Union of increasing $\sigma$-algebras which is no $\sigma$-Algebra
117185
The union of a strictly increasing sequence of $\sigma$-algebras is not a $\sigma$-algebra
128711
$(\Bbb Z\oplus \Bbb Z)[X] \not\cong \Bbb Z[X]\oplus \Bbb Z[X]\,$?
157597
Homeomorphism between $\mathbb{Q}$ and $\mathbb{Q}(>0)$, and $\mathbb{Q}(\ge 0)$
29756
Classsifying 1- and 2- dimensional Algebras, up to Isomorphism
53642
Cardinality of Borel sigma algebra
89990
Direct limit of arbitrary family of tensor products of A-algebras
84460
If a set is $F_\sigma$ and is of first category then it is a countable union of closed nowhere dense sets
70298
Are quotients of chain groups $C_n(X)/C_n(A)$ still free?
86542
Group of positive rationals under multiplication not isomorphic to group of rationals
116955
Showing the existence of a certain kind of extension of a finitely generated algebra
55717
cardinality of the Borel $\sigma$-algebra of a second countable space
14647
Conditional expectation on more than one sigma-algebra
129194
Correspondences between Borel algebras and topological spaces
120273
Showing that a collection of sets is a $\sigma$-algebra: either set or complement is countable
120442
Prove Borel sigma-algebra translation invariant
16048
Approximating a $\sigma$-algebra by a generating algebra
146006
Uncountable $\sigma$-algebra