$\mathbb Z[\sqrt{-5}]$ is not a UFD
24413
Why is $\mathbb{Z}[\sqrt{-n}], n\ge 3$ not a UFD?
78351
Why is $(2, 1+\sqrt{-5})$ not principal?
104486
Ideal in $\mathbb Z[x]$ which is not two-generated
85032
$Q(\mathbb{Z}[t]) / \mathbb{Z}[t]$ is not injective
1020
Proving that $\mathbb{Z}[\sqrt{2}]$ is a Euclidean domain
119545
Nonzero rationals under multiplication are not a cyclic group
79056
Prove that $\mathbb Z^n$ is not isomorphic to $\mathbb Z^m$ for $m\neq n$
50241
Cardinality of the quotient ring $\mathbb Z_5[i]/\langle 1+i\rangle$
11935
Is $\mathbb{Z} \left[ \frac{1+i \sqrt{7}}{2} \right]$ euclidean?
17991
Nonconstant polynomials do not generate maximal ideals in $\mathbb Z[x]$
85586
Proving $\mathbb{Q}$ isn't homeomorphic to $\mathbb{Z}$
151439
$\sqrt{1-x^2}$ is not a rational function
76032
Prove that if m is a square integer then m is neither congruent to 2 modulo 5 nor congruent to 3 modulo 5
75970
$\mathbb Z[1/2]$ is not finitely generated?
135241
Show that $\mathbb{Z}/n^2\mathbb{Z}$ is not isomorphic to other group
116630
Antiderivative $1/z$ on $\mathbb C$
3296
Proof That $\mathbb{R} \setminus \mathbb{Q}$ Is Not an $F_{\sigma}$ Set