The critical points of a cubic function are its stationary points, that is the points where the slope of the function is zero. Thus the critical points of a cubic function defined by occur at values of such that the derivative of the cubic function is zero. The solutions of this equation are the -values of the critical points and are given, using the quadratic formula, by The sign of the expression inside the square root determines the number of critical points. If it is positive, then there are two critical points, one is a local maximum, and the other is a local minimum. If, then there is only one critical point, which is an inflection point. If, then there are no critical points. In the two latter cases, that is, if is nonpositive, the cubic function is strictly monotonic. See the figure for an example of the case. The inflection point of a function is where that function changes concavity. An inflection point occurs when the second derivative is zero, and the third derivative is nonzero. Thus a cubic function has always a single inflection point, which occurs at
Classification
The graph of a cubic function is an example of a cubic curve. Although cubic functions depend on four parameters, their graph can have only very few shapes. In fact, the graph of a cubic function is always similar to the graph of a function of the form This similarity can be built as the composition of translations parallel to the coordinates axes, a homothecy, and, possibly, a reflection with respect to the -axis. A further non-uniform scaling can transform the graph into the graph of one among the three cubic functions This means that there are only three graphs of cubic functions up to an affine transformation. The above geometric transformations can be built in the following way, when starting from a general cubic function Firstly, if, the change of variable allows supposing. After this change of variable, the new graph is the mirror image of the previous one, with respect of the -axis. Then, the change of variable provides a function of the form This corresponds to a translation parallel to the -axis. The change of variable corresponds to a translation with respect to the -axis, and gives a function of the form The change of variable corresponds to a uniform scaling, and give, after multiplication by a function of the form which is the simplest form that can be obtained by a similarity. Then, if, the non-uniform scaling gives, after division by where has the value 1 or –1, depending on the sign of. If one defines the latter form of the function applies to all cases.
Symmetry
For a cubic function of the form the inflection point is thus the origin. As such a function is an odd function, its graph is symmetric with respect to the inflection point, and invariant under a rotation of a half turn around the inflection point. As these properties are invariant by similarity, the following is true for all cubic functions. The graph of a cubic function is symmetric with respect to its inflection point, and is invariant under a rotation of a half turn around the inflection point.
Collinearities
The tangent lines to the graph of a cubic function at three collinear points intercept the cubic again at collinear points. This can be seen as follows. As this property is invariant under a rigid motion, one may suppose that the function has the form If is a real number, then the tangent to the graph of at the point is the line So, the intersection point between this line and the graph of can be obtained solving the equation, that is which can be rewritten and factorized as So, the tangent intercepts the cubic at So, the function that maps a point of the graph to the other point where the tangent intercepts the graph is This is an affine transformation that transforms collinear points into collinear points. This proves the claimed result.
Cubic interpolation
Given the values of a function and its derivative at two points, there is exactly one cubic function that has the same four values, which is called a cubic Hermite spline. There are two standard ways for using this fact. Firstly, if one knows, for example by physical measurement, the values of a function and its derivative at some sampling points, one can interpolate the function with a continuously differentiable function, which is a piecewise cubic function. If the value of a function is known at several points, cubic interpolation consists in approximating the function by a continuously differentiable function, which is piecewise cubic. For having a uniquely defined interpolation, two more constraints must be added, such as the values of the derivatives at the endpoints, or a zero curvature at the endpoints.
