# Inverse function theorem

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

### Simple version at a specific point

Suppose $f$ is a function of one variable that is a one-one function and $a$ is in the domain of $f$. Suppose $f$ is [differentiable function|differentiable]] at $a$ and $b = f(a)$. Suppose further that the derivative $f'(a)$ is nonzero, i.e., $f'(a) \ne 0$. Then:

The inverse function $f^{-1}$ is differentiable at $b$, and further: $(f^{-1})'(b) = \frac{1}{f'(a)}$

### Simple version at a generic point

Suppose $f$ is a function of one variable that is a one-one function. Then, the formula for the derivative of the inverse function is as follows: $(f^{-1})'(x) = \frac{1}{f'(f^{-1}(x))}$

with the formula applicable at all points in the range of $f$ for which $f'(f^{-1}(x))$ exists and is nonzero.

### One-sided versions

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### Infinity-sensitive versions

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