Question Number 54830 by maxmathsup by imad last updated on 12/Feb/19
$${let}\:{V}_{{n}} =\:\int_{\mathrm{0}} ^{\infty} \:\:\frac{{cos}\left({nx}\right)}{{n}\:+{x}^{\mathrm{2}} }{dx}\:\:\:{with}\:{n}\:{integr}\:{nstural}\:{not}\:\mathrm{0}\:. \\ $$$$\left.\mathrm{1}\right)\:{calculate}\:{V}_{{n}} \\ $$$$\left.\mathrm{2}\right){calculate}\:{lim}_{{n}\rightarrow+\infty} {nV}_{{n}} \\ $$$$\left.\mathrm{3}\right)\:{calculate}\:{the}\:{sum}\:\sum_{{n}=\mathrm{0}} ^{\infty} \:{V}_{{n}} \\ $$
Commented by maxmathsup by imad last updated on 13/Feb/19
$$\left.\mathrm{1}\right)\:{we}\:{have}\:{V}_{{n}} =_{{x}=\sqrt{{n}}{t}} \:\:\:\:\:\int_{\mathrm{0}} ^{\infty} \:\:\frac{{cos}\left({n}\sqrt{{n}}{t}\right)}{{n}\left(\mathrm{1}+{t}^{\mathrm{2}} \right)}\:\sqrt{{n}}{dt}\:=\frac{\mathrm{1}}{\:\sqrt{{n}}}\:\int_{\mathrm{0}} ^{\infty} \:\:\frac{{cos}\left({n}\sqrt{{n}}{t}\right)}{\mathrm{1}+{t}^{\mathrm{2}} }{dt}\:\Rightarrow \\ $$$$\mathrm{2}\sqrt{{n}}{V}_{{n}} =\int_{−\infty} ^{+\infty} \:\:\frac{{cos}\left({n}\sqrt{{n}}{t}\right)}{{t}^{\mathrm{2}} \:+\mathrm{1}}{dt}\:={Re}\:\left(\:\int_{−\infty} ^{+\infty} \:\:\frac{{e}^{{in}\sqrt{{n}}{t}} }{{t}^{\mathrm{2}} \:+\mathrm{1}}{dt}\right)\:\:{let}\:\varphi\left({z}\right)=\frac{{e}^{{in}\sqrt{{n}}{z}} }{{z}^{\mathrm{2}} \:+\mathrm{1}} \\ $$$${the}\:{poles}\:{of}\:\varphi\:{are}\:{i}\:{and}\:−{i}\:\:{residus}\:{theorem}\:{give} \\ $$$$\int_{−\infty} ^{+\infty} \:\varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:{Res}\left(\varphi,{i}\right)\:{but}\:\varphi\left({z}\right)\:=\frac{{e}^{{in}\sqrt{{n}}{z}} }{\left({z}−{i}\right)\left({z}+{i}\right)} \\ $$$${Res}\left(\varphi,{i}\right)\:={lim}_{{z}\rightarrow{i}} \left({z}−{i}\right)\varphi\left({z}\right)\:=\:\frac{{e}^{−{n}\sqrt{{n}}} }{\mathrm{2}{i}}\:\Rightarrow\:\int_{−\infty} ^{+\infty} \:\varphi\left({z}\right){dz}\:=\mathrm{2}{i}\pi\:\frac{{e}^{−{n}\sqrt{{n}}} }{\mathrm{2}{i}} \\ $$$$=\pi\:{e}^{−{n}\sqrt{{n}}} \:\:\:\Rightarrow\mathrm{2}\sqrt{{n}}{V}_{{n}} =\pi\:{e}^{−{n}\sqrt{{n}}} \:\Rightarrow\:{V}_{{n}} =\frac{\pi}{\mathrm{2}\sqrt{{n}}}\:{e}^{−{n}\sqrt{{n}}} \\ $$$$\left.\mathrm{2}\right)\:{lim}_{{n}\rightarrow+\infty} {n}\:{V}_{{n}} ={lim}_{{n}\rightarrow+\infty} \:\:\frac{\pi}{\mathrm{2}}\:\sqrt{{n}}\:{e}^{−{n}\sqrt{{n}}} \:\:=\mathrm{0} \\ $$