Horizontal line test
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In mathematics, the horizontal line test is a test used to determine if a function is injective, surjective or bijective.
Suppose there is a function f : X → Y with a graph., and you have a horizontal line of X x Y :[y_0 \in Y, \ = (X \times y_0) ] .
- If the function is injective, then it can be visualized as one whose graph is never intersected by any horizontal line more than once.
- Iff f is surjective any line will intersect the graph at least at one point
- If f is bijective any line will intersect the graph at exactly one point.
 Passes the test (injective) |  Fail the test (not injective) |
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