You can find out if a function is injective by graphing it.An injective function must be continually increasing, or continually decreasing. from increasing to decreasing), so it isn’t injective. \$\begingroup\$ See Horizontal line test: "we can decide if it is injective by looking at horizontal lines that intersect the function's graph." If the horizontal line crosses the function AT LEAST once then the function is surjective. The \horizontal line test" is a (simplistic) tool used to determine if a function f: R !R is injective. Example. In the example shown, =+2 is surjective as the horizontal line crosses the function … If a horizontal line can intersect the graph of the function only a single time, then the function … With this test, you can see if any horizontal line drawn through the graph cuts through the function more than one time. Injective = one-to-one = monic : we say f:A –> B is one-to-one if “f passes a horizontal line test”. You can also use a Horizontal Line Test to check if a function is surjective. "Line Tests": The \vertical line test" is a (simplistic) tool used to determine if a relation f: R !R is function. An injective function can be determined by the horizontal line test or geometric test. The horizontal line test lets you know if a certain function has an inverse function, and if that inverse is also a function. The first is not a function because if we imagine that it is traversed by a vertical line, it will cut the graph in two points. If f(a1) = f(a2) then a1=a2. Look for areas where the function crosses a horizontal line in at least two places; If this happens, then the function changes direction (e.g. This means that every output has only one corresponding input. See the horizontal and vertical test below (9). Horizontal Line Testing for Surjectivity. The function f: R !R given by f(x) = x2 is not injective as, e.g., ( 21) = 12 = 1. \$\endgroup\$ – Mauro ALLEGRANZA May 3 '18 at 12:46 1 The second graph and the third graph are results of functions because the imaginary vertical line does not cross the graphs more than once. If no horizontal line intersects the function in more than one point, the function is one-to-one (or injective). If a horizontal line intersects the graph of the function, more than one time, then the function is not mapped as one-to-one. Only one-to-one functions have inverses, so if your line hits the graph multiple times then don’t bother to calculate an inverse—because you won’t find one. Examples: An example of a relation that is not a function ... An example of a surjective function … All functions pass the vertical line test, but only one-to-one functions pass the horizontal line test. 2. 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