Cambridge IGCSE Additional Mathematics · Syllabus 0606 · Functions
Horizontal Line Test
What is Horizontal Line Test?
A graphical test for invertibility: a function has an inverse function over a stated domain exactly when every horizontal line meets its graph at most once. A horizontal line meeting the graph twice exhibits two different inputs sharing one output, which makes the function many-one and therefore not invertible on that domain.
This definition is part of the Functions chapter in Cambridge IGCSE Additional Mathematics.