$\mathbb{Q}$ is not isomorphic to $\mathbb{Q}^+$
86542
Group of positive rationals under multiplication not isomorphic to group of rationals
99884
Are $\mathbb{Q}_p$ and $\mathbb{Q}_q$ homeomorphic?
64179
What is the difference between the standard representants of $\mathbb Z/q\mathbb Z$?
143831
Showing $\mathbb{Q}$ is homeomorphic to $\mathbb{Q}^2$
156679
$K[X,Y]/(XY-1)$ not isomorphic to $K[T]$
69046
Proof of $\mathbb{Q}$ is not cyclic
134686
$D_3\oplus D_4$ not isomorphic to $D_{24}$
93169
Show that $(\mathbb Z[x],+)$ and $(\mathbb Q_{>0},\cdot)$ are isomorphic groups
75970
$\mathbb Z[1/2]$ is not finitely generated?
82046
prove that this double transpose $T''$ it's not injective.
84114
Does there exist a surjective homomorphism from $(\mathbb R,+)$ to $(\mathbb Q,+)$ ?
5996
Are $(\mathbb{R},+)$ and $(\mathbb{C},+)$ isomorphic as additive groups?
45627
Find a sub-ring of M(2, $\mathbb{Q}$) which is isomorphic to $\mathbb{Q}$
124864
A not free $\mathbb{Z}$-module
157915
Isomorphism between the group $(\mathbb Z[x], +)$ and $(\mathbb Q_{>0}, .)$
132386
Show that $(\mathbb{Q}^*,\cdot)$ and $(\mathbb{R}^*,\cdot)$ aren't cyclic
98731
Show that there is no epimorphism from ($\mathbb{Q}$, +) to ($\mathbb{Z}$, +)