Horizontal Line Test (Leaving Cert Mathematics): Revision Notes

Horizontal Line Test

The horizontal line test determines whether a function is injective (one-to-one). If any horizontal line intersects the graph more than once, the function is not injective.

$y=x^2$ is not an injective function.

$y=x^3$ is an injective function.

Since any bijective function is also injective, it also passes the horizontal line test.