Buchstab function

The Buchstab function is a continuous function $\omega \colon [1,\infty) \rightarrow (0,\infty)$ defined by the initial value problem $$\dfrac{d}{du}(u\omega(u-1)); u \geq 2$$ and for $1 \leq u \geq 2$, $\omega(u)=\dfrac{1}{u}$.