= Horizontal line test
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For a real-valued <function> drawn in the plane, the <horizontal line test> asks whether every horizontal <line> intersects its <function graph> at most once. This is exactly <injectivity>, since two intersections at the same height mean two inputs have the same output. A missing height is compatible with <injectivity>, so an <inverse function> is then defined on the <image of a function>; it is an inverse on the entire specified <codomain> only when <surjectivity> also holds.
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