Power Rule for Derivatives/Fractional Index

Theorem
Let $n \in \N^*$.

Let $f: \R \to \R$ be the real function defined as $f \left({x}\right) = x^{1/n}$.

Then:
 * $f^{\prime} \left({x}\right) = n x^{n-1}$

everywhere that $f \left({x}\right) = x^n$ is defined.

When $x = 0$ and $n = 0$, $f^{\prime} \left({x}\right)$ is undefined.

Proof from First Principles
Let $n \in \N^*$.

Thus, let $f \left({x}\right) = x^{1/n}$.

From the definition of power for rational numbers, or alternatively from the definition of the root of a number, $f \left({x}\right)$ is defined when $x \ge 0$.

(However, see the special case where $x = 0$.)

From Continuity of Root Function, $f \left({x}\right)$ is continuous over the open interval $\left({0 .. \infty}\right)$, but not at $x = 0$ where it is continuous only on the right.

Let $y > x$.

From Inequalities Concerning Roots, we have:
 * $\forall n \in \N^*: X Y^{1/n} \ \left|{x - y}\right| \le n X Y \ \left|{x^{1/n} - y^{1/n}}\right| \le Y X^{1/n} \ \left|{x - y}\right|$

where $x, y \in \left[{X .. Y}\right]$.

Setting $X = x$ and $Y = y$, this reduces (after algebra) to:


 * $\displaystyle \frac 1 {n y} y^{1/n} \le \frac {y^{1/n} - x^{1/n}} {y - x} \le \frac 1 {n x} x^{1/n}$

From the Squeeze Theorem, it follows that:
 * $\displaystyle \lim_{y \to x^+} \ \frac {y^{1/n} - x^{1/n}} {y - x} = \frac 1 {n x} x^{1/n} = \frac 1 n x^{\frac 1 n - 1}$

A similar argument shows that the left hand limit is the same.

Thus the result holds for $f \left({x}\right) = x^{1/n}$.

Proof using Derivative of Inverse
Let $n \in \N^*$.

Thus, let $f \left({x}\right) = y = x^{1/n}$.

Thus $f^{-1} \left({y}\right) = x = y^n$ from the definition of root.

So: