# Discontinuous derivative

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Revision as of 11:00, 13 February 2019 by A WASSERMANN (talk | contribs)

Consider the function

- [math] f: \mathbb{R} \to \mathbb{R}, x \mapsto \begin{cases} x^2\sin(1/x),& x\neq 0\\ 0, x=0 \end{cases}\,. [/math]

[math]f[/math] is a continous and differentiable. But the derivative of [math]f\lt/\ltmath\gt is the function :\ltmath\gt f': \mathbb{R} \to \mathbb{R}, x \mapsto \begin{cases} 2\sin(1/x) - \cos(1/x), &x \neq 0\\ 0, x=0 \end{cases}\,. \ltmath\gt We observe that \ltmath\gtf'(0) = 0[/math] but [math]\lim_{x\to0}f'(x)[/math] does not exist.

Therefore, [math]f'[/math] is an example of a derivative which is not continuous.

### The underlying JavaScript code

```
var board = JXG.JSXGraph.initBoard('jxgbox', {axis:true, boundingbox:[-1/2,1/2,1/2,-1/2]});
var g = board.create('functiongraph', ["2*sin(1/x) - cos(1/x)"], {strokeColor: 'red'});
var f = board.create('functiongraph', ["x^2*sin(1/x)"], {strokeWidth:2});
```