# Second derivative rule for inverse function

This article is about adifferentiation rule, i.e., a rule for differentiating a function expressed in terms of other functions whose derivatives are known.

View other differentiation rules

## Statement

### Simple version at a specific point

Suppose is a one-one function and is a point in the domain of such that is twice differentiable at and where denotes the derivative of . Suppose .

Then, we have the following formula for the second derivative of the inverse function :

### Simple version at a generic point

Suppose is a one-one function. Then, we have the following formula:

where the formula is applicable for all in the range of for which is twice differentiable at and the *first* derivative of at is nonzero.

MORE ON THE WAY THIS DEFINITION OR FACT IS PRESENTED: We first present the version that deals with a specific point (typically with a subscript) in the domain of the relevant functions, and then discuss the version that deals with a point that is free to move in the domain, by dropping the subscript. Why do we do this?

The purpose of the specific point version is to emphasize that the point is fixed for the duration of the definition, i.e., it does not move around while we are defining the construct or applying the fact. However, the definition or fact applies not just for a single point but for all points satisfying certain criteria, and thus we can get further interesting perspectives on it byvaryingthe point we are considering. This is the purpose of the second,generic pointversion.