A construction of sigma-algebras - surely not new, right?
92309
The $\sigma$-algebra of subsets of $X$ generated by a set $\mathcal{A}$ is the smallest sigma algebra including $\mathcal{A}$
41937
Can one construct a non-measurable set without Axiom of choice?
80996
A canonical construction proving $\left|HK\right| = \left|H\right|\left|K\right| / \left|H \cap K\right|$?
80735
Show that $C_0([a, b], \mathbb{R})$ is not $\sigma$-compact
10702
Show that $\left\{A\subset\Omega: A~\text{ is countable or }A^C\text{ is countable}\right\}$ is a $\sigma$-algebra
106101
Why there is much interest in the study of $\operatorname{Gal}\left(\overline{\mathbb Q}/\mathbb Q\right)$?
20789
If $\Sigma \vdash \varphi$ then $\Sigma \vdash \forall x \varphi$. Why when $(\varphi \implies \forall x \varphi)$ is not true?
136083
Show that $\sigma\circ\phi : G_1 \to G_3$ is an isomorphism
153542
Fixed Field of $\sigma, \tau$
101453
Existence of a Boolean algebra with a unique ultrafilter in ZF
74162
Show intersection of two algebras are not a $\sigma$-algebra
151251
A simple algebra that is not semisimple
26369
Proving seemingly obvious equations with measure theory
43894
If $S$ is an infinite $\sigma$ algebra on $X$ then $S$ is not countable
53642
Cardinality of Borel sigma algebra
51134
If $E \in \sigma(\mathcal{C})$ then there exists a countable subset $\mathcal{C}_0 \subseteq \mathcal{C}$ with $E \in \sigma(\mathcal{C}_0)$
48524
Existence proof of the tensor product using the Adjoint functor theorem.