Is $A = \{ B \subset E: B \text{ countable or } B^c \text{ countable}\}$ a sigma algebra?
10702
Show that $\left\{A\subset\Omega: A~\text{ is countable or }A^C\text{ is countable}\right\}$ is a $\sigma$-algebra
51134
If $E \in \sigma(\mathcal{C})$ then there exists a countable subset $\mathcal{C}_0 \subseteq \mathcal{C}$ with $E \in \sigma(\mathcal{C}_0)$
110635
Proving $\mathbb{N}^k$ is countable
155063
Is Cl(f(A)) a subset of f(Cl(A))?
147136
Subset of a countable set is itself countable
147486
Cartesian product of two $\sigma$-algebras is generally not a $\sigma$-algebra?
67637
How come $\lambda(\mathbb{Q})= \inf\left\{\sum_{n \ge 1} \lambda([a_n,b_n)) : \mathbb{Q} \subset \bigcup_{n \ge 1} [a_n,b_n)\right\} = 0$?
118595
Proving that a set is countable by finding a bijection
155168
Is the collection of finite subsets of $\mathbb{Z}$ countable?
37398
Proof that$ (a+A)\cap A=\varnothing$ if $A$ is countable
55717
cardinality of the Borel $\sigma$-algebra of a second countable space
63218
Is conditional expectation with respect to two sigma algebra exchangeable?
110450
probability measure on $\sigma$-algebra
146951
Every countably generated sigma algebra is generated by a random variable
16892
Reference request: product Borel $\sigma$-algebra of non-separable metric spaces
10247
What is the minimum $ \sigma$-algebra that contains open intervals with rational endpoints
133266
If $(X, \mathcal{T})$ has a countable subbasis, then it has a countable basis