\longleftrightarrowA long double-headed arrow pointing both left and right, often used to denote logical equivalence or a bidirectional relationship between two mathematical expressions.
Logical equivalence between statements A and B
A \longleftrightarrow BBidirectional relationship between variables x and y
x \longleftrightarrow yEquivalence of predicates P and Q for variable x
P(x) \longleftrightarrow Q(x)