Question Number 33355 by caravan msup abdo. last updated on 15/Apr/18
$${let}\:{a}\geqslant\mathrm{1}\:{find}\:{the}\:{radius}\:{of} \\ $$$$\sum_{{n}\geqslant\mathrm{1}} {arc}\:{cos}\left(\mathrm{1}−\frac{\mathrm{1}}{{n}^{{a}} }\right){z}^{{n}} \\ $$