 # A function whose derivative at 0 is one but which is not increasing near 0

From the mean value theorem, a real function whose derivative is strictly positive at every point of an interval is strictly increasing. In particular, a continuously differentiable function $$f$$ defined in a non-degenerate interval $$I$$ with a strictly positive derivative at a point $$a$$ of the interval is strictly increasing near that point. For the proof, we just have to notice that as $$f^\prime$$ is continuous and $$f^\prime(a) > 0$$, $$f^\prime$$ is strictly positive within an interval $$J \subset I$$ containing $$a$$. By the mean value theorem, $$f$$ is strictly increasing on $$J$$.

We now suppose that $$f$$ is differentiable on an interval $$I$$ containing $$0$$ with $$f^\prime(0)>0$$. For $$x>0$$ sufficiently close to zero we have $$\displaystyle \frac{f(x)-f(0)}{x-0} > \frac{f^\prime(0)}{2}>0$$, hence $$f(x)>f(0)$$. But that doesn’t imply that $$f$$ is strictly increasing in a neighborhood of zero. Let’s prove it with a counterexample.

Consider the function:
$\begin{array}{l|rcl} f : & \mathbb{R} & \longrightarrow & \mathbb{R} \\ & 0 & \longmapsto & 0 \\ & x & \longmapsto & x + 4x^2 \sin(\frac{1}{x}) \text{ for } x \neq 0 \end{array}$ $$f$$ is continuous and differentiable at all non-zero points. $$f$$ is continuous at $$0$$ as for $$x \in \mathbb{R}$$ we have $$\vert f(x) – f(0) \vert = \vert f(x) \vert \le \vert x \vert + 4 \vert x \vert^2$$. The function $$g: x \mapsto x^2 \sin(\frac{1}{x})$$ is differentiable at $$0$$ as for $$x \neq 0$$ we have the inequality:
$\left\vert \frac{g(x)-g(0)}{x-0} \right\vert=\left\vert \frac{g(x)}{x}\right\vert \le \vert x \vert$ proving that $$g$$ is differentiable at $$0$$ with a derivative equal to zero at that point. As $$f(x)=4g(x)+x$$, $$f$$ is differentiable at $$0$$ with $$f^\prime(0)=1$$. For all $$x \neq 0$$ we have:
$f^\prime(x)=1-4 \cos(\frac{1}{x})+8x \sin(\frac{1}{x})$ Therefore for $$\vert x \vert \le 1/8$$ we get:
$-1 \le f^\prime(x)-\left (1-4 \cos(\frac{1}{x}) \right ) \le 1$ and
$-4 \cos(\frac{1}{x}) \le f^\prime(x)\le 2-4 \cos(\frac{1}{x})$ Consequently for $$n \ge 1$$ integer:

• when $$\displaystyle \frac{1}{x} \in (2n\pi + \frac{2\pi}{3},2n\pi + \frac{4\pi}{3})$$ we have $$\displaystyle \cos(\frac{1}{x}) \le -\frac{1}{2}$$ hence for $$\displaystyle x \in (\frac{1}{2n\pi + \frac{4\pi}{3}},\frac{1}{2n\pi + \frac{2\pi}{3}})$$: $$f^\prime(x) \ge -4 \cos(\frac{1}{x}) \ge 2$$. Which implies that $$f$$ is strictly increasing on each interval $$(\frac{1}{2n\pi + \frac{4\pi}{3}},\frac{1}{2n\pi + \frac{2\pi}{3}})$$.
• Similarly for $$\displaystyle \frac{1}{x} \in ((2n+1)\pi + \frac{5\pi}{6},(2n+1)\pi + \frac{7\pi}{6})$$ we have $$\displaystyle \cos(\frac{1}{x}) \ge \frac{\sqrt{3}}{2}$$ hence for $$\displaystyle x \in (\frac{1}{(2n+1)\pi + \frac{7\pi}{6}},\frac{1}{(2n+1)\pi + \frac{5\pi}{6}})$$: $$f^\prime(x) \le 2-4 \cos(\frac{1}{x}) \le 2(1-\sqrt{3}) <0$$. Which implies that $$f$$ is strictly decreasing on each interval $$(\frac{1}{(2n+1)\pi + \frac{7\pi}{6}},\frac{1}{(2n+1)\pi + \frac{5\pi}{6}})$$.

Finally $$f$$ is not strictly increasing in any neighborhood of $$0$$ although $$f^\prime (0) >0$$.