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In mathematics, the horizontal line test is a test used to determine whether a function is injective (i.e., one-to-one). ==In calculus== A ''horizontal line'' is a straight, flat line that goes from left to right. Given a function (i.e. from the real numbers to the real numbers), we can decide if it is injective by looking at horizontal lines that intersect the function's graph. If any horizontal line intersects the graph in more than one point, the function is not injective. To see this, note that the points of intersection have the same y-value (because they lie on the line ) but different x values, which by definition means the function cannot be injective.〔 Variations of the horizontal line test can be used to determine whether a function is surjective or bijective: *The function ''f'' is surjective (i.e., onto) if and only if its graph intersects any horizontal line at least once. *''f'' is bijective if and only if any horizontal line will intersect the graph exactly once. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Horizontal line test」の詳細全文を読む スポンサード リンク
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