∀
i
∈
R
\forall i \in R
∀i∈R,
∃
i
∈
R
\exists i \in R
∃i∈R,
i
⊂
R
i \subset R
i⊂R,
∅
\empty
∅,
A
∩
B
A\cap B
A∩B,
A
∪
B
A\cup B
A∪B,