Derivative of Monotone Function

Theorem

Let $f$ be a real function which is continuous on the closed interval $\left[{a \,.\,.\, b}\right]$ and differentiable on the open interval $\left({a \,.\,.\, b}\right)$.

• If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \ge 0$, then $f$ is increasing on $\left[{a \,.\,.\, b}\right]$.
• If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) > 0$, then $f$ is strictly increasing on $\left[{a \,.\,.\, b}\right]$.

• If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \le 0$, then $f$ is decreasing on $\left[{a \,.\,.\, b}\right]$.
• If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) < 0$, then $f$ is strictly decreasing on $\left[{a \,.\,.\, b}\right]$.

Proof

Let $c, d \in \left[{a \,.\,.\, b}\right]: c < d$.

Then $f$ satisfies the conditions of the Mean Value Theorem on $\left[{c \,.\,.\, d}\right]$.

Hence:

$\exists \xi \in \left({c \,.\,.\, d}\right): f' \left({\xi}\right) = \dfrac {f \left({d}\right) - f \left({c}\right)} {d - c}$

If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) \ge 0$, then $f^{\prime} \left({\xi}\right) \ge 0$ and hence:

$f \left({d}\right) \ge f \left({c}\right)$

Thus $f$ is increasing on $\left[{a \,.\,.\, b}\right]$

If $\forall x \in \left({a \,.\,.\, b}\right): f' \left({x}\right) > 0$, then $f' \left({\xi}\right) > 0$ and hence:

$f \left({d}\right) > f \left({c}\right)$

Thus $f$ is strictly increasing on $\left[{a \,.\,.\, b}\right]$.

The other cases follow similarly.

$\blacksquare$

Sources

• 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 12.7$