Question Number 34721 by abdo mathsup 649 cc last updated on 10/May/18
$${let}\:\xi\left({x}\right)\:=\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\frac{\mathrm{1}}{{n}^{{x}} }\:\:{with}\:{x}>\mathrm{1} \\ $$$$\left.\mathrm{1}\right){prove}\:{that}\:\:\frac{\mathrm{1}}{{x}−\mathrm{1}}\:\leqslant\xi\left({x}\right)\leqslant\:\frac{{x}}{{x}−\mathrm{1}} \\ $$$$\left.\mathrm{2}\right)\:{find}\:{lim}_{{x}\rightarrow\mathrm{1}^{+} } \left({x}−\mathrm{1}\right)\xi\left({x}\right)\:. \\ $$