Partially Ordered Set that is NOT a lattice?
39129
Simplest Example of a Poset that is not a Lattice
68230
The cardinality of a countable union of sets with less than continuum cardinality
100919
Examples of non-measurable sets in $\mathbb{R}$
44430
Is there an open dense set $S \subset [0,1]$ such that $m(S)<1$?
133762
A set which satisfies all conditions for a Group except associativity
70143
Mutually exclusive countable subsets of a countable set
90826
Constructing a subset not in $\mathcal{B}(\mathbb{R})$ explicitly
4213
The set that only contains itself
85338
Equivalence relations on classes instead of sets
19924
Uncountable ordinals without power set axiom
52451
Ordered Triples and Set Theory
104550
Countable union of countable sets(ZF)
33117
Cyclic containment of sets
28804
No. of possible dense subsets of a metric space
89749
Example of a not recursively enumerable set $A \subseteq \mathbb{N}$
130771
Sets and the principle of inclusion and exclusion
124732
Finding a nonempty subset that is not a subgroup