A rectangular hyperbola has asymptotes that are perpendicular to each other. These two points are specified by the slopes of the two asymptotes of the hyperbola. Because a skew reflection leaves the hyperbola fixed, the pair of asymptotes is fixed, too. There the asymptotes of the unit hyperbola form a light cone. For example, one may identify the asymptotes to the unit hyperbola in this manner. Instead, they can change concavity around vertical asymptotes or discontinuities. Polynomials do not have horizontal asymptotes; such asymptotes may however occur with rational functions. A sigmoid function is constrained by a pair of horizontal asymptotes as formula 2. "Horizontal asymptotes" are horizontal lines that the graph of the function approaches as . At formula 10 the asymptotes are at right angles. This function has three vertical asymptotes formula 43 a continuous and monotone increasing function. In the case of the curve formula 1 the asymptotes are the two coordinate axes. There are three kinds of asymptotes&colon; "horizontal", "vertical" and "oblique". In this case, the curve has vertical asymptotes and this limits the span to . At distances greater than 25 pipe diameters the error asymptotes to zero.