Assume that $f$ lies in the class of starlike functions of order $\alpha \in [0,1)$, that is, which are regular and univalent for $|z|<1$ and such that $${\rm Re} \left (\frac{zf'(z)}{f(z)} \right ) > \alpha ~\mbox{ for } |z|<1. $$ In this paper we show that for each $\alpha \in [0,1)$, the following sharp inequality holds: $$ |f(re^{i\theta})|^{-1} \int_{0}^{r}|f'(ue^{i\theta})| du \leq \frac{\Gamma (\frac12)\Gamma (2-\alpha )}{\Gamma (\frac32-\alpha )} ~\mbox {for every $r<1$ and $\theta$}... (read more)

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