$\{\}$

symbol of set

$U$

universal set

$n(X)$

cardinal number of set

$b ∈ A$

$b$ is element of $A$

$a ∉ B$

$a$ is not element of $B$

$∅$

null set

$A U B$

union of $A$ and $B$

$A ∩ B$

intersection of $A$ and $B$

$A ⊆ B$

$A$ is subset of $B$

$B ⊇ A$

set $B$ is superset of $A$