How the order of $G/2G$ is $2^n$? Let $G $ be an abelian group. In this section we shall use additive notation
for $G$, so the group operation will be denoted by ' $+$ ' the identity by $0$,
the inverse of $g$ by $-g$ and powers of $g$ by $2g, 3g, ....$
Let $2G$ be the subgroup of $G$ consisting of all elements of the form $g + g$ $(g \in  G)$. If $G$ has a basis of $n$ elements, then $G/2G$ is a group of order $2^n$.
How the order of $G/2G$ is  $2^n$? Plz show in an elementary way.
This an excerpt from "Algebraic number theory and Fermats last theorem by  Ian Stewart, David Tall (3rd ed.).
 A: "... G has a basis of $n$ elements..." to me implies that $G$ is a free Abelian group of rank $n$, i.e. there is a basis made of $n$ linearly independent generators $g_1, g_2,\ldots,g_n\in G$; every element of $G$ can be uniquely represented as $m_1g_1+m_2g+\ldots+m_ng_n$ where $m_i\in\mathbb Z$ are "co-ordinates" in this basis.
(Recall we use the same terms in linear algebra, but the coefficients there come from a field, while here they come from the ring $\mathbb Z$ which is not a field. This makes calculations slightly more complicated.)
Now, note $2G$ is the subset of those elements of $G$ which have even co-ordinates, i.e. all $m_i$'s are even. That means that $G/2G$ is made up of "cosets" of the form $\lambda_1g_1+\ldots+\lambda_ng_n+2G$ where $\lambda_i\in\{0,1\}$. (Every $m_1g_1+\ldots+m_ng_n$ belongs to the coset $\lambda_1g_1+\ldots+\lambda_ng_n+2G$ where $\lambda_i =\text{ remainder in dividing }m_i\text{ by }2$). Thus, the number of those cosets (which is the order of $G/2G$) is $2^n$.
Hope this helps!
