Checking that U(n) is a set that is closed under multiplication modulo n
90332
Prove that that $U(n)$ is an abelian group.
94825
Open set as a finite union of closed sets
121304
Is the set of polynomials of degree less than or equal to $n$ closed?
148824
Is associative binary operator closed on this subset?
140103
If product of two sets $A\times B$ is closed, are $A$ and $B$ closed?
21431
The set of all functions from $\mathbb{N} \to \{0, 1\}$ is uncountable?
143457
Is product of two closed sets closed?
153072
Guessing a subset of $\{1,...,N\}$
78117
Closed property of nonempty finite set
70959
Prove that a set $E$ is closed iff it's complement $E^{c}$ is open
55614
Prove that a set consisting of a sequence and its limit point is closed
48162
Is the subset $[0, \sqrt2] ∩\mathbb{Q} ⊂ \mathbb{Q}$ closed, bounded, compact?
72402
Summation of natural number set with power of $m$
153934
How to prove that a set R\Z is open
47697
Russell's paradox: a set cannot contain its own powerset
30079
Is for open connected $U$ the set $U_\varepsilon$ for small $\varepsilon$ connected?
124060
Are these subsets of the powerset P(N) countable?