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In differential calculus, an inflection point, point of inflection, flex, or inflection (inflexion) is a point on a curve at which the curve changes from being concave (concave downward) to convex (concave upward), or vice versa.
A point where the curvature vanishes but does not change sign is sometimes called a point of undulation or undulation point.
In algebraic geometry an inflection point is defined slightly more generally, as a point where the tangent meets the curve to order at least 3, and an undulation point or hyperflex is defined as a point where the tangent meets the curve to order at least 4.
A differentiable function has an inflection point at (x, f(x)) if and only if its first derivative, f′, has an isolated extremum at x. (This is not the same as saying that f has an extremum). That is, in some neighborhood, x is the one and only point at which f′ has a (local) minimum or maximum. If all extrema of f′ are isolated, then an inflection point is a point on the graph of f at which the tangent crosses the curve.
For an algebraic curve, a non singular point is an inflection point if and only if the multiplicity of the intersection of the tangent line and the curve (at the point of tangency) is odd and greater than 2.
For a twice differentiable function, an inflection point is a point on the graph at which the second derivative changes sign.
A necessary but not sufficient condition
If x is an inflection point for f then the second derivative, f″(x), is equal to zero if it exists, but this condition does not provide a sufficient definition of a point of inflection. One also needs the lowest-order (above the second) non-zero derivative to be of odd order (third, fifth, etc.). If the lowest-order non-zero derivative is of even order, the point is not a point of inflection, but an undulation point. However, in algebraic geometry, both inflection points and undulation points are usually called inflection points. An example of such an undulation point is y = x4 for x=0.
It follows from the definition that the sign of f′(x) on either side of the point (x,y) must be the same. If this is positive, the point is a rising point of inflection; if it is negative, the point is a falling point of inflection.
Categorization of points of inflection
Points of inflection can also be categorized according to whether f′(x) is zero or not zero.
- if f′(x) is zero, the point is a stationary point of inflection, also known as a saddle-point
- if f′(x) is not zero, the point is a non-stationary point of inflection
An example of a saddle point is the point (0,0) on the graph y = x3. The tangent is the x-axis, which cuts the graph at this point.
A non-stationary point of inflection can be visualised if the graph y = x3 is rotated slightly about the origin. The tangent at the origin still cuts the graph in two, but its gradient is non-zero.
Functions with discontinuities
Some functions change concavity without having points of inflection. Instead, they can change concavity around vertical asymptotes or discontinuities. Take, for example, the function 2x2/(x2 – 1). It is concave when |x| > 1 and convex when |x| < 1. However, it has no points of inflection because 1 and -1 are not in the domain of the function.
- Critical point (mathematics)
- Vertex (curve), a local minimum or maximum of curvature
- Hesse configuration formed by the nine inflection points of an elliptic curve
References and Sources
- Weisstein, Eric W., "Inflection Point", MathWorld.
- Hazewinkel, Michiel, ed. (2001), "Point of inflection", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4