Every abelian group is a $\mathbb{Z}$-module. I was curious what it would take for one to also be a $\mathbb{Q}$-module. This would mean in particular that an element of the group would have to multiplied by fractions $1/n$, and associativity of scalar multiplication requires that for any $g \in G$, $$ n \left(\frac{1}{n} \left( g \right) \right) = \left( n \cdot \frac{1}{n} \right) g = 1_\mathbb{Q} g = g $$ Therefore, for every integer $n > 0$ and $g \in G$, there must be an element $h$ such that $nh = g$. Equivalently, the map $f_n : G \to G$ sending $g \mapsto ng$ must be a surjection. It follows immediately that $G$ cannot be a periodic group; otherwise $f_{e}$ (where $e$ is the exponent of $G$) would send every $g$ to $1_G$. In particular, it contains an isomorphic copy of $\mathbb{Z}$ via the embedding $n \mapsto n \cdot 1_G$. My question is, does it also contain an isomorphic copy of $\mathbb{Q}$? It seems intuitively clear that it must, since it contains the integers and also must contain a multiplicative inverse of every integer -- i.e. an element $n^{-1} = \frac{1}{n} \cdot 1_G$ such that $n \cdot n^{-1} = n^{-1} + \cdots + n^{-1} = 1$. However, I can imagine that maybe for some groups $G$ there are other constructions that add in such inverses without ending up with an embedding of $\mathbb{Q}$.
2026-04-09 00:48:58.1775695738
Does every $\mathbb{Q}$-module contain a copy of $\mathbb{Q}$?
51 Views Asked by Bumbble Comm https://math.techqa.club/user/bumbble-comm/detail AtRelated Questions in GROUP-THEORY
- What is the intersection of the vertices of a face of a simplicial complex?
- Group with order $pq$ has subgroups of order $p$ and $q$
- How to construct a group whose "size" grows between polynomially and exponentially.
- Conjugacy class formula
- $G$ abelian when $Z(G)$ is a proper subset of $G$?
- A group of order 189 is not simple
- Minimal dimension needed for linearization of group action
- For a $G$ a finite subgroup of $\mathbb{GL}_2(\mathbb{R})$ of rank $3$, show that $f^2 = \textrm{Id}$ for all $f \in G$
- subgroups that contain a normal subgroup is also normal
- Could anyone give an **example** that a problem that can be solved by creating a new group?
Related Questions in MODULES
- Idea to make tensor product of two module a module structure
- $(2,1+\sqrt{-5}) \not \cong \mathbb{Z}[\sqrt{-5}]$ as $\mathbb{Z}[\sqrt{-5}]$-module
- Example of simple modules
- $R$ a domain subset of a field $K$. $I\trianglelefteq R$, show $I$ is a projective $R$-module
- $S_3$ action on the splitting field of $\mathbb{Q}[x]/(x^3 - x - 1)$
- idempotent in quiver theory
- Isomorphism of irreducible R-modules
- projective module which is a submodule of a finitely generated free module
- Exercise 15.10 in Cox's Book (first part)
- direct sum of injective hull of two modules is equal to the injective hull of direct sum of those modules
Related Questions in RATIONAL-NUMBERS
- What is the smallest integer $N>2$, such that $x^5+y^5 = N$ has a rational solution?
- I don't understand why my college algebra book is picking when to multiply factors
- Non-galois real extensions of $\mathbb Q$
- A variation of the argument to prove that $\{m/n:n \text{ is odd },n,m \in \mathbb{Z}\}$ is a PID
- Almost have a group law: $(x,y)*(a,b) = (xa + yb, xb + ya)$ with rational components.
- When are $\alpha$ and $\cos\alpha$ both rational?
- What is the decimal form of 1/299,792,458
- Proving that the sequence $\{\frac{3n+5}{2n+6}\}$ is Cauchy.
- Is this a valid proof? If $a$ and $b$ are rational, $a^b$ is rational.
- What is the identity element for the subgroup $H=\{a+b\sqrt{2}:a,b\in\mathbb{Q},\text{$a$ and $b$ are not both zero}\}$ of the group $\mathbb{R}^*$?
Trending Questions
- Induction on the number of equations
- How to convince a math teacher of this simple and obvious fact?
- Find $E[XY|Y+Z=1 ]$
- Refuting the Anti-Cantor Cranks
- What are imaginary numbers?
- Determine the adjoint of $\tilde Q(x)$ for $\tilde Q(x)u:=(Qu)(x)$ where $Q:U→L^2(Ω,ℝ^d$ is a Hilbert-Schmidt operator and $U$ is a Hilbert space
- Why does this innovative method of subtraction from a third grader always work?
- How do we know that the number $1$ is not equal to the number $-1$?
- What are the Implications of having VΩ as a model for a theory?
- Defining a Galois Field based on primitive element versus polynomial?
- Can't find the relationship between two columns of numbers. Please Help
- Is computer science a branch of mathematics?
- Is there a bijection of $\mathbb{R}^n$ with itself such that the forward map is connected but the inverse is not?
- Identification of a quadrilateral as a trapezoid, rectangle, or square
- Generator of inertia group in function field extension
Popular # Hahtags
second-order-logic
numerical-methods
puzzle
logic
probability
number-theory
winding-number
real-analysis
integration
calculus
complex-analysis
sequences-and-series
proof-writing
set-theory
functions
homotopy-theory
elementary-number-theory
ordinary-differential-equations
circles
derivatives
game-theory
definite-integrals
elementary-set-theory
limits
multivariable-calculus
geometry
algebraic-number-theory
proof-verification
partial-derivative
algebra-precalculus
Popular Questions
- What is the integral of 1/x?
- How many squares actually ARE in this picture? Is this a trick question with no right answer?
- Is a matrix multiplied with its transpose something special?
- What is the difference between independent and mutually exclusive events?
- Visually stunning math concepts which are easy to explain
- taylor series of $\ln(1+x)$?
- How to tell if a set of vectors spans a space?
- Calculus question taking derivative to find horizontal tangent line
- How to determine if a function is one-to-one?
- Determine if vectors are linearly independent
- What does it mean to have a determinant equal to zero?
- Is this Batman equation for real?
- How to find perpendicular vector to another vector?
- How to find mean and median from histogram
- How many sides does a circle have?