# Derivative of Identity Function

## Theorem

Let $X$ be either set of either the real numbers $\R$ or the complex numbers $\C$.

Let $I_X: X \to X$ be the identity function.

Then:

$I_X' \left({x}\right) = 1$

where $I_X' \left({x}\right)$ denotes the derivative of $I_X$ with respect to $x$.

This can be presented for each of $\R$ and $\C$:

### Derivative of Identity Function for Real Numbers

Let $I_\R: \R \to \R$ be the identity function.

Then:

$I_\R' \left({x}\right) = 1$

### Derivative of Identity Function for Complex Numbers

Let $I_\C: \C \to \C$ be the identity function.

Then:

$\map {I_\C'} z = 1$

## Corollary

$\dfrac {\mathrm d} {\mathrm d x} \left({c x}\right) = c$

where $c$ is a constant.

## Also presented as

Note that this can be written:

$D_x \left({x}\right) = 1$

Using Leibniz's notation for derivatives it can also be stated as:

$\dfrac {\mathrm d x} {\mathrm d x} = 1$