Which elements in $\mathbb{Z}_{26}$ do not have multiplicative inverses?
31917
If $q$ is coprime to $a$ then $a\mid nq-1,\,$ so $q$ is invertible mod $a$
24413
Why is $\mathbb{Z}[\sqrt{-n}], n\ge 3$ not a UFD?
143069
What are the direct summands in $\mathbb{Z}^n$?
94747
Show that the rings $2\mathbb{Z}$ and $3\mathbb{Z}$ are not isomorphic.
154845
Irreducible elements in  $\mathbb{Z}[\sqrt{-2}]$ and is it a Euclidean domain?
38911
Squares in $\mathbb{Z}_p$ that are not squares in $\mathbb{Z}$
132386
Show that $(\mathbb{Q}^*,\cdot)$ and $(\mathbb{R}^*,\cdot)$ aren't cyclic
93031
How to show that $\mathbb{Z}$ with the operation $m*n=m+(-1)^m n$ is associative?
14003
$(\mathbb{Z}/2^n \mathbb{Z})^*$ is not cyclic Group for $n\geq 3$
8434
Order of $\mathbb{Z}[i]/(1+i)$
125469
What are the generators of $\mathbb{Z}_9^*$?
125604
Proof that $\mathbb{Z}$ has no zero divisors
135241
Show that $\mathbb{Z}/n^2\mathbb{Z}$ is not isomorphic to other group
35034
Multiplicative inverses for $Z_n$
61292
How to know which elements are in the Quotient Ring $\mathbb{Z}[x]/\langle x, 2\rangle$?
88645
Prove that the additive groups $\mathbb{R}$ and $\mathbb{Q}$ are not isomorphic.
138258
What is the injective envelope of $\mathbb{Z}/n\mathbb{Z}$?