A proof that $\mathbb{Z}[\frac{1 +i\sqrt{19}}{2}]$ is not Euclidean.
24351
The ring $\Bbb Z\left [\frac{-1+\sqrt{-19}}{2}\right ]$ is not a Euclidean domain
1020
Proving that $\mathbb{Z}[\sqrt{2}]$ is a Euclidean domain
75970
$\mathbb Z[1/2]$ is not finitely generated?
14447
Direct proof that $\pi$ is not constructible
18484
Show that $k[x,y,z]/(xz-y^2)$ is not a UFD.
23879
If $m$ and $n$ are distinct positive integers, then $m\mathbb{Z}$ is not ring-isomorphic to $n\mathbb{Z}$
64130
Prove that $\mathbb{Q}^{\times}$ not isomorphic to $\mathbb{Z}^{n}$
40400
Finding an isomorphism between $\mathbb{Z}[i]/(7)$ and $\mathbb{Z}[\sqrt{-2}]/(7)$
66238
Proving $\mathbb{Z}[\sqrt {10}]$ is not a UFD
24601
Proving that a natural number made entirely of 6's and 0's is not a square.
112043
Show that the ideal $ (2, 1 + \sqrt{-7} ) $ in $ \mathbb{Z} [\sqrt{-7} ] $ is not principal
35569
Prove that $[0,1]$ isn't homeomorphic to $\mathbb{R}$
1720
Show that $\langle 2,x \rangle$ is not a principal ideal in $\mathbb Z [x]$
41051
How to show $e^{e^{e^{79}}}$ is not an integer
70576
Elementary proof that $\mathbb{R}^n$ is not homeomorphic to $\mathbb{R}^m$
8589
$\sqrt[b_1]{a_1}+\sqrt[b_2]{a_2}+\cdots+\sqrt[b_k]{a_k}$ not an integer
67605
Different proof that $\sqrt{2}$ is irrational