\twoheadrightarrowThe symbol \twoheadrightarrow is used to denote a surjective (onto) function or mapping in mathematics. It is an arrow with two heads, indicating that every element in the codomain is mapped to by at least one element in the domain.
A surjective function from set X to set Y
f: X \twoheadrightarrow YCanonical projection from a group G to its quotient group G/N
\pi: G \twoheadrightarrow G/NA surjective linear transformation from vector space V to vector space W
\phi: V \twoheadrightarrow W