seen people use the term "finitely generated $k$-algebra," which looked fine to me at first until I realized the term may have two different meanings. Let $A$ be ...