Long Rightwards Arrow From Bar

It says the expression on the left is assigned to the right.

Read x ⟼ x² as “x maps to x squared”: x is the input, and x² is the output assigned to it. A mapsto arrow marks this input-to-output assignment in a formula.

How do I type longmapsto?

In LaTeX math mode, use \longmapsto.

MathML, a markup format for mathematical notation, recognizes longmapsto as this operator’s name. The LaTeX mathtools package also provides \xmapsto{...}; its arrow stretches to fit a label above it.

Why is the mapsto arrow long?

Its long span marks a mapping form.

Unicode’s mathematical guidance associates long arrows like ⟼ with mappings; shorter arrows are often used with limits, which describe values a formula approaches. The short counterpart ↦ has the same maps-to reading, while ⟼ spans a longer expression.

Function-arrow notation developed over time; no single inventor of the barred form is securely established. A function-arrow notation appears in a paper by mathematicians Witold Hurewicz and Norman Steenrod submitted in 1940. Around 1959, mathematician Alexander Grothendieck used an earlier form without the bar. Bourbaki, a collective of French mathematicians, was using the barred notation in its materials in 1963–64; a 1965 edition of its treatise also uses it.