The LaTeX command for twoheadleftarrow (↞) is \twoheadleftarrow.
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Write \twoheadleftarrow inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amssymb package, so add \usepackage{amssymb} to your preamble.
Arrows are relations in LaTeX, so \twoheadleftarrow gets the same thick spacing as an equals sign. With unicode-math under XeLaTeX or LuaLaTeX, typing ↞ in a formula gives the same symbol.
$A \twoheadleftarrow B$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\swarrow\nwarrow\mapsto\longmapsto\hookrightarrow\hookleftarrow\twoheadrightarrow\rightharpoonup\rightharpoondown\leftrightharpoons\rightleftharpoons\dashrightarrow\circlearrowleft\circlearrowrightArrows in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \twoheadleftarrow in math mode after adding \usepackage{amssymb} to your preamble. For example, $A \twoheadleftarrow B$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.