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A horizontal line is a straight, flat line that goes from left to right. Given a function f : R → R {\displaystyle f\colon \mathbb {R} \to \mathbb {R} } (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 .
A graphical approach for a real-valued function of a real variable is the horizontal line test. If every horizontal line intersects the curve of f ( x ) {\displaystyle f(x)} in at most one point, then f {\displaystyle f} is injective or one-to-one.
In mathematics, injections, surjections, and bijections are classes of functions distinguished by the manner in which arguments (input expressions from the domain) and images (output expressions from the codomain) are related or mapped to each other. A function maps elements from its domain to elements in its codomain.
The vertical line test, shown graphically. The abscissa shows the domain of the (to be tested) function. In mathematics, the vertical line test is a visual way to determine if a curve is a graph of a function or not. A function can only have one output, y, for each unique input, x.
The term "horizontal line test" is sometimes used in calculus (it is the same idea as Vertical line test).However, this article is written in very general terms and is claimed to be part of set theory, which doesn't make any sense to me, because the bit about graphs and horizontal lines seems to require a real-valued function of a real variable.
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The graph of a function with a horizontal (y = 0), vertical (x = 0), and oblique asymptote (purple line, given by y = 2x) A curve intersecting an asymptote infinitely many times In analytic geometry , an asymptote ( / ˈ æ s ɪ m p t oʊ t / ) of a curve is a line such that the distance between the curve and the line approaches zero as one or ...
For example, y = x 2 fails the horizontal line test: it fails to be one-to-one. The inverse is the algebraic "function" x = ± y {\displaystyle x=\pm {\sqrt {y}}} . Another way to understand this, is that the set of branches of the polynomial equation defining our algebraic function is the graph of an algebraic curve .