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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). Asymptotic curve hvo1.svg
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. Asymptote02 vectorial.svg
A curve intersecting an asymptote infinitely many times.

In analytic geometry, an asymptote ( /ˈæsɪmptt/ ) of a curve is a line such that the distance between the curve and the line approaches zero as one or both of the x or y coordinates tends to infinity. In projective geometry and related contexts, an asymptote of a curve is a line which is tangent to the curve at a point at infinity. [1] [2]


The word asymptote is derived from the Greek ἀσύμπτωτος (asumptōtos) which means "not falling together", from ἀ priv. + σύν "together" + πτωτ-ός "fallen". [3] The term was introduced by Apollonius of Perga in his work on conic sections, but in contrast to its modern meaning, he used it to mean any line that does not intersect the given curve. [4]

There are three kinds of asymptotes: horizontal, vertical and oblique. For curves given by the graph of a function y = ƒ(x), horizontal asymptotes are horizontal lines that the graph of the function approaches as x tends to + or . Vertical asymptotes are vertical lines near which the function grows without bound. An oblique asymptote has a slope that is non-zero but finite, such that the graph of the function approaches it as x tends to + or .

More generally, one curve is a curvilinear asymptote of another (as opposed to a linear asymptote) if the distance between the two curves tends to zero as they tend to infinity, although the term asymptote by itself is usually reserved for linear asymptotes.

Asymptotes convey information about the behavior of curves in the large, and determining the asymptotes of a function is an important step in sketching its graph. [5] The study of asymptotes of functions, construed in a broad sense, forms a part of the subject of asymptotic analysis.


{\displaystyle f(x)={\tfrac {1}{x}}}
graphed on Cartesian coordinates. The x and y-axis are the asymptotes. Hyperbola one over x.svg
graphed on Cartesian coordinates. The x and y-axis are the asymptotes.

The idea that a curve may come arbitrarily close to a line without actually becoming the same may seem to counter everyday experience. The representations of a line and a curve as marks on a piece of paper or as pixels on a computer screen have a positive width. So if they were to be extended far enough they would seem to merge, at least as far as the eye could discern. But these are physical representations of the corresponding mathematical entities; the line and the curve are idealized concepts whose width is 0 (see Line). Therefore, the understanding of the idea of an asymptote requires an effort of reason rather than experience.

Consider the graph of the function shown in this section. The coordinates of the points on the curve are of the form where x is a number other than 0. For example, the graph contains the points (1,1), (2, 0.5), (5, 0.2), (10, 0.1), ... As the values of become larger and larger, say 100, 1,000, 10,000 ..., putting them far to the right of the illustration, the corresponding values of , .01, .001, .0001, ..., become infinitesimal relative to the scale shown. But no matter how large becomes, its reciprocal is never 0, so the curve never actually touches the x-axis. Similarly, as the values of become smaller and smaller, say .01, .001, .0001, ..., making them infinitesimal relative to the scale shown, the corresponding values of , 100, 1,000, 10,000 ..., become larger and larger. So the curve extends farther and farther upward as it comes closer and closer to the y-axis. Thus, both the x and y-axis are asymptotes of the curve. These ideas are part of the basis of concept of a limit in mathematics, and this connection is explained more fully below. [6]

Asymptotes of functions

The asymptotes most commonly encountered in the study of calculus are of curves of the form y = ƒ(x). These can be computed using limits and classified into horizontal, vertical and oblique asymptotes depending on their orientation. Horizontal asymptotes are horizontal lines that the graph of the function approaches as x tends to +∞ or ∞. As the name indicates they are parallel to the x-axis. Vertical asymptotes are vertical lines (perpendicular to the x-axis) near which the function grows without bound. Oblique asymptotes are diagonal lines such that the difference between the curve and the line approaches 0 as x tends to +∞ or ∞.

Vertical asymptotes

The line x = a is a vertical asymptote of the graph of the function y = ƒ(x) if at least one of the following statements is true:

where is the limit as x approaches the value a from the left (from lesser values), and is the limit as x approaches a from the right.

For example, if ƒ(x) = x/(x–1), the numerator approaches 1 and the denominator approaches 0 as x approaches 1. So

and the curve has a vertical asymptote x = 1.

The function ƒ(x) may or may not be defined at a, and its precise value at the point x = a does not affect the asymptote. For example, for the function

has a limit of +∞ as x 0+, ƒ(x) has the vertical asymptote x = 0, even though ƒ(0) = 5. The graph of this function does intersect the vertical asymptote once, at (0, 5). It is impossible for the graph of a function to intersect a vertical asymptote (or a vertical line in general) in more than one point. Moreover, if a function is continuous at each point where it is defined, it is impossible that its graph does intersect any vertical asymptote.

A common example of a vertical asymptote is the case of a rational function at a point x such that the denominator is zero and the numerator is non-zero.

If a function has a vertical asymptote, then it isn't necessarily true that the derivative of the function has a vertical asymptote at the same place. An example is

at .

This function has a vertical asymptote at because



The derivative of is the function


For the sequence of points


that approaches both from the left and from the right, the values are constantly . Therefore, both one-sided limits of at can be neither nor . Hence doesn't have a vertical asymptote at .

Horizontal asymptotes

The arctangent function has two different asymptotes Asymptote03.png
The arctangent function has two different asymptotes

Horizontal asymptotes are horizontal lines that the graph of the function approaches as x ±. The horizontal line y = c is a horizontal asymptote of the function y = ƒ(x) if

or .

In the first case, ƒ(x) has y = c as asymptote when x tends to , and in the second ƒ(x) has y = c as an asymptote as x tends to +∞.

For example, the arctangent function satisfies


So the line y = –π/2 is a horizontal asymptote for the arctangent when x tends to –∞, and y = π/2 is a horizontal asymptote for the arctangent when x tends to +∞.

Functions may lack horizontal asymptotes on either or both sides, or may have one horizontal asymptote that is the same in both directions. For example, the function ƒ(x) = 1/(x2+1) has a horizontal asymptote at y = 0 when x tends both to and +∞ because, respectively,

Other common functions that have one or two horizontal asymptotes include x ↦ 1/x (that has an hyperbola as it graph), the Gaussian function the error function, and the logistic function.

Oblique asymptotes

In the graph of
{\displaystyle f(x)=x+{\tfrac {1}{x}}}
, the y-axis (x = 0) and the line y = x are both asymptotes. 1-over-x-plus-x.svg
In the graph of , the y-axis (x = 0) and the line y = x are both asymptotes.

When a linear asymptote is not parallel to the x- or y-axis, it is called an oblique asymptote or slant asymptote. A function ƒ(x) is asymptotic to the straight line y = mx + n (m  0) if

In the first case the line y = mx + n is an oblique asymptote of ƒ(x) when x tends to +∞, and in the second case the line y = mx + n is an oblique asymptote of ƒ(x) when x tends to ∞.

An example is ƒ(x) = x + 1/x, which has the oblique asymptote y = x (that is m = 1, n = 0) as seen in the limits

Elementary methods for identifying asymptotes

The asymptotes of many elementary functions can be found without the explicit use of limits (although the derivations of such methods typically use limits).

General computation of oblique asymptotes for functions

The oblique asymptote, for the function f(x), will be given by the equation y = mx + n. The value for m is computed first and is given by

where a is either or depending on the case being studied. It is good practice to treat the two cases separately. If this limit doesn't exist then there is no oblique asymptote in that direction.

Having m then the value for n can be computed by

where a should be the same value used before. If this limit fails to exist then there is no oblique asymptote in that direction, even should the limit defining m exist. Otherwise y = mx + n is the oblique asymptote of ƒ(x) as x tends to a.

For example, the function ƒ(x) = (2x2 + 3x + 1)/x has

and then

so that y = 2x + 3 is the asymptote of ƒ(x) when x tends to +∞.

The function ƒ(x) = lnx has

and then
, which does not exist.

So y = lnx does not have an asymptote when x tends to +∞.

Asymptotes for rational functions

A rational function has at most one horizontal asymptote or oblique (slant) asymptote, and possibly many vertical asymptotes.

The degree of the numerator and degree of the denominator determine whether or not there are any horizontal or oblique asymptotes. The cases are tabulated below, where deg(numerator) is the degree of the numerator, and deg(denominator) is the degree of the denominator.

The cases of horizontal and oblique asymptotes for rational functions
deg(numerator)−deg(denominator)Asymptotes in generalExampleAsymptote for example
< 0
= 0y = the ratio of leading coefficients
= 1y = the quotient of the Euclidean division of the numerator by the denominator
> 1noneno linear asymptote, but a curvilinear asymptote exists

The vertical asymptotes occur only when the denominator is zero (If both the numerator and denominator are zero, the multiplicities of the zero are compared). For example, the following function has vertical asymptotes at x = 0, and x = 1, but not at x = 2.

Oblique asymptotes of rational functions

Black: the graph of
{\displaystyle f(x)=(x^{2}+x+1)/(x+1)}
. Red: the asymptote
{\displaystyle y=x}
. Green: difference between the graph and its asymptote for
{\displaystyle x=1,2,3,4,5,6} SlantAsymptoteError.svg
Black: the graph of . Red: the asymptote . Green: difference between the graph and its asymptote for

When the numerator of a rational function has degree exactly one greater than the denominator, the function has an oblique (slant) asymptote. The asymptote is the polynomial term after dividing the numerator and denominator. This phenomenon occurs because when dividing the fraction, there will be a linear term, and a remainder. For example, consider the function

shown to the right. As the value of x increases, f approaches the asymptote y = x. This is because the other term, 1/(x+1), approaches 0.

If the degree of the numerator is more than 1 larger than the degree of the denominator, and the denominator does not divide the numerator, there will be a nonzero remainder that goes to zero as x increases, but the quotient will not be linear, and the function does not have an oblique asymptote.

Transformations of known functions

If a known function has an asymptote (such as y=0 for f(x)=ex), then the translations of it also have an asymptote.

If a known function has an asymptote, then the scaling of the function also have an asymptote.

For example, f(x)=ex-1+2 has horizontal asymptote y=0+2=2, and no vertical or oblique asymptotes.

General definition

(sec(t), cosec(t)), or x + y = (xy) , with 2 horizontal and 2 vertical asymptotes. Graph of sect csct.svg
(sec(t), cosec(t)), or x + y = (xy) , with 2 horizontal and 2 vertical asymptotes.

Let A : (a,b) R2 be a parametric plane curve, in coordinates A(t) = (x(t),y(t)). Suppose that the curve tends to infinity, that is:

A line ℓ is an asymptote of A if the distance from the point A(t) to ℓ tends to zero as t  b. [7] From the definition, only open curves that have some infinite branch can have an asymptote. No closed curve can have an asymptote.

For example, the upper right branch of the curve y = 1/x can be defined parametrically as x = t, y = 1/t (where t > 0). First, x  ∞ as t  ∞ and the distance from the curve to the x-axis is 1/t which approaches 0 as t  ∞. Therefore, the x-axis is an asymptote of the curve. Also, y  ∞ as t  0 from the right, and the distance between the curve and the y-axis is t which approaches 0 as t  0. So the y-axis is also an asymptote. A similar argument shows that the lower left branch of the curve also has the same two lines as asymptotes.

Although the definition here uses a parameterization of the curve, the notion of asymptote does not depend on the parameterization. In fact, if the equation of the line is then the distance from the point A(t) = (x(t),y(t)) to the line is given by

if γ(t) is a change of parameterization then the distance becomes

which tends to zero simultaneously as the previous expression.

An important case is when the curve is the graph of a real function (a function of one real variable and returning real values). The graph of the function y = ƒ(x) is the set of points of the plane with coordinates (x,ƒ(x)). For this, a parameterization is

This parameterization is to be considered over the open intervals (a,b), where a can be ∞ and b can be +∞.

An asymptote can be either vertical or non-vertical (oblique or horizontal). In the first case its equation is x = c, for some real number c. The non-vertical case has equation y = mx + n, where m and are real numbers. All three types of asymptotes can be present at the same time in specific examples. Unlike asymptotes for curves that are graphs of functions, a general curve may have more than two non-vertical asymptotes, and may cross its vertical asymptotes more than once.

Curvilinear asymptotes

x +2x+3 is a parabolic asymptote to (x +2x +3x+4)/x Nonlinear asymptote.svg
x +2x+3 is a parabolic asymptote to (x +2x +3x+4)/x

Let A : (a,b) R2 be a parametric plane curve, in coordinates A(t) = (x(t),y(t)), and B be another (unparameterized) curve. Suppose, as before, that the curve A tends to infinity. The curve B is a curvilinear asymptote of A if the shortest distance from the point A(t) to a point on B tends to zero as t  b. Sometimes B is simply referred to as an asymptote of A, when there is no risk of confusion with linear asymptotes. [8]

For example, the function

has a curvilinear asymptote y = x2 + 2x + 3, which is known as a parabolic asymptote because it is a parabola rather than a straight line. [9]

Asymptotes and curve sketching

Asymptotes are used in procedures of curve sketching. An asymptote serves as a guide line to show the behavior of the curve towards infinity. [10] In order to get better approximations of the curve, curvilinear asymptotes have also been used [11] although the term asymptotic curve seems to be preferred. [12]

Algebraic curves

A cubic curve, the folium of Descartes (solid) with a single real asymptote (dashed). Folium Of Descartes.svg
A cubic curve, the folium of Descartes (solid) with a single real asymptote (dashed).

The asymptotes of an algebraic curve in the affine plane are the lines that are tangent to the projectivized curve through a point at infinity. [13] For example, one may identify the asymptotes to the unit hyperbola in this manner. Asymptotes are often considered only for real curves, [14] although they also make sense when defined in this way for curves over an arbitrary field. [15]

A plane curve of degree n intersects its asymptote at most at n2 other points, by Bézout's theorem, as the intersection at infinity is of multiplicity at least two. For a conic, there are a pair of lines that do not intersect the conic at any complex point: these are the two asymptotes of the conic.

A plane algebraic curve is defined by an equation of the form P(x,y) = 0 where P is a polynomial of degree n

where Pk is homogeneous of degree k. Vanishing of the linear factors of the highest degree term Pn defines the asymptotes of the curve: setting Q = Pn, if Pn(x, y) = (axby) Qn1(x, y), then the line

is an asymptote if and are not both zero. If and , there is no asymptote, but the curve has a branch that looks like a branch of parabola. Such a branch is called a parabolic branch, even when it does not have any parabola that is a curvilinear asymptote. If the curve has a singular point at infinity which may have several asymptotes or parabolic branches.

Over the complex numbers, Pn splits into linear factors, each of which defines an asymptote (or several for multiple factors). Over the reals, Pn splits in factors that are linear or quadratic factors. Only the linear factors correspond to infinite (real) branches of the curve, but if a linear factor has multiplicity greater than one, the curve may have several asymptotes or parabolic branches. It may also occur that such a multiple linear factor corresponds to two complex conjugate branches, and does not corresponds to any infinite branch of the real curve. For example, the curve x4 + y2 - 1 = 0 has no real points outside the square , but its highest order term gives the linear factor x with multiplicity 4, leading to the unique asymptote x=0.

Asymptotic cone

Hyperbolas, obtained cutting the same right circular cone with a plane and their asymptotes. Conic section hyperbola.gif
Hyperbolas, obtained cutting the same right circular cone with a plane and their asymptotes.

The hyperbola

has the two asymptotes

The equation for the union of these two lines is

Similarly, the hyperboloid

is said to have the asymptotic cone [16] [17]

The distance between the hyperboloid and cone approaches 0 as the distance from the origin approaches infinity.

More generally, consider a surface that has an implicit equation where the are homogeneous polynomials of degree and . Then the equation defines a cone which is centered at the origin. It is called an asymptotic cone, because the distance to the cone of a point of the surface tends to zero when the point on the surface tends to infinity.

See also

Related Research Articles

Derivative Operation in calculus

In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value with respect to a change in its argument. Derivatives are a fundamental tool of calculus. For example, the derivative of the position of a moving object with respect to time is the object's velocity: this measures how quickly the position of the object changes when time advances.

Exponential function Class of specific mathematical functions

In mathematics, the exponential function is the function where the base e = 2.71828... is Euler's number and the argument x occurs as an exponent. More generally, an exponential function is a function of the form where the base b is a positive real number.

LHôpitals rule Mathematical rule for evaluating certain limits

In mathematics, more specifically calculus, L'Hôpital's rule or L'Hospital's rule is a theorem which provides a technique to evaluate limits of indeterminate forms. Application of the rule often converts an indeterminate form to an expression that can be easily evaluated by substitution. The rule is named after the 17th-century French mathematician Guillaume de l'Hôpital. Although the rule is often attributed to L'Hôpital, the theorem was first introduced to him in 1694 by the Swiss mathematician Johann Bernoulli.

Uniform continuity Uniform restraint of the change in functions

In mathematics, a function f is uniformly continuous if, roughly speaking, it is possible to guarantee that f(x) and f(y) be as close to each other as we please by requiring only that x and y be sufficiently close to each other; unlike ordinary continuity, where the maximum distance between f(x) and f(y) may depend on x and y themselves.

In mathematics, the affinely extended real number system is obtained from the real number system by adding two infinity elements: and where the infinities are treated as actual numbers. It is useful in describing the algebra on infinities and the various limiting behaviors in calculus and mathematical analysis, especially in the theory of measure and integration. The affinely extended real number system is denoted or or It is the Dedekind–MacNeille completion of the real numbers.

Taylors theorem Approximation of a function by a truncated power series

In calculus, Taylor's theorem gives an approximation of a k-times differentiable function around a given point by a polynomial of degree k, called the kth-order Taylor polynomial. For a smooth function, the Taylor polynomial is the truncation at the order k of the Taylor series of the function. The first-order Taylor polynomial is the linear approximation of the function, and the second-order Taylor polynomial is often referred to as the quadratic approximation. There are several versions of Taylor's theorem, some giving explicit estimates of the approximation error of the function by its Taylor polynomial.

Algebraic curve Curve defined as zeros of polynomials

In mathematics, an affine algebraic plane curve is the zero set of a polynomial in two variables. A projective algebraic plane curve is the zero set in a projective plane of a homogeneous polynomial in three variables. An affine algebraic plane curve can be completed in a projective algebraic plane curve by homogenizing its defining polynomial. Conversely, a projective algebraic plane curve of homogeneous equation h(x, y, t) = 0 can be restricted to the affine algebraic plane curve of equation h(x, y, 1) = 0. These two operations are each inverse to the other; therefore, the phrase algebraic plane curve is often used without specifying explicitly whether it is the affine or the projective case that is considered.

In mathematics, the limit of a function is a fundamental concept in calculus and analysis concerning the behavior of that function near a particular input.

In mathematics, a rational function is any function that can be defined by a rational fraction, which is an algebraic fraction such that both the numerator and the denominator are polynomials. The coefficients of the polynomials need not be rational numbers; they may be taken in any field K. In this case, one speaks of a rational function and a rational fraction over K. The values of the variables may be taken in any field L containing K. Then the domain of the function is the set of the values of the variables for which the denominator is not zero, and the codomain is L.

Projectile motion Motion of an object given an initial velocity which then follows a path determined entirely by gravity

Projectile motion is a form of motion experienced by a launched object. Ballistics is the science of dynamics that deals with the flight, behavior and effects of projectiles, especially bullets, unguided bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.

In mathematics, a divergent series is an infinite series that is not convergent, meaning that the infinite sequence of the partial sums of the series does not have a finite limit.

Projectively extended real line

In real analysis, the projectively extended real line, is the extension of the set of the real numbers, by a point denoted . It is thus the set with the standard arithmetic operations extended where possible, and is sometimes denoted by The added point is called the point at infinity, because it is considered as a neighbour of both ends of the real line. More precisely, the point at infinity is the limit of every sequence of real numbers whose absolute values are increasing and unbounded.

Critical point (mathematics)

Critical point is a wide term used in many branches of mathematics.

In mathematics, the (linear) Peetre theorem, named after Jaak Peetre, is a result of functional analysis that gives a characterisation of differential operators in terms of their effect on generalized function spaces, and without mentioning differentiation in explicit terms. The Peetre theorem is an example of a finite order theorem in which a function or a functor, defined in a very general way, can in fact be shown to be a polynomial because of some extraneous condition or symmetry imposed upon it.

The Gompertz curve or Gompertz function is a type of mathematical model for a time series, named after Benjamin Gompertz (1779–1865). It is a sigmoid function which describes growth as being slowest at the start and end of a given time period. The right-hand or future value asymptote of the function is approached much more gradually by the curve than the left-hand or lower valued asymptote. This is in contrast to the simple logistic function in which both asymptotes are approached by the curve symmetrically. It is a special case of the generalised logistic function. The function was originally designed to describe human mortality, but since has been modified to be applied in biology, with regard to detailing populations.

Vertical tangent

In mathematics, particularly calculus, a vertical tangent is a tangent line that is vertical. Because a vertical line has infinite slope, a function whose graph has a vertical tangent is not differentiable at the point of tangency.

In applied mathematics, polyharmonic splines are used for function approximation and data interpolation. They are very useful for interpolating and fitting scattered data in many dimensions. Special cases include thin plate splines and natural cubic splines in one dimension.

In numerical analysis, Aitken's delta-squared process or Aitken Extrapolation is a series acceleration method, used for accelerating the rate of convergence of a sequence. It is named after Alexander Aitken, who introduced this method in 1926. Its early form was known to Seki Kōwa and was found for rectification of the circle, i.e. the calculation of π. It is most useful for accelerating the convergence of a sequence that is converging linearly.

In statistical modeling, polynomial functions and rational functions are sometimes used as an empirical technique for curve fitting.

Most of the terms listed in Wikipedia glossaries are already defined and explained within Wikipedia itself. However, glossaries like this one are useful for looking up, comparing and reviewing large numbers of terms together. You can help enhance this page by adding new terms or writing definitions for existing ones.


General references
Specific references
  1. Williamson, Benjamin (1899), "Asymptotes", An elementary treatise on the differential calculus
  2. Nunemacher, Jeffrey (1999), "Asymptotes, Cubic Curves, and the Projective Plane", Mathematics Magazine, 72 (3): 183–192, CiteSeerX , doi:10.2307/2690881, JSTOR   2690881
  3. Oxford English Dictionary, second edition, 1989.
  4. D.E. Smith, History of Mathematics, vol 2 Dover (1958) p. 318
  5. Apostol, Tom M. (1967), Calculus, Vol. 1: One-Variable Calculus with an Introduction to Linear Algebra (2nd ed.), New York: John Wiley & Sons, ISBN   978-0-471-00005-1 , §4.18.
  6. Reference for section: "Asymptote" The Penny Cyclopædia vol. 2, The Society for the Diffusion of Useful Knowledge (1841) Charles Knight and Co., London p. 541
  7. Pogorelov, A. V. (1959), Differential geometry, Translated from the first Russian ed. by L. F. Boron, Groningen: P. Noordhoff N. V., MR   0114163 , §8.
  8. Fowler, R. H. (1920), The elementary differential geometry of plane curves, Cambridge, University Press, hdl:2027/uc1.b4073882, ISBN   0-486-44277-2 , p. 89ff.
  9. William Nicholson, The British enciclopaedia, or dictionary of arts and sciences; comprising an accurate and popular view of the present improved state of human knowledge, Vol. 5, 1809
  10. Frost, P. An elementary treatise on curve tracing (1918) online
  11. Fowler, R. H. The elementary differential geometry of plane curves Cambridge, University Press, 1920, pp 89ff.(online at
  12. Frost, P. An elementary treatise on curve tracing, 1918, page 5
  13. C.G. Gibson (1998) Elementary Geometry of Algebraic Curves, § 12.6 Asymptotes, Cambridge University Press ISBN   0-521-64140-3,
  14. Coolidge, Julian Lowell (1959), A treatise on algebraic plane curves, New York: Dover Publications, ISBN   0-486-49576-0, MR   0120551 , pp. 4044.
  15. Kunz, Ernst (2005), Introduction to plane algebraic curves, Boston, MA: Birkhäuser Boston, ISBN   978-0-8176-4381-2, MR   2156630 , p. 121.
  16. L.P. Siceloff, G. Wentworth, D.E. Smith Analytic geometry (1922) p. 271
  17. P. Frost Solid geometry (1875) This has a more general treatment of asymptotic surfaces.