Intersection of two $\sigma$-algebras is not an algebra
74162
Show intersection of two algebras are not a $\sigma$-algebra
88474
$\mathbb{A}^{2}$ not isomorphic to affine space minus the origin
149412
Every prime ideal of a finitely generated $\mathbb{R}$-algebra is an intersection of maximal ideals?
1720
Show that $\langle 2,x \rangle$ is not a principal ideal in $\mathbb Z [x]$
28819
A union of two subspaces not equal to the vector space.
64118
Number of elements in a finite $\sigma$-algebra
105333
Generated $\sigma$-algebras with cylinder set doesn't contain the space of continuous functions
70373
Intersection of normal subgroups proof
138419
Sigma algebra and algebra difference
132653
How to show that $\mathfrak{sl}_n(\mathbb{R})$ and $\mathfrak{sl}_n(\mathbb{C})$ are simple?
46121
Does the intersection of two finite index subgroups have finite index?
11301
Union of two vector subspaces not a subspace?
104308
Proving a set is a Sigma Algebra
146006
Uncountable $\sigma$-algebra
80302
A module $M$ whose submodules and factor modules are semisimple but not semisimple itself
88099
Not $\sigma$-compact set without axiom of choice
47159
Wanted: example of an increasing sequence of $\sigma$-fields whose union is not a $\sigma$-field