Is $f(x)=x\sin(1/x)$ with $f(0)=0$ of bounded variation on $[0,1]$? – Problem with abs. continuous

absolute-continuitybounded-variation

I am having the following trouble:

  1. From Is $f(x)=x\sin(\frac{1}{x})$ with $f(0)=0$ of bounded variation on $[0,1]$?, $x\sin(1/x)$ has not bounded variation in $[0,1]$.

  2. $x\sin(1/x)$ has derivative $-\cos(1/x)/x + \sin(1/x)$ a.e. and $\int_0^1 (-\cos(1/x)/x + \sin(1/x)) dx=\sin(1)$, i.e., $x\sin(1/x)$ is abs. continuous.

  3. Every abs. continuous function is of bounded variation.

What is the error?

Thank you, really much.

Best Answer

$$g(x) = -\frac{1}{x}\cos\left(\frac{1}{x}\right) + \sin\left(\frac{1}{x}\right)$$ is not integrable on $[0,1]$. Hence the equality

$$\int_0^1 (-\cos(1/x)/x + \sin(1/x)) dx=\sin(1)$$ is wrong and $g$ is not absolutely continuous.

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